Rear Suspension Toe Link Removal

[AndrewS]

I'm in charge of the suspension geometry for my team's car this year. I already have the front design pretty solid, I based it off last year's and used WinGeo3 to confirm the last team's calculations and to make a few adjustments of my own.

For the rear though, we have a big empty area in the frame behind the diff that serves no purpose but to mount the suspension to. So the big thing I'm looking to change is to bring the rearmost chassis points forward as much as I can. We want to have the diff hanging halfway out the back.

The current design is a double wishbone SLA pullrod type with the A-arms being isosceles triangles with the hub centered. And the toe links are mounted parallel behind the lower A-arms about a half an inch.

So I was basically looking to have the A-arms become right triangles by moving the rear chassis mounts to be in line with the wheel center. This is ok and dealing with the changing of the A-arm loads shouldn't be problem. But I don't like having the toe links behind that making the lower chassis a half inch longer and they seem pointless to me in the end.

What I am interested in doing is one of two things. I've seen designs with the toe links attached to the lower A-arm and I think that may be a good way to eliminate at lease half of them. Just have them triangulate to the midpoint of the lower arm and weld them to it. This seems like a pretty reasonable design and I'm keeping it in mind.

My other idea is to get rid of the toe link all together and have the A-arm become an H-arm. Instead of having the two links of the lower arm come to a single ball joint, one would go to the current position and the other would go the where the toe link mounted. More or less that is, the exact position may change. I may also need to weld the two links together with a cross brace but I'm not sure if that is needed.

I was wondering if anyone had tried an H-arm type rear suspension or has their toe link attached to their arm and if they have any advice. I can't even model this with WinGeo3 as it doesn't seem to recognize that as being a valid design. And I'm having a lot of trouble finding any information on this in detail pretty much anywhere. Both these ideas were mentioned in passing in "Race Car Vehicle Dynamics" but no details were given.

Any ideas on how to design a rear suspension with the chassis points far forward of the wheels and diff would also be a big help. I know I've seen teams with this sort of setup.

Thanks in advance.

[AndrewS]

I'm in charge of the suspension geometry for my team's car this year. I already have the front design pretty solid, I based it off last year's and used WinGeo3 to confirm the last team's calculations and to make a few adjustments of my own.

For the rear though, we have a big empty area in the frame behind the diff that serves no purpose but to mount the suspension to. So the big thing I'm looking to change is to bring the rearmost chassis points forward as much as I can. We want to have the diff hanging halfway out the back.

The current design is a double wishbone SLA pullrod type with the A-arms being isosceles triangles with the hub centered. And the toe links are mounted parallel behind the lower A-arms about a half an inch.

So I was basically looking to have the A-arms become right triangles by moving the rear chassis mounts to be in line with the wheel center. This is ok and dealing with the changing of the A-arm loads shouldn't be problem. But I don't like having the toe links behind that making the lower chassis a half inch longer and they seem pointless to me in the end.

What I am interested in doing is one of two things. I've seen designs with the toe links attached to the lower A-arm and I think that may be a good way to eliminate at lease half of them. Just have them triangulate to the midpoint of the lower arm and weld them to it. This seems like a pretty reasonable design and I'm keeping it in mind.

My other idea is to get rid of the toe link all together and have the A-arm become an H-arm. Instead of having the two links of the lower arm come to a single ball joint, one would go to the current position and the other would go the where the toe link mounted. More or less that is, the exact position may change. I may also need to weld the two links together with a cross brace but I'm not sure if that is needed.

I was wondering if anyone had tried an H-arm type rear suspension or has their toe link attached to their arm and if they have any advice. I can't even model this with WinGeo3 as it doesn't seem to recognize that as being a valid design. And I'm having a lot of trouble finding any information on this in detail pretty much anywhere. Both these ideas were mentioned in passing in "Race Car Vehicle Dynamics" but no details were given.

Any ideas on how to design a rear suspension with the chassis points far forward of the wheels and diff would also be a big help. I know I've seen teams with this sort of setup.

Thanks in advance.

[Mark_W]

try looking for Z-arms.

[J.R.]

A few things to think about:
1. If you use an H lower member, your uprights will have to be able to attach to it, making them heavier, your uprights will have to be shorter to package it inside the wheel and you wont be able to adjust rear toe at all. IMO, bad idea.

2. If you place the toe link half way down the a-arm, how will this effect the loading in the a-arm?

3. What is your toe base currently? (Distance between LBJ and toe link longitudinally) It sounds like it may be ~1 in? If it is, compliance rear steer can be a huge issue.

This all being said, we run a Satchell Link rear so I may be missing some information that you independent teams know about. And, BTW, if you really want to save weight at the rear end of your car, you can run a solid rear axle, and completely drop 99% of your frame behind the rear roll hoop. http://fsae.com/groupee_common/emoticons/icon_smile.gif

[AndrewS]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">try looking for Z-arms. </div></BLOCKQUOTE>
I'm not finding anything about those.

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by J.R.:
A few things to think about:
1. If you use an H lower member, your uprights will have to be able to attach to it, making them heavier, your uprights will have to be shorter to package it inside the wheel and you wont be able to adjust rear toe at all. IMO, bad idea. </div></BLOCKQUOTE>
The uprights may need to heavier to deal with the load, but they may just have the weight shift from one area to another. And they definitely won't need to be shorter, I still have 1/2" to an inch between the sides and the wheel at the widest point. We're running 13" wheels, BTW.

As far as toe adjustment, it won't be as straightforward, but it can still be adjustable. Worst case, we can use shims at the chassis mounts to get toe, but more likely we can design it into the H-arm ends or perhaps the upright.

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">2. If you place the toe link half way down the a-arm, how will this effect the loading in the a-arm? </div></BLOCKQUOTE>
This definitely important to consider, that link may need to be significantly bigger to deal with this.

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">3. What is your toe base currently? (Distance between LBJ and toe link longitudinally) It sounds like it may be ~1 in? If it is, compliance rear steer can be a huge issue. </div></BLOCKQUOTE>
I described the toe link wrong, its not parallel like I was thinking. The toe base is about 3.2"

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">This all being said, we run a Satchell Link rear so I may be missing some information that you independent teams know about. And, BTW, if you really want to save weight at the rear end of your car, you can run a solid rear axle, and completely drop 99% of your frame behind the rear roll hoop. http://fsae.com/groupee_common/emoticons/icon_smile.gif </div></BLOCKQUOTE>
That would have its advantages, but the way our drive train is laid out, I'm pretty sure it would all have to be completely redesigned to do that.

Thanks for your thoughts.

[woodsy96]

The first car our team built in 2004 had the toe link mounted on the arm. The tubing is not huge, but you want the line of force acting through the pickup point to reduce bending in the arm.

I personally think a toe link would be better than a z-arm because toe adjustment would be significantly easier than using shims.

Susprog3D has provision for arm mounted toe control links.

[J.R.]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by woodsy96:
The first car our team built in 2004 had the toe link mounted on the arm. The tubing is not huge, but you want the line of force acting through the pickup point to reduce bending in the arm.

I personally think a toe link would be better than a z-arm because toe adjustment would be significantly easier than using shims.

Susprog3D has provision for arm mounted toe control links. </div></BLOCKQUOTE>

Agreed, it can definately be done, my point was to get him to think if cutting 1/2 the weight of a toe link by mounting it halfway up the a-arm (I think what he said) you may have to add weight elsewhere to make it work. I made a similar mistake last year with our suspension.

[BilletB]

A toe rod attached to your a-arm is the solution for you. All you need to do is pay attention to the bending put in the control arm, so be smart with that. We've run the toe rod mounted to the a-arm in the past and it's a viable solution.

Next, if you try that H-arm thing with two independent links you leave two degrees of freedom. The one you want and another that you don't. You need another link, constraint, or truly rigid "H" arm to constrain the 5 degrees of freedom. I'd speculate a rigid H arm that doesn't weigh twice that of a typical a-arm would be a struggle.

[Adambomb]

Yes, I had a lot to say, but J.R. pretty much said it all already. I have seen an H-arm setup before...it was on one of our old baja cars though...just sayin. They had no toe adjustment, and also poor manufacturing tolerances, resulting in about 1/2-1" of toe in that they had no ability to correct.

I've seen a lot of z-arms (or z-bars) out there, in fact we're considering those next year for the same reasons you are. There are a lot of good pictures of these around, may have to do a lot of random digging though. I personally prefer just taking lots of "spy pics" at competition http://fsae.com/groupee_common/emoticons/icon_biggrin.gif