Time attack S13 brake design

Hello!

The brake design course arrived with some rare good timing as I'm in the process of working out a better brake setup for my Sil80 (180SX) time attack car.

Usually a course comes out just after I've spent a lot of time working things out for myself!

The car is fairly basic, but nothing is really standard except most of the chassis and suspension geometry.

Front brakes are currently R34 GT-T (305mm rotors with 4 pot Sumitomo calipers)

Rear brakes are R32 GTS-T (297mm rotors with 2 pot Sumitomo calipers)

I'm using a Wilwood 7:1 pedal with balance bar and dual master cylinder setup.

This originally came with two 3/4" master cylinders but I had a lot of pedal travel and had to run the bias almost all the way forward.

Pedal travel was long and squishy and didn't inspire confidence.

Coupled with being my first non boosted brake setup and having ~20 years experience driving low powered cars with a minimal braking style, I didn't feel I was getting the most out of it.

The car has more than its weight in downforce and I found I was happily cornering at 2G but braking around 1G.

The front master cylinder always had a lot of travel and the rear was very solid.

I was tempted to increase the size of the front master but figured increasing the rear one would allow the bias setting to be closer to the middle and leverage would help reduce the travel.

I swapped the rear master to 1" diameter and it definitely helped. I'm running much closer to middle on the balance bar and it feels better - but still not great.

Plugging the figures into the HPA brake system calculator showed that the current setup should definitely be better than the old setup - but I'm not 100% sure about the calculation of the bias shift.

There will be a load shift to the front due to deceleration and COG, but I think there should also be a large component of weight transfer as a direct result of the braking torque. eg 2500Nm at the front wheel would result in about 100kg weight shift to the front tyre - although it might not show up in shock pot data depending on the suspension design.

Adding downforce into the calculator rapidly pushes the pressures to crazy levels so I have been using zero downforce and 1.5G as my deceleration target (would 1.3G be more realistic for this on A050s?) to at least get comparisons between setups.

I think once you have serious aero, there is so much drag at high speeds that it's not where you need to target braking performance.

I have been running Project Mu HC+ pads the whole time so last time out I tried WinMax W7s in the front and I think it helped. The pedal feels more solid and I was pulling 1.3-1.4G in braking with less pedal travel in the data (I have a pedal travel sensor).

I didn't get any clear laps in the dry so it's not the best comparison, but it seemed to be an improvement.

In the meantime I have been looking at larger front brakes as a way to sort out the compromise between pedal force and travel, and have some larger diameter rotors on the way to play with.

While I'd like a set of proper motorsport Endless, Brembo, AP Racing, etc calipers my budget is more at the bigger OEM caliper end of the spectrum.

I was also looking at Wilwood calipers but apart from there being a huge number of options to choose from, they're generally not considered to be super rigid - and they're all 2+ piece designs.

I have a potential source of Tesla Brembos but they're super heavy and use weird rotor sizes.

In the last few days I picked up a D2 forged monoblock 6 piston setup with 356mm rotors and it looks like the specs line up pretty well in the calculator (attached).

I have a 7/8" master cylinder ready to swap in for the fronts.

I also noticed that in the calculator, enabling the proportioning valve seems to make things line up nicely and would be a very good thing - but I have never seen a proportioning valve used in a balance bar setup.

This also causes the rear brake pressures to go very high (which I assume is the master cylinder pressure increasing to to being able to make use of more front brake pressure and increasing the pedal effort)

It rapidly gets beyond the 1200psi max of the Wilwood cylinders. (edit: today it seems to make no difference at all. The google docs thing is pretty flaky. Sometimes it updates immediately, sometimes it take several minutes)

When playing with options I often seem to end up with the rear brakes being the limiting factor - especially with downforce added in.

My rear brakes are not huge, but there's not much weight on the rear and the better the front brakes, the less the rears should be doing!

I don't have brake temp sensors, but my rear brakes don't run hot and don't appear to have worn at all in 4 years or so of use - so I don't think they're working hard!

I generally set the bias by driving around a medium speed corner and hitting the brakes fairly hard and seeing what the car does - then move the bias forward until the tail doesn't come out.

Does anyone have experience with a similar setup or comments on my current plan / use of the brake calculator?

thanks!

Andrew

"There will be a load shift to the front due to deceleration and COG, but I think there should also be a large component of weight transfer as a direct result of the braking torque. eg 2500Nm at the front wheel would result in about 100kg weight shift to the front tyre - although it might not show up in shock pot data depending on the suspension design."

Those are the same thing.

Actually, a quick read sugests there are quite a few mis-understandings, I'll try and address them tomorrow, if i have the time.

thanks :)

My reasoning for the brake torque weight shift is based on having calculated this stuff manually before for an autonomous robot vehicle project - which seemed to match real world testing.

My gut feeling with the race car is that the brake calculator is underestimating the front bias requirement shift under braking, but obviously it's hard to measure and there are many variables including driver preference and perception of stability / brake bias.

The sanity check for me though is that even if you had a COG height of zero, there would be weight shift when braking due to the wheel torque.

Setting the COG height to zero in the calculator results in zero weight transfer, which is not realistic.

First off, sorry for the calculator being flakey. The iterative calculation is one of those things that works well in excel, but not so great on a google sheet that's being shared. We are working on a website version that should address all of this.

In my mind (and happy to be proven wrong and learn here), the brake torque reaction results in a force at the contact patch. This in itself isn't load transfer. It's happening at both the front and rear tires. But this causes the load transfer about the COG.

If the COG height was at zero, the tire forces would change under braking, but I'm not sure the weight distribution would.

I think geometric and elastic load transfer factors into this, and how much of this force is transferred through the suspension arms vs the springs, but that was all outside the scope of the course.

The calculator is a simplified tool with some clear assumptions to allow you to quickly find a suitable starting point or find clear issues with the sizing of components in the brake system. Without having to include your suspension geometry to include anti-dive etc. We do have some information on calculating anti-dive in our suspension course if you are interested.

In terms of your brake setup though, it looks like you're on the right path to fix the issues.

Is it possible to get a downloadable excel sheet or .ods version?

I tried exporting it from google docs but the bias graph section didn't work.

The way I see the wheel torque thing is that if you apply a torque to an axle, you're lifting or pushing down one end of the car, and that weight must be cancelled out at the other end.

Depending on suspension design it may or may not be seen in the spring loads, but the extra weight is on (or off) the tyre, which affects the braking calculations.