[wally8]

So I want my car to be pretty low. Kind of like the way Troy Trepanier does it.

I watched the Rides show where he built Sick Fish (the silver cuda) and noted how he went about getting the stance of the car the way he wanted.

I set my frame up on wooden blocks on the floor. I put a 4.5in. block under the front corners of the frame (the corner right behind the front wheels) and 5in blocks under the corners just in front of the rear wheels (this is a 65 LeMans BTW)

Next I set the rearend in on blocks that hold it to the heighth that it would be if I were using 28in. tires. The rearend is just a half inch away from hitting the frame at that point so I'll have to do a C notch and gusset on the top of the frame. I'm planning on 3 inches or so of suspension compression from ride height.

I originally wanted to use a truck arm suspension but that is looking like it will be impossible with the radically low setup. I would have to use something like 4 in. lowering blocks to bolt the rearend to the arms. Not a good solution I don't think.

So now the question is where to go from here. I can't use the stock suspension (that design sucks anyway), it looks like a 4 link would intrude quite a ways into the backseat space (I want a usable back seat) and I'm not sure a 3 link would work either for the same reason. I could split the back seats and add a console but that seems like an excessive amount of work.

For a solution I'm looking at using the original lower frame mounts for the lower control arms and building a torque arm to create a 2 link torque arm rear suspension (have to add a panhard bar too of course). I'll need a crossmember for that but I was planning for that anyway for the truck arm.

Anybody have any experience building an exceptionally low car?

Any comments on my ideas?

I think it's always good to run these things past some other people for input. I'm sure I haven't thought of everything I can post some pictures for clarification too if needed.

Thanks for any and all input. Sorry it's a long one.

Later....

Wally

 

[Triaged]

I happen to like the torque arm type suspensions.

If you want to find the Instant Center (IC) it is the intersection of a vertical line through the torque arm mount (near the trans output) and a line through the control arms.

With control arms parallel from above the roll center will be about the height of the center of the Panhard bar and the axle roll axis will be at the same angle as the control arms.

That said I would try to make the control arms parallel with the ground or sloping down slightly (no more then 5deg) twards the front. If you want to adjust the anti-squat adjust it by the length of the torque arm. I'm guessing that you will want the torque arm a bit shorter then a factory F-body one...but would have to see where everything ends up...

 

[wally8]

You're right on Triaged. That's pretty much exactly where I was headed with the design. I'm going to give the lower links the ability to adjust 5 degrees up or down and parallel. I'll use a full length panhard bar across the back.

I also think you're right about the torque arm; it will be shorter than an F-body. I have some figuring to do yet but I'll probably come up with a length and then maybe try to make the cross member adjustable front to rear. At least a little bit anyway. That way I can fine tune it a little. I figure I can make as many torque arms as it takes to get it right.

I guess I'll calculate the length based on the Instant Center location I think it should have. I need to do a little research on that. Any ideas? From my experience the IC is not always an intersection through the mounting point. It can be a virtual IC just like a 4 link.

I think I'll mount the end of the torque arm in a polyurethane bushing sandwich. Kind of like a sway bar end link. That way I can tune it a bit with softer or harder bushings.

One reason I picked this setup is that I have quite a bit of experience with it in a circle track environment. However, the application here is very limited compared to a circle track car. My experience may not be worth much in this case.

Thanks for your input.


Wally

 

[Triaged]

My understanding of finding the IC is that the the 2 control arms are obvious but the 3rd link is a "phantom" link. The reason for this is that the torque arm is considered as "part of the axle housing". The joint at the front of the torque arm is a slider. When you analyze a slider you take it as a "link of infinite length perpendicular to the slider". In this case that means vertical. If however your slider has some friction in it (which I'm sure it will) that "link of infinite length" stuff goes out the window. In trying to fit everything under a low floor I'm sure there won't be much vertical distance between the slider and the front mount of the control arms so I don't think the angle of the phantom link will make a HUGE difference in IC location...but I don't know for sure.

 

[comet boy]

This is headed a different direction but you could do like I am going to do and use a Chris Alstin truck Four link. It is designed to stay under the bed of the truck. so nothing has to be cut. Thus leaving room for a backseat and trunk.

 

[wally8]

Thanks Guys.

Triaged,

I can see what you mean by that. I'll have to do a little more research. I'm still pretty up in the air about what to do.

Comet Boy,

That could work but I think the biggest issue is intruding on the back seat space which would be the case with that truck setup as well.

Guess I have some more thinking to do.


Wally

 

[Triaged]

I was poking around and found this...

http://www.totalcontrolproducts.com/rearcoil.html

 

[wally8]

I found that link too in my searches. I swear I've done every variation of torque arm suspension and design you can think of on google. I found some interesting info on corner-carvers.com that put what you were saying into perspective.

It makes sense to break it down to the very simple components. The torque arm is controlling axle rotation. The LCA's are pushing the car forward and thus their angle is really the driving factor for the IC. This is me thinking aloud here. Let me know what you think of this way of thinking about it.

This would also coincide with how I've actually tuned our modified that had a torque arm on it. We messed with LCA angles to change forward bite. and separately tuned the spring and shock on the TA to apply the torque over time.

If this is the correct way of thinking about it (which I believe it is) I feel good about the decision now. This was my initial reasoning for the adjustment positions of +/- 5 degrees.

I need to do some more mockup to see what the packaging will come out to be. I also need to think about TA length. It will mostly be a factor of the mechanical advantage needed to make the bushing deflection work properly.

Again, any and all feedback / criticism is appreciated.

Thanks Triaged.

Later...

Wally

 

[Triaged]

Because of the angles involved the control arms and the torque arm both have alot to do with the IC location. However changing the angle of the control arms will change the axle 'roll axis', roll center height (very slightly), as well as the IC. Changing the torque arm length will only change the IC so that is what I would use to tune the suspension. It wouldn't be as quick and easy as changing the angle of the control arms but it is less confusing to only change one thing at a time.

One other comment
I have no idea why GM mounted the torque arm to the driver's side Doing so will cause the right tire to have even less load on it under acceleration then it would otherwies have. Putting it on the passenger's side would help to counteract some of the driveshaft torque to equal out the tire loads on acceleration. I'm sure it is a small difference because the torque arm isn't far from the axle's center but it still doesn't seem right to me