3D Modeling & CAD for Motorsport: Using Rod Ends
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Using Rod Ends
07.40
| 00:00 | - Using CAD software to design suspension components like control arms for example, can be a really rewarding task because it allows us to analyse and optimise the geometry virtually before bringing it into the real world. |
| 00:13 | And if we're going to be designing suspension components from scratch, it's very likely we'll be incorporating rod ends into our designs. |
| 00:20 | Also known as heim or rose joints, rod ends are a common component in motorsport and the aftermarket automotive industry. |
| 00:27 | Particularly when it comes to suspension. |
| 00:30 | There are a few advantages to using a rod end over a rubber or urethane suspension bush. |
| 00:35 | Firstly they're less compliant when loads are applied through the suspension and can reduce parasitic drag and that means more consistent alignment and handling. |
| 00:45 | Due to the threaded body of the rod end, it's very easy to make subtle adjustments to the length of the suspension arm or a component and quickly modify the alignment. |
| 00:55 | If we design a toe control arm with a left hand threaded rod end at one end and a right hand threaded rod end at the other, the length of the arm can be easily adjusted without needing to remove the component. |
| 01:07 | When designing components with rod ends, it's always best to model the rod end as well so we can get an accurate representation of our design. |
| 01:16 | Ideally we first want to specify the appropriate size rod end for our application, considering the forces discussed in the previous module. |
| 01:24 | And from there, we can model the rest of the component to fit. |
| 01:27 | Usually the dimensional information for rod ends is available on the supplier's website. |
| 01:33 | If possible, it's best to order the rod ends first so we can take measurements from a physical sample to get the most accurate model. |
| 01:40 | To be clear, this module isn't intended to be a complete guide to rod ends but there are a few aspects that are important to understand to ensure that these components are used as per their design intention for the best functionality, longevity and safety. |
| 01:55 | The first consideration is that rod ends are available in a range of different materials and price points. |
| 02:01 | The cheaper option might seem attractive if you're on a budget but as per usual, you get what you pay for. |
| 02:08 | We can most commonly find rod ends manufactured in four main materials. |
| 02:12 | Low carbon steel, chromoly, aluminium and stainless steel. |
| 02:16 | The low carbon rod ends are the cheapest, however they're not well suited to critical applications due to their lower load capacity which increases the risk for bending or fatigue failure. |
| 02:28 | To clarify, by mission critical, we're talking about anything that supports the suspension or related components. |
| 02:34 | 7075 T6 alloy rod ends are good for saving weight and they've been used with success in suspension arms but they do have a similar load capacity to a low carbon steel. |
| 02:44 | They also have poor elongation properties and don't give or bend under high loads which means they're really not the best choice for critical applications and are better suited to something like a strut brace where it doesn't see much movement but still needs corrosion resistance. |
| 03:00 | Stainless rod ends can be produced in low or high strength materials so you really need to understand what you're buying. |
| 03:07 | While these may seem like an attractive option due to their ability to resist corrosion, that doesn't necessarily make them the best choice if strength is important. |
| 03:16 | With all the above in mind, for critical applications, a chromoly rod end is usually the best option. |
| 03:22 | Generally for applications where movement is expected, the teflon lined rod end is preferred. |
| 03:27 | With this style of rod end, the ball runs against a teflon liner which is self lubricating, meaning that we don't need to use grease which attracts dirt and debris that will cut down on the life expectancy of the part. |
| 03:41 | Teflon lined rod ends also allow for tighter tolerances and provide less movement which is always a good thing. |
| 03:47 | If you've ever dealt with rod ends before, then none of what we've just covered is probably anything new but the way the rod end is incorporated into a part is often overlooked and misunderstood, even by professional engineers so this is really something we need to discuss in finer detail. |
| 04:04 | This really comes down to what we discussed in the last module, a rod end is very strong in compression and tension but not designed to handle bending loads. |
| 04:13 | The failure point ends up being at the threaded section due to the concentration of stresses. |
| 04:18 | Let's take a quick look at a couple of scenarios where the rod end could be positioned that it'll be subject to bending forces. |
| 04:25 | Here is a double wishbone suspension system with a push rod attached to the lower wishbone. |
| 04:30 | We now have a bending load placed into the rod ends of the lower wishbone. |
| 04:35 | This could be avoided easily by connecting the rod end to the suspension upright instead of the lower wishbone. |
| 04:42 | It's a relatively subtle change that has a very important impact on the forces that the rod end will be subjected to. |
| 04:48 | Next let's look at the design of an upper wishbone for the suspension. |
| 04:52 | In this situation, it can be very tempting to install the rod end like we see here. |
| 04:57 | Since it provides quick and easy adjustability for camber. |
| 05:00 | You might think that this makes sense since at first glance, the rod end does seem to be working in compression and tension. |
| 05:07 | But we also need to consider the brake torque reaction which happens when the driver applies the brakes. |
| 05:13 | The brake system creates a brake torque that slows the rotation of the wheels which in turn slows down the car. |
| 05:19 | The problem is that the suspension arms that locate the upright or hub need to provide an equal and opposite torque, otherwise the hub or upright would rotate when the brakes are applied. |
| 05:30 | With this in mind, we can see that the brake torque reaction has the effect of applying a bending load to the outer rod end which is just asking for trouble. |
| 05:37 | A better solution is to use an encapsulated spherical bearing at the outer point instead of a rod end. |
| 05:43 | We'd lose the ease of adjustability that the rod end provides but it's worth it when dealing with critical components like this. |
| 05:49 | One more subtle thing to consider is the way that the rod ends are integrated with the suspension arms. |
| 05:56 | Back to our first example, we can see a relatively common wishbone design. |
| 06:00 | Overlooking the brake torque reaction for a moment, we can see that the force is applied in a line that runs between the centre of each rod end which isn't directly in line with the wishbone structure. |
| 06:10 | This also has the effect of applying a bending load to the road end. |
| 06:15 | At a lesser degree to the previous examples but still not ideal. |
| 06:18 | Keep in mind that there are plenty of cars out there successfully using rod ends that are subject to bending loads that there are a lot of people that'll say that if the rod end is big enough, it'll hold up just fine and they could be right, but they're still using a component in a way that it wasn't designed to be used. |
| 06:34 | If the system was designed correctly and bending loads were avoided, this would allow smaller and lighter rod ends to be used which ultimately leads to weight savings and performance advantage. |
| 06:45 | We've gone over quite a lot in this module so let's quickly summarise the main important points to remember. |
| 06:52 | Rod ends are commonly used in performance parts, particularly suspension components because they offer some key performance advantages over standard rubber bushes that we find in most factory applications. |
| 07:04 | There are some critical aspects of rod ends to understand if we want to use them in our own designs and assure we safely get the most out of them. |
| 07:13 | Teflon lined chromoly rod ends are usually the best choice for mission critical components due to their self lubrication properties, minimal play from tight tolerances, strength and corrosion resistance. |
| 07:25 | Lastly, the threaded section of the rod end is more prone to failure under bending loads so some careful consideration needs to be made when designing suspension components to avoid positioning rod ends in bending. |
