instantaneous center outside of car

[swong46]

I was looking at an excerpt from "How to make your car handle" by Fred Puhn on google and saw that he had a picture of a setup where the upper arms are higher on the inboard side. This makes the instantaneous center outside the car instead of inside.

I can't seem to find information on its effects or at least I am not using the correct keywords.

Can someone tell me where I can find info on this?

[swong46]

I was looking at an excerpt from "How to make your car handle" by Fred Puhn on google and saw that he had a picture of a setup where the upper arms are higher on the inboard side. This makes the instantaneous center outside the car instead of inside.

I can't seem to find information on its effects or at least I am not using the correct keywords.

Can someone tell me where I can find info on this?

[Dash]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by swong46:
I was looking at an excerpt from "How to make your car handle" by Fred Puhn on google and saw that he had a picture of a setup where the upper arms are higher on the inboard side. This makes the instantaneous center outside the car instead of inside.

I can't seem to find information on its effects or at least I am not using the correct keywords.

Can someone tell me where I can find info on this? </div></BLOCKQUOTE>


This is purely a mechanisms problem. If the center of rotation is on the outside of the car, it is highly likely that you would gain POSITIVE camber under bump.

[thewoundedsoldier]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">If the center of rotation is on the outside of the car, it is highly likely that you would gain negative camber under bump. </div></BLOCKQUOTE>

Do you mean highly unlikely?

When i first started designing with 10" wheels, I kept on making the compromise you are talking about, swong. I would induce camber with loads of caster instead of suspension travel. You can get camber gain (or should i say loss? how bout change rate?) pretty close to neutral with upper arms pointing upward.

As for other effects, the IC being outboard is going to put your roll center well below ground, leading to a huge roll moment. Watch out for that. RCVD page 610+ really addresses further effects pretty well.

[Dash]

My bad, I meant to type Positive camber. Had to re-read my comment. :P

[Demon Of Speed]

For a clarifications, are you talking about diverging control arms, as apposed to the more traditional converging control arms?

[Zac]

GM ran a setup like this on the 49-54 passenger cars and consequently the 53-62 corvettes. First gen corvettes aren't exactly known to be agile handling vehicles or have an extensive racing history. You might want to look into the reasons why.

[thewoundedsoldier]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Zac:
GM ran a setup like this on the 49-54 passenger cars and consequently the 53-62 corvettes. First gen corvettes aren't exactly known to be agile handling vehicles or have an extensive racing history. You might want to look into the reasons why. </div></BLOCKQUOTE>

This is a great observation and I like the analogy. I think the great performance of the sting ray had more to do with the increase in engine displacement than the suspension change. It's important to note the first generations had solid beam axles too. i think if you are going with a solid beam axle on 10" wheels all around, the diverging a-arms are a great solution because the caster-induced weight jacking and LLT get you where you need to be. On all other styles of cars, it is probably a compromise.

The only muscle cars I am familiar with are the ray and the mustang, but it'd be cool to hear what other cars have ran similar setups. I wouldn't imagine it would be highly desired on any IRS car.

[DFT]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by thewoundedsoldier:
As for other effects, the IC being outboard is going to put your roll center well below ground, leading to a huge roll moment. Watch out for that. RCVD page 610+ really addresses further effects pretty well. </div></BLOCKQUOTE>

Having the ICs outside the wheels does not necessarily mean your RC will be below the ground. Off the top of my head, I can't think of any good kinematic reasons for putting your IC on the outside if you care much about maintaining negative camber. Although it is sometimes done for other reasons. F1 cars, for example, have their ICs on the outside of their wheels, but this is because of aerodynamic constraints. They have to put their control arms really high to get them out of the airflow and it seems that they put their ICs on the outside to allow them to maintain a reasonable height for their RCs.

[swong46]

Thanks for the reply guys.
I do have one more thing, I read that high instantaneous center helps in anti-squat and also it will be closer to the CG height to help with body roll.

But also saw that too high will cause the rear to lift, how do I know when it is too high?

[thewoundedsoldier]

Don't confuse the instantaneous center and the roll center. Having the IC high does not mean the roll center will be high, such as in the case of the diverging a-arms you first asked about.

You're questions are simple and straightforward, but the answers to them are pretty complex. There is no "easy" button for where to place I.C.'s and other suspension parameters. Start by trying to understand how the roll axis of the vehicle affects lateral load transfer, and what effects lateral load transfer will have on handling. Don't be afraid to make basic assumptions, and then iterate over and over and over.

[swong46]

Well I say that because making the IC higher on both ends will cause the roll center to move up (at least in my sketch model)

So there will be less lateral weight transfer because the moment arm is small; I guess I just need to find how much the roll center will move up under full compression

[thewoundedsoldier]

WinGeo is awesome for quickly finding all the deltas like that. Such a pain in the ass to do all the hand calcs, then slightly change a point, and have to start all over!

Side note: limiting roll center migration in the SAE z-axis during bump and y-axis during steering will help make the car more predictable. This can be a measured quantity described as a variable change rate (i.e., you want a small change rate over a large distance of travel).

[murpia]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by thewoundedsoldier:
Don't confuse the instantaneous center and the roll center. Having the IC high does not mean the roll center will be high, such as in the case of the diverging a-arms you first asked about. </div></BLOCKQUOTE>
Care to justify that statement?

Regards, Ian

[thewoundedsoldier]

So steering forces not withstanding, if an IC is outboard with respect to its own wheel, it will create a line intersecting with the tire contact patch which points downwards and towards the center of the car. The intersection of both of these lines (the RC) must be below ground because of the direction of the intersection line.

With ICs outboard, a lower IC will mean a higher RC. With ICs inboard (like normal), the opposite is true.

Not sure if I'm missing something...I think I'm seeing it correctly?

[Zac]

Can't you position the IC (for the wheel) either above or below ground though? and won't that have an influence on where the RC is?

[thewoundedsoldier]

Of course. Baja seems to be a good application for an outboard, subterranean IC (because they can place less emphasis on the importance of gaining negative camber in bump). I haven't seen it done on a formula student car, but I'm sure it's happened. Still, if the upper a-arms point upward going into the chassis and the lower a-arms point downward going into the chassis, you will have an outboard, above-ground IC.

Thinking about it more, I would be very impressed to see a well set-up car that has an outboard, subterranean IC. The lower and upper a-arms would both have to point upwards going into the chassis, but the lower would have to be at a smaller angle. The fine-tuning sounds intense and completely unworth it to me (too simple of a person http://fsae.com/groupee_common/emoticons/icon_razz.gif ).

[Dash]

Not to mention it would probably be a nightmare trying to fit a steering system into it that would pass template and not have the most bump-steer ever. OR you would have insane amounts of ground clearance, OR be running something bigger than a 13 inch rim.