Suspension Tuning & Optimization: Lateral Load Transfer Calculations
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Lateral Load Transfer Calculations
36.10
| 00:00 | - In this module, we're talking all about lateral load transfer calculations. |
| 00:03 | Now that's a little bit of a mouthful so let's dig into a little bit of a refresher to talk about exactly what that means. |
| 00:10 | So lateral load transfer, you should know from the course by now that it is a really important thing to control the balance of your car and it's all about taking advantage of the tyre load vertical sensitivity. |
| 00:21 | Now what do we mean by that? It's all about how the tyre forces are generated with respect to how much vertical load is on them and we know from the course that when we add more vertical load to one side of the car when we corner, while we are adding cornering force to the outside of the car, we are losing more on the outside than we gain on the inside due to the way the coefficient of friction works on tyres. |
| 00:42 | Now that is a big over simplification really on how tyres are actually working but it's still a relatively descriptive term, useful term for understanding things from a basic perspective. |
| 00:52 | Now what we're doing when we are tuning lateral load transfer distribution is affecting how that vertical load is reacted throughout the car. |
| 01:00 | So whether the front of the car or the front axle or the rear axle is taking more or less load. |
| 01:05 | That's really how we're making balance changes to the car by taking advantage of that vertical tyre load sensitivity. |
| 01:11 | Now the tools we have available to us that we've already gone through in the course, we know that there are 3 main components of that lateral load transfer that we have available to us, some which are more practical to tune than others. |
| 01:23 | The first is the non sprung mass lateral load transfer. |
| 01:26 | Now we know that the component of load transfer that comes from the unsprung mass, and that's really just dictated by the wheels, tyres, the uprights, the brakes, the suspension, all that stuff that makes up your unsprung mass, assuming you're not going to make any big changes to either the position or the location of that stuff then the non sprung mass lateral load transfer isn't really something that we're going to be using as a tuning tool on the car because it really is made up by the base components that we've got fitted to the car. |
| 01:51 | The next two components get a little bit more interesting, that's where we've got the elastic and the geometric weight transfer of the sprung mass individually. |
| 01:59 | So the elastic load transfer, that's all coming from things like our springs and our anti roll bars and to some extent our bump stops if we've got them engaged. |
| 02:06 | And from a steady state point of view we're ignoring the dampers so it's really just maybe springs, anti roll bars and to some extent bump stops. |
| 02:13 | On the geometric load transfer, this is really talking about the way our suspension geometry and our kinematics work. |
| 02:19 | Now the tools we've used to describe this throughout the course so far are really boiling things down to the roll centre and the roll axis. |
| 02:27 | Obviously as we know from the course, depending on where we place the roll centre with respect to the centre of gravity, that's going to affect the amount of geometric weight transfer we have. |
| 02:36 | Now the calculations involved with understanding the different tuning effects, whether it's changing a spring, maybe we're increasing or decreasing the stiffness from an anti roll bar, raising or lowering the roll centre at one end of the car, the calculations for each one of these components are actually pretty simple, there's nothing too complex about them. |
| 02:52 | It just really, when we put all of them together into one place, it's the contribution and understanding how they all interrelate to each other, that's when it does get a little bit complex. |
| 03:02 | So what we've done instead is I've put together a spreadsheet tool that you guys can download and use for yourself that has all of the calculations and everything taken care of for you, all you need to do is go ahead and put in a few inputs that describe everything about your car. |
| 03:17 | If you want to make use of these sorts of calculations, for some people there is going to be a little bit of input work required, whether it's going ahead and measuring some aspects of your car or working out maybe things like roll centre positions and stuff like that if you don't already know them But I promise you if you're interested in this sort of stuff, this does become an incredibly powerful tool and it is worthwhile putting in the effort to try and characterise some of these aspects so we can use them as inputs to this particular tool. |
| 03:42 | Once you have this sort of tool up and running for your car, it is an incredibly powerful tool for going ahead and understanding how to keep something like the lateral load transfer distribution static if you want to get the same balance. |
| 03:56 | If you want to make a spring change for one reason, you want to make up for an anti roll bar, alternatively if you're going to make a geometry change, what you might need to do with your springs and anti roll bars to keep or change your balance a certain amount and that's what you'll see as we dig into understanding this tool and going through a few examples with it. |
| 04:12 | If you're interested in playing with this sheet as we go through this section, I encourage you to go ahead and download it, you'll find a link to it just below the video. |
| 04:18 | If you download that now, it's a Google sheet, it should be really simple for you guys to download and use and you'll be able to go ahead and play with it as we go and see how the sheet reacts and hopefully you'll get more out of it as a result. |
| 04:29 | Now it's important to understand that the calculations in this sheet are really heavily simplified from what's going on in reality. |
| 04:35 | It's certainly not a full on intense sort of vehicle dynamics simulation, this is really stripped down, there's a lot of assumptions built into it that I'll go through in a second but even though it is really stripped down and simplified, this is still a really useful tool. |
| 04:51 | Lots of race teams, even at moderately high levels will still be using tools relatively similar to this with not a lot more levels of sophistication to make setup decisions on their cars throughout race weekends so I definitely encourage you to put the effort in to start playing around with a tool like this for yourself. |
| 05:08 | So let's jump across now to my laptop screen and we'll go through the spreadsheet and I'll explain every single part of it as we go. |
| 05:13 | So when you go ahead and download this sheet, this is the first page that you'll land on. |
| 05:16 | The first thing to understand here is that there are two tabs that make up the spreadsheet down the bottom here. |
| 05:22 | One is the instructions tab that we're on now and the other is the main tab and we'll get to both of those in due course. |
| 05:28 | Now I just want to go through a couple of the basic instructions up here. |
| 05:30 | The first one up here is that all inputs and visualisations are found in the main tab which is this one down here that I pointed out. |
| 05:37 | So that's where all of the inputs and the calculations are going to happen down there. |
| 05:41 | The next thing I've got written there is that all of the inputs are shown as grey boxes. |
| 05:45 | So that's what I've got highlighted over here is the only things you need to change in this spreadsheet are the boxes that are highlighted in grey like that. |
| 05:54 | Everything else is a result that you don't need to worry about, it's a calculation that's going on in the background, all you really need to worry about are the inputs, everything else will be calculated and spat out for you. |
| 06:04 | Now we have got the ability to hover over some of the cells that have got comments which have got further explanations so if you're not sure what some of the terms mean you can always just take your mouse and hover over any of those cells which we'll go through in a second and it'll bring up a little bit more explanation about what's going on in that cell. |
| 06:20 | Now some of the assumptions and notes I've got down here, the first one here is that calculations are only valid when all wheels are in contact with the ground. |
| 06:27 | So what that means, and this is something that a lot of people will already be familiar with and this is actually something that's happening in reality. |
| 06:33 | When you've got a lot of cornering force and maybe a big split or some split in the roll stiffness distribution, it's quite normal to see cars 3 wheeling where you've got one wheel off the ground, whether it's an inside front or an inside rear, depending on where your roll stiffness distribution is, for example on front wheel drive cars it's really normal to see the inside rear wheel lifted off the ground just because you're trying to leave, trying to have relatively low front roll stiffness in order to promote traction and grip on the front end so you end up with quite high rear anti roll stiffness. |
| 07:00 | So that's an example where you might have 3 wheels on the ground and one wheel off the ground. |
| 07:06 | It's important to understand about the simulation and we'll see it in a couple of examples here is that this calculation is only valid when we have all 4 wheels on the ground and that means we've got a positive force on all 4 tyres on the ground. |
| 07:17 | After that, because it's simplifying assumptions and the maths that's going on in the background, the calculation is no longer valid at the point where one wheel comes off the ground. |
| 07:24 | Now that's not, for this particular model it's not really a big limitation but I just wanted to mention, it's something, we do have some warnings built in there but it is something to take note of. |
| 07:33 | The next one down here is vertical tyre stiffness is ignored and tyres are assumed rigid so that's something I've harped on a few times throughout the course is that we are not considering the vertical stiffness of the tyres in these equations, it's not actually that difficult to take the vertical stiffness into account as part of a model like this but it is just another piece of information you're probably not likely to have about your car, it's not impossible to get but I just wanted, in order to keep things as simple as possible and make this sheet as usable as possible for people, just keep it consistent with the rest of the course and not consider the tyres as part of these calculations. |
| 08:06 | The last note down here is that the model is linear. |
| 08:09 | Now that's really maybe doesn't have too much meaning for some people but really what it means is mathematically all of the assumptions in the model are linear. |
| 08:18 | So that means whether we have 0.1° of roll angle or 10° of roll angle, the model is treated as linear, it'll linearly interpolate between those two points. |
| 08:28 | So all that really means, the important thing to understand about the linear model, it's really only valid in something like this, in a situation, a dynamic race car, linear models are pretty limited in their use, they're usually only limited in use to one specific piece of simulation or maybe one specific viewpoint or part of a corner, you can't extrapolate because the entire behaviour of the car is highly non linear but for the sort of narrow window we're looking at today, which is all about looking at the lateral load transfer distribution in steady state which is when the car has already taken its set in the corner and we're only looking at the mid corner once everything has settled out then this is perfectly valid to look at a linear model, at least perfectly useful to look at a linear model but it's just an important assumption to understand there. |
| 09:17 | OK now we're going to head across to the main tab which is really where most of the interesting stuff is happening on this spreadsheet. |
| 09:23 | Now there are a few different sections I'll go through before I dig into the detail, we've got this one up here, the input car data. |
| 09:29 | Now this is the only place in the spreadsheet you actually need to input anything about your car. |
| 09:33 | These are the grey boxes I was talking about down here, so these are the only parts you actually need to input any information. |
| 09:39 | The next section down her is intermediate calculations. |
| 09:41 | Now the reason I've shown those, I could have hidden those in the background but I think that they are a useful thing for the user to see as you change things in the inputs, you can see how some of the intermediate calculations which we'll go through in a second, you can see how some of those are changing, it's important to take note of those 'cause it is a useful learning tool. |
| 09:58 | Down the bottom in the results here, this is where we've got really the most interesting stuff that we're going to be interested in most of the time. |
| 10:04 | Things like the different contributions to the lateral load transfer distribution, what the actual lateral load transfer distribution numbers are, we'll go through all of those in more detail. |
| 10:15 | And down the bottom, it's really just a visual representation of the exact same thing, this is simply just the exact same thing that's shown in the results but in a plot format down here so it's just a different way to visualise the exact same information so let's head back up the top and go through each one of these things in detail. |
| 10:33 | So as I said, some of this stuff you are going to have to go through and make some measurements of your car, some of these are simpler than others, some of this information you already have, some of it you might not but it is absolutely worthwhile going ahead and getting this stuff because it shouldn't be too difficult for most cases. |
| 10:46 | So let's start with this one here, wheel base, pretty simple, this is just obviously the linear distance between the front and rear axles and usually it's easiest to measure on both sides and take an average because there's usually a bit of error associated with that, so that's just the length of your wheel base. |
| 11:03 | And then we have the front and rear track width and you'll see down here as well there's a little hint for each one of these, for the units you need to input in both of those. |
| 11:11 | So here I've used our Toyota GT86 as an example here, we've got almost 2.6 metres wheel base, we've got 1560 on the front track and 1540 on the rear track, all in mm obviously. |
| 11:22 | Now the next section down here is mass that we're talking about so from top to bottom here we have the total mass of the car, all in kg so that's just the total weight, fully loaded with driver and everything in the car. |
| 11:34 | Next one down below that is the weight distribution so how much of that weight is distributed onto the front axle. |
| 11:42 | Now we have the non sprung mass and both for the front and the rear axle so that's just understanding how much of the total mass is contributing towards the unsprung mass on the front and how much unsprung mass on the rear. |
| 11:55 | It's pretty typical to have a higher unsprung mass on the driven axle, obviously this is a rear wheel drive car so we've got roughly 50 kg of unsprung mass on the rear and roughly 45 kg there we can see of unsprung mass on the front. |
| 12:07 | Now some of these values, if you don't have the ability to measure this or calculate it for yourself, it's fine just to estimate this for a start, somewhere in that 40 to 50 kg range is usually about right for most road cars. |
| 12:19 | But if you do have the ability to measure it for yourself, awesome, got for it but if you don't and you just want to get started with playing around with some numbers, use that anywhere in that 40 to 50 kg range is just fine. |
| 12:30 | Now this is, next one down here is the total centre of gravity height. |
| 12:34 | So this is this value here as well. |
| 12:37 | So remember when we did this in one of the earlier parts of the course, that's where we got a value of 506mm I think, that was with a full tank of fuel. |
| 12:45 | That is the total centre of gravity height so that's not just for the sprung mass, that's for the entire car, everything as it sits on the ground and that's, one of the things that just popped up there you'll see as I hover over each one of these things, this is what I was talking about before with the little pointers just giving you a little bit more of an explanation about what each one of these parameters are as you head through if you need a little bit more of a pointer on it. |
| 13:07 | Now the next two, the last two here in the mass section, the heights of the non sprung masses both for the front and the rear axles respectively. |
| 13:16 | Now again, that's something you might not have for your car but in pretty much every case, a really good starting point or an assumption if you don't have that information for where the centre of gravity non sprung mass height, it's almost always pretty much at the geometric centre of the wheel. |
| 13:30 | So what does that mean? Half the diameter or the radius of your tyre, that's going to give you a pretty good starting point for where the vertical height of your non sprung mass at each end of the car's going to be so we've got the same value there, roughly 325 mm which is the loaded radius of our Michelin 18" that we've got fitted to the car. |
| 13:51 | Now next couple across is all about the elastic stiffness so when we're considering elastic stiffness in these sorts of calculations, we're talking about the springs and the anti roll bars in almost all cases. |
| 14:01 | So here I've got 4 lines, the top two are all about the springs and the bottom two are all about the anti roll bars so here we have the front spring stiffness and the rear spring stiffness. |
| 14:11 | Both in newtons per mm. |
| 14:13 | So in this case, I'm doing a simulation with the same springs fitted front to rear. |
| 14:18 | Now down here I've got the front and rear anti roll bar stiffness. |
| 14:23 | So these values are coming from what we measured in one of the previous modules talking all about how to measure the anti roll bar stiffness. |
| 14:29 | Our situation was the same as what most people were in where we go have aftermarket anti roll bars fitted to the car but we don't have any stiffness information, that's not something the manufacturer was able or at least willing to give us so these values are taken from the measurements we did on the car where we've got 59 newtons per millimetre on the front and 47 on the rear. |
| 14:48 | Now the next one down from that is talking all about the motion ratio. |
| 14:51 | So obviously we know from the course that these are a really important piece of the puzzle, it's one thing to have a spring or an anti roll bar in one position but that doesn't usually have a linear or a 1:1 motion ratio between the wheel and the spring or the wheel and the anti roll bar so this is where we input that stuff here. |
| 15:05 | Obviously we're assuming these things are constant throughout their travel. |
| 15:08 | This is another example of a simplification within this model, all of these ratios and rates, all of these things are assumed to be constant or linear throughout the entire travel of the suspension. |
| 15:20 | So down here, the first one here is the front spring motion ratio. |
| 15:24 | So we have a MacPherson strut in this car so it's almost exactly 1 so I've input it here as 1. |
| 15:31 | It's essentially what it is, a 1:1 motion ratio which is pretty typical for a MacPherson strut. |
| 15:35 | What can influence the motion ratio on a MacPherson strut more than most things is actually the inclination angle or the, how leaned over that strut axis is depending on that will tend to have the biggest influence on how much the motion ratio changes on a MacPherson strut. |
| 15:50 | It's not actually exactly 1 but in our case it's pretty close to 1 so I've got a 1 there. |
| 15:54 | Now the rear motion ratio is a little bit more intersting. |
| 15:56 | We've got that 1.35 so that means that the wheel is moving 1.35 times more than the spring for a given amount of movement and that's just based on the damping spring being inboard from the wheel. |
| 16:07 | That's your pivot point, that's your wheel, the damper and springs are somewhere about here, the wheel moves through a larger arc than the spring and obviously we're linearising that, treating it as a constant motion ratio over its entire travel, so 1.35. |
| 16:17 | Then down at the bottom here we have the anti roll bar motion ratios, both for the front and the rear anti roll bars. |
| 16:23 | So again we've been through that in a previous section but that's all about understanding how far the anti roll bar moves with respect to the wheel. |
| 16:31 | Now we're getting onto the last of the inputs up here, we have the roll centres up here. |
| 16:35 | So the front and rear roll centres respectively, so in this car we've got 70 mm to the front and 120 mm for the rear. |
| 16:42 | The process you go through is going to vary depending on what sort of suspension you've got, obviously we went through that in detail in the course, depending on what type of suspension you've got fitted to your car, different ways of working that out, whether you're doing just pencil and paper and measuring this thing with a tape measure on the ground, that's absolutely a valid way to go ahead and certainly get yourself in the ballpark anyway. |
| 17:02 | Maybe you're going to go a bit further and measure all your hard points, do some more simulation work, whatever it is, it's going to depend a little bit on your situation but that's where that information is coming from as far as getting the roll centre heights. |
| 17:11 | The other thing is that for a lot of cars, particularly high performance cars or cars that tend to be modified extensively, a lot of this information's actually available online for you to get started with anyway so you don't necessarily even need to measure this stuff for yourself, at least to get started and start playing around with this tool. |
| 17:24 | The last input down there is the lateral acceleration. |
| 17:29 | Now this isn't, I'll come back and play around with this a little bit more in detail but essentially because this is a linear model it's not actually going to matter to some extent whether you are using 0.2 of a G or 2 Gs. |
| 17:42 | The results as far as the lateral load transfer distribution are not going to change, you're going to get the same results no matter what. |
| 17:48 | What it does change is you can see how much, we'll go through that in a little bit more detail, how much the car's actually rolling for a given amount of input and you're going to be able to estimate, or see at which position one tyre's going to start to come off the ground as well and that's what I discussed before, the simulation's only valid for a point where all 4 tyres have got, they're actually touching the ground, you haven't got 1 lifting off the ground. |
| 18:10 | At that point these calculations become invalid and we'll see as we modify that lateral G, we can see we'll get one of those wheels to pop off the ground by using a bit more lateral G. |
| 18:20 | Now let's get onto the intermediate calcualtions. |
| 18:23 | So this is something, again you don't need to worry about how to calculate this stuff, it's all done for you but this is spitting out the numbers. |
| 18:28 | So for the inputs that we've put in there so far, we've got a front non sprung mass distribution. |
| 18:32 | So that's just saying, just considering the non sprung mass only, how is that distributed between the front and rear axle, we've got just over 47% on the front, obviously we've got more of our non sprung mass on the rear because we've got a driven rear axle. |
| 18:46 | So the total sprung mass, so that's just the total amount of mass that's part of the sprung mass, so that's the sprung mass minus the total unsprung mass, 1296 kg there and just when we consider the sprung mass only, it has a 56.6% of it is concentrated on the front axle of this car. |
| 19:05 | So that's just the sprung mass, not the total mass.. |
| 19:08 | Now looking at stiffness at the wheel, so obviously this is taking into account our stiffnesses so let's just, I'm just going to go back up to the elastic stiffness. |
| 19:15 | So that's looking at these stiffnesses here of our front and rear springs and our front and rear anti roll bars and then converting them into effective stiffnesses at the wheel by considering the motion ratios that go with them. |
| 19:25 | So these are the numbers that we probably care the most about in terms of how they're affecting the car at the wheel rather than at the spring or at the anti roll bar. |
| 19:34 | At the wheel is where we really start to be able to do useful things with them. |
| 19:37 | Now down the bottom here, this is where we start to spit out a few useful pieces of information. |
| 19:43 | Well sometimes some of these are useful some of them are more academic but still interesting. |
| 19:48 | So we've got the roll moment arm here of 441 mm so that's saying that's the difference between if we take the roll axis where it goes between the front and the rear roll centres and if we've got the centre of mass between them, that's the moment arm that we've discussed in the course, between that roll axis and the centre of gravity, how far away or how big is that roll moment arm that's acting, obviously our roll centres are what's having the biggest effect on that. |
| 20:14 | If we raise our roll centre so much that the roll moment arm gets small, we're obviously reducing, so we're increasing the amount of geometric load transfer and as we drop it we are reducing the amount of geometric load transfer. |
| 20:25 | The next one under that is the roll moment. |
| 20:28 | So that is the total roll moment including the force acting on the centre of gravity and the roll moment that's pivoting about that roll axis so that's just maybe more academically interesting but either way I've put it in there so you guys can visualise it as well. |
| 20:43 | Now what this is predicting is a sprung mass roll angle here of 1.54° for a lateral acceleration of 1.2 G. |
| 20:50 | The one below that is the roll gradient which is simply dividing how much roll by how many Gs. |
| 20:56 | So that's just a way, essentially a way of quantifying your roll stiffness. |
| 21:01 | So that's a really common way you'll see lots of race engineers using something like the roll gradient, it's just a way to sort of have a normalised perspective of how much roll stiffness you have in terms of an angular measurement rather than a linear measurement so it's just a way of understanding what your roll stiffness is, just to maybe make that a little bit clearer, if I just come up to my lateral G input up here and just put a value of 1 in here so we're just doing 1 G. |
| 21:30 | You'll see here that these values are now the same because we have 1 G of input, so that, when you have 1G of input your roll angle and your roll gradient will always have an equal value because you're only putting a value of 1 in so if we stay with our value of 1.2 in here, you'll see they will change again. |
| 21:48 | So now coming down to the results, this is really when things start to get really interesting and useful and how we really want to start, the things we really want to start paying attention to when we're using a tool like this. |
| 21:58 | So to start off with, these ones here are all about the lateral load transfer contributions. |
| 22:02 | Now what do I mean by that? This is saying of each of those 3 components, whether it's the non sprung mass, whether it's the elastic or whether it's the geometric sprung mass contributions, which ones of those tend to be, well are the most dominant for our particular situation? 'Cause this is really, well for me anyway this is when things start to get really interesting. |
| 22:20 | So if we're looking at the total non sprung mass, 8.2% is what it's saying there is our total contribution of the sprung mass so that's a good thing in that we went through this in the course as far as a non sprung mass not being the biggest component but because we can't really do much about it without redesigning a whole lot of components in the car, it's a good thing it's a relatively small component because we can't do much with it anyway. |
| 22:42 | The next one down there is elastic and this is obviously our spring and anti roll bars contribution so this is where almost 76% of the entire lateral load transfer contribution is coming from our elastic elements. |
| 22:54 | So what we mean by that is as far as the weight that's laterally transferred, how it's distributed between the front and the rear axle, 76% of that is being determined by our spring and anti roll bar stiffnesses and that's great because that means we've got a lot of authority over changing the balance of the car with these elements. |
| 23:13 | The next one down there, let's say roughly 16%, 15.8% there on geometric. |
| 23:20 | So that means, that's how much influence our roll centres are having on our lateral load transfer distribution. |
| 23:27 | So not insignificant but certainly not the last largest either. |
| 23:30 | So what that means straight away, it obviously, we're not comparing like quantities here when we're talking about changing a roll centre height versus changing a spring stiffness and it's going to depend on the magnitudes of each one of those you change but we can probably expect there for reasonable changes in either roll centre versus a spring or anti roll bar change, we're probably going to have more authority and more effect on the car by making elastic changes in the situation than having geometric changes. |
| 23:54 | That doesn't mean that geometric changes are valid or they're useless or anything like that, it just gives us some understanding of the sort of magnitudes and effect that we can have on the car by making something like a roll centre change. |
| 24:07 | Now in the centre here we've got our lateral load transfer distributions and this really is where we're getting to the crux of what we want to use this tool for. |
| 24:15 | This is just, in the first 3 sections here, this is all just saying what the lateral load transfer distribution for each one of these components is. |
| 24:24 | So it's saying for the non sprung mass it's 47% front, for elastic it's 61.8% front and for geometric it's 42.9% front. |
| 24:33 | If we come down to the total which is the one shown in bold here it's saying we've got 57.6. |
| 24:39 | Now what does that mean? That means that we're saying for these calculations, that as far as the total roll stiffness distribution, 57.6% of it is concentrated on the front axle. |
| 24:51 | We know from the course that that means that most of our, well we have more lateral load transfer happening on the front axle than on the rear. |
| 24:59 | Now to some extent, we can kind of think of this as being, we want it to be somewhat similar in theory anyway to our mass distribution, how much weight we've got on the front axle versus how much we've got on the rear. |
| 25:12 | Now if we come up, back up to the front up here, up to the top sorry, we had roughly 55% total weight distribution on the front. |
| 25:20 | So this number of 57% doesn't seem crazy for a car that's relatively balanced and working pretty well on track. |
| 25:28 | I do need to put another caveat here, is these numbers, I wouldn't get too carried away with trying to get them lying on top of each other, you shouldn't necessarily be shooting for a target or a lateral load transfer distribution that is equal to your weight distribution on your car. |
| 25:45 | The reason is you've got so many things muddying the water there. |
| 25:48 | As I said, there's a lot of simplifications and assumptions when it comes to these calculations. |
| 25:53 | You also don't in reality necessarily want the lateral load transfer distribution to match your real weight distribution. |
| 26:00 | You've got all sorts of things like chassis compliance, highly non linear tyre behaviour, different aero balance, different driver preferences, doesn't necessarily mean that you want these two things to lie on top of each other, it just means that we need to treat this number more as a way to measure changes or a way to anticipate how much the balance of the car's going to changes, versus trying to get obsessed with getting a specific number. |
| 26:25 | It's more about looking at the delta or the changes as we go rather than that absolute final number of what that lateral load transfer distribution number is. |
| 26:34 | Now we will come back and play with a couple of things, we'll see how that's going to get affected. |
| 26:38 | Before I do I just want to point out this last one here which is our vertical wheel loads which is just telling us, looking from a bird's eye view there, so we've got the, looking at the, starting with the front left and the front right and the rear left and the rear right, that's showing us, for this situation, this model, it's saying for a 1.2 G corner, that is giving us a prediction of what the vertical load is going to be on each tyre. |
| 27:01 | Now you can see there's a huge weight transfer going on here, we've got almost 10 times the weight on the front right than we do on the front left. |
| 27:07 | But we can see the thing that's really clear to us is that front right is the one that's by far taking the most load out of all of these things, we've got 747 kg on the right front and only let's say 580 on the right rear. |
| 27:20 | It's a huge difference in the amount of load that's being supported by those tyres. |
| 27:24 | Particularly comparing the inside to the outside. |
| 27:27 | But that's something to keep an eye on when you are putting different numbers into this, if any of those numbers go negative, that's straight away telling you that at least one of the tyres is off the ground and the simulation is no longer valid. |
| 27:38 | Now down the bottom here, this is just what I was talking about before, a visual representation of the exact same numbers that we've got shown in these two sections of the results up here, all the numbers are exactly the same but again it's just nice to get a visual representation, rather than, some people prefer to look at tables, some people prefer to look at graphs with colours on them, myself, I probably react better to looking at the graphs, gives me a bit better idea of exactly what's going on. |
| 28:03 | So let's look at these. |
| 28:05 | Go back to our lateral load transfer contributions which is understanding which of those three components of lateral load transfer is the most dominant? And it's really clear here we can see here obviously our elastic one roughly 76%, geometric down here at roughly 16% and our non sprung mass down here on just over 8%. |
| 28:21 | Again we can see that our elastic is going to be a really powerful thing to change as far as changing the balance of our car. |
| 28:27 | And then with the distributions, again the same thing but just looking in a visual form, we can see how each one of those 3 components is distributing its lateral load transfer between the front and the rear axle so non sprung mass, elastic, geometric and our all important total down here at 57.6%. |
| 28:48 | Now we've done that, let's look at a couple of different things we can change and let's look at how the results change as we go. |
| 28:55 | So let's say we've got a situation where we want to make a, let's say we've got some we want to reduce the amount the front's diving, maybe we've got some oversteer, we're going to try fitting a stiffer front spring into the car, maybe we need a little bit more front support for whatever reason, we're going to put a stiffer front spring in the car, we want to understand how much that's going to change our total lateral load transfer distribution. |
| 29:18 | Now one of the things to keep in mind here is that when we're looking at these percentage numbers, it's good to have an idea of how much of a percentage change is a reasonable amount to change in order for making a balance change on the car. |
| 29:30 | So typically what I would look at, if you're looking to make a balance change on the car and you're looking to calculate how much of a balance change you're going to need, if you're looking at making a moderate balance change, you're probably talking in the range of between 0.5 and 1%. |
| 29:44 | Anything over that 1% range is really starting to get a pretty big shift in balance so if you're looking, maybe you've got a moderate mid corner understeer somewhere, you might be looking at making 0.5% of change. |
| 29:54 | Let's just try and get a feeling for how big these changes need to be. |
| 29:57 | So we've got 57.6 down here. |
| 29:59 | If I was to then, change the front spring by 10 newtons per millimetre, so let's say 108. |
| 30:09 | So we've gone from 57.6 to 58.9 so almost 1.5% change there just by that one spring change so that's a pretty big change. |
| 30:21 | That's certainly above that 1% range where I would start to be expecting the car to feel relatively different as far as its balance. |
| 30:28 | But the great thing about that is if we then know the different roll stiffness changes that we're going to get from going down in the anti roll bar, let's say that for arguments sake if we were to be able to go down to let's say 50 in the front axle, so we drop down from, let's go back there, from 0.9 to 0.3 by going down to 50, let's say we've got the option to go to 40 on that front anti roll bar and we're getting back down to 57.7 which is pretty close to our original. |
| 30:56 | So that's an example of how you might be able to say, OK I want to go up in spring for this reason, what sort of anti roll bar change do I need to make in order to account for that to keep the same stiffness distribution? So hopefully that's really clear to you guys how that works now. |
| 31:09 | So I'm going to put those back to our original numbers. |
| 31:12 | In the same way we can come in here and change anything else as well. |
| 31:15 | We can make changes to our front and rear track width or what I'm actually going to do which is looking more interesting is make a change to our roll centres. |
| 31:22 | So if we look at the roll centre here, let's say if I was going to put the roll centre down at, let's say drop it quite a lot, drop it maybe to 40 mm so a 30 mm drop so we're going from 57.6, this is the number to watch down here, if I enter that, now I've got to 56.5 so by dropping the front roll centre height 30 mm, we've essentially shifted that balance 1% to the rear, we've shifted our lateral load transfer distribution 1% to the rear axle. |
| 31:52 | So again that's going to be a pretty significant balance change but again that's just giving you that tool to understand exactly how much you can expect the balance change for a given change. |
| 32:01 | So you can see hopefully here how poweful this tool is by giving you the ability to punch in these different numbers without just guessing and really truly understand how much of a balance shift you expect to see in the car. |
| 32:13 | So the last thing I wanted to point out here was what happens to these wheel loads when we start getting too much lateral G into the car or we end up in a situation where we've only got 3 wheels in the car and the model becomes invalid at that point. |
| 32:26 | So I'm going to put a stupid lateral acceleration in here, well not a, if I put say 3 Gs which clearly this car is never going to achieve, we'll see down here we've got some negative numbers shown in the wheel loads. |
| 32:37 | So straight away that tells us that we've got, our model is no longer valid, we can't be trusting these calculations anymore. |
| 32:46 | We've also got this little message that'll come up down here saying caution, invalid, 1 or more wheels off the ground, reduce the lateral G input. |
| 32:52 | So that means if you want to run this particular setup on the car, and you want to get a valid input out of it you need to reduce the lateral G input. |
| 32:57 | Again it's not going to make a difference to, all of this lateral G input is not going to make a difference to this number here, this, or any of these distribution numbers down here because the model is leaner. |
| 33:09 | So you can always still understand your same setup by just pulling back that lateral G number and it'll still give you an understanding of what your stiffness distribution is. |
| 33:20 | It just means that you need to run a lower lateral G number to understand that particular setup. |
| 33:25 | So let's come back to 1.2 here. |
| 33:27 | And let's see how much acceleration we can get away with before we start lifting a wheel on this model. |
| 33:32 | So I'm going up to 1.3, we need to go up a little bit more so let's jump up to 1.5. |
| 33:36 | Straight away we've got -7 has dropped on there so let's see if I can get away with 1.4. |
| 33:42 | So that looks like around 1.4, maybe somewhere between 1.4 and 1.5 is the maximum lateral G we can get away with for this particular car setup in this model before we start lifting the wheel. |
| 33:52 | Now there's nothing wrong necessarily with having 3 wheels on the ground, having 1 wheel off the ground, that doesn't mean it's an incorrect condition for the car to be running in, it just means for this particular simulation that's a limitation of the model. |
| 34:03 | So if you're using a tool like this to calculate all of your components from scratch. |
| 34:07 | So for example if you were designing a car from scratch or specifying all the components for your car before you head out onto track which is a perfectly reasonable thing to do, probably the starting point for that lateral load transfer distribution I would be starting to tune for would be roughly 5% forward of your static weight distribution so for our case if we've got roughly 55% front mass distribution on the front axle, I'd be starting my tuning point for 60% front is what I would want my total lateral load transfer distribution to be and my first prediction. |
| 34:39 | The reason you do that is as I said, don't get too hung up on the magnitudes but the idea here is that we're trying to start the car in a position of understeer, we want it to be stable, we don't want the car to be unsafe or undrivable when it heads out on track for the first time and 5% is usually a good safe starting point to aim for as far as having it 5% further ahead of your static weight distribution. |
| 35:02 | An important thing to understand about a model like this is that the model itself is always wrong. |
| 35:06 | I want to stress here that it's not about the model being exactly right, it's just about giving ourselves a useful tool to be able to start understanding how we might change some of the magnitudes of some of the parameters that we have access to actually changing. |
| 35:20 | It's not about getting the exact right number, it's just about understanding the trends, the directions, the relative influence of each component of the car. |
| 35:28 | Now whether that's things through having errors in incorrect data, the model not being properly linear, obviously we're assuming things like constant roll centres that aren't moving which isn't reality. |
| 35:39 | There's all sorts of things we are not taking into account here. |
| 35:42 | It's just about giving ourselves some sort of yard stick where we can take some of the inputs for this car and understand how it's likely to affect the car as we make changes to it on track. |
| 35:51 | So hopefully guys, you guys can see that is going to be a really useful tool for you to be able to go around and play around with. |
| 35:58 | If you haven't already, get down there and download it for yourself, start playing around with it, if you've got any questions about it, feel free to jump into the forums, I'll be happy to answer your questions there. |
