Revised suspension geometry, anti-lift/dive discussion

[Reaper1]

This thread is inspired by the release of the tubular K-frame by Brian Slowe and the revised suspension geometry that is available with it. The modifications are based on a technical SAE paper that studied the effects of this alltered geometry on FWD drag cars. The intent here is to discuss the actual geometry, it's changes, theory, ect. This is NOT limited to drag racing and I would actually preffer it to include street cars as well as autocross and road racing.

I believe this community can bennefit as a whole with this knowledge, so please, let's have a good informative discussion!

[shackwrrr]

My thoughts are for a drag setup you would want pro lift. At launch the pro lift would slam the tires into the pavement. You would want to run struts with a pretty high rebound dampening to try and spread the load the whole 60 ft. This is the same idea as the instant center in rear geometry. An instant center that promotes anti squat will separate the rear axle from the chassis, pushing the tires into the ground.

[Warren Stramer]

I dunno, I read the entire SAE paper and didnt find any new earth shattering information. Just common sense stuff, but they showed their math.
Also, remember it is computer modeling, not real data from a real car. I do believe their modeling to be quite realistic though. A good read.

[bakes]

Anyone have a link to this paper ?

[Shadow]

I dunno, I read the entire SAE paper and didnt find any new earth shattering information. Just common sense stuff, but they showed their math.
Also, remember it is computer modeling, not real data from a real car. I do believe their modeling to be quite realistic though. A good read.

The Single most significant thing that I can see, and the entire reason I went ahead and asked Brian to make me one of these cross members is because of what I saw his car do after his cross member was installed. (ie. 60' better than it ever has before)

The real beauty of what brian did (in my mind) is build and Prove the working model in the real world works as well (if not better) than the "computer model".

[Warren Stramer]

The Single most significant thing that I can see, and the entire reason I went ahead and asked Brian to make me one of these cross members is because of what I saw his car do after his cross member was installed. (ie. 60' better than it ever has before)

The real beauty of what brian did (in my mind) is build and Prove the working model in the real world works as well (if not better) than the "computer model".

I agree with that.

[135sohc]

Anyone have a link to this paper ?

http://papers.sae.org/2005-01-0421/

[bakes]

http://papers.sae.org/2005-01-0421/

thanks for link.

[vipernbox]

Subscribed... even though I am going a totally different direction with the current project, and your "pro lift" geometry would end up being anti-squat in my setup That is exactly what I need....

Or at least think I need...



Something to think about though... With that pro lift setup... You are also adding a little pro dive under braking...

[shackwrrr]

Something to think about though... With that pro lift setup... You are also adding a little pro dive under braking...

Doesn't matter when you got laundry to drop and a long shutdown, lol.

[Shadow]

Something to think about though... With that pro lift setup... You are also adding a little pro dive under braking...

Correct me if I'm wrong, Pro lift = anti dive under braking which is why it's a +/+ for road racing.

[Reaper1]

I thought by what I read in the brief description in the Suspensino book that if you introduce pro-lift, it will also induce more dive under braking due to the suspension actually traveling more. Hence why I would think that this is a good thing, but you need to revise the spring rates, and possibly the roll bar diameter as the roll rate will also change. The revised geometry WILL work, but it requires an ENTIRE package, not just the one part, IMHO.

[vipernbox]

This might be the link Brain posted in the other thread....



http://www.orldsm.com/showthread.php?13797-Anti-Squat-and-Pro-Lift\



The idea of pro lift is basically the opposite of anti-dive. Under heavy braking the front of the car wants to squat down, ant-dive geometry in the front suspension uses braking forces to counteract this and jack against the squat to stop the dive = anti-dive. Unfortunately, braking forces are opposite acceleration forces in the same system so this means the same anti-dive characteristic creates anti-lift in reaction to acceleration forces = squat for the front suspension.

[vipernbox]

And here is a kinda interesting discussion primarily about the other end of the car...


http://www.eng-tips.com/viewthread.c...=87498&page=18


That is a great forum BTW...

[turboshad]

I dunno, I read the entire SAE paper and didnt find any new earth shattering information. Just common sense stuff, but they showed their math.
Also, remember it is computer modeling, not real data from a real car. I do believe their modeling to be quite realistic though. A good read.

Sometimes common sense comes from experience. If you asked me before I would have said anti-lift to keep the body and CG lower and I feel I have a reasonable amount of common sense..........maybe. But after reading into it I can see how that reduces the normal force at the tires. While reading the article I couldn't help but wonder what the effect of pro-lift and a 'Stramer' anti-roll bar mod would do. I personally would have liked to see more equations in the paper and not just explanations of variables.

One thing to note which I do fell is common sense was that the wheelie bar showed the largest gains since it puts the chassis' pivot point so far back.

My thoughts are for a drag setup you would want pro lift. At launch the pro lift would slam the tires into the pavement. You would want to run struts with a pretty high rebound dampening to try and spread the load the whole 60 ft. This is the same idea as the instant center in rear geometry. An instant center that promotes anti squat will separate the rear axle from the chassis, pushing the tires into the ground.

I understood it that the raising of the body was the reaction force to the increased normal force at the tire. So despite the body reaction, the tire still sees the same normal proportional to the change in acceleration. With that understanding it shouldn't matter how fast the body reacts as the tire force is independent but I would agree on a higher rebound to keep the body movement quiet.

Something to think about though... With that pro lift setup... You are also adding a little pro dive under braking...

That is my only concern, just like with Warrens roll bar mod. Since I don't just drag my car I don't want to end up with a car body that is moving all over the place. In their computer simulation it looks like they got an extra 2.5deg of pitch angle taking it to 6.5deg from the amount of pro-lift they added so you could assume similar changes under breaking, and that was with a wheelie bar. Since you can conceivably get more breaking forces than acceleration that means in their case they could see over 13deg of total pitch movement from full acceleration to full breaking. At the same point Brian said could notice a difference and if you want videos of his car it looks very stable so I guess I will just have to try it.

[Shadow]

The different angles of the LCA's keeps messing with my brain. (trying to picture what's happening through the components) I think it's easier for me to think of the spring becoming more active as a result of the changed geometry and therefore becoming "softer" as far as suspension movement which would translate into more dive during braking as you guys have pointed out.

I keep picturing upgraded springs to dial the new geometry back in, so I seem to forget about what the "old springs" in the new set-up are going to be doing.

The more I think about the springs becoming more active in the suspension, the more I think about the actual "intended use" of a swaybar and I see it like this.

A properly tuned suspension does not need or have a use for a swaybar in Any type of motorsport. This is because the proper spring rates and tuned suspension are doing their job for whatever conditions they were ment for. That being said, a full on road race car is going to be pretty stiff to ride in and you certainly wouldn't want to take it on long road trips!

So, from my limited understanding, a swaybar is an "in-between". A way to soften up the spring rates to give a more comfortable ride under normal driving conditions while limiting body roll under hard cornering the way stiffer spring normaly would.

---------- Post added at 10:37 AM ---------- Previous post was at 10:28 AM ----------

One thing to note which I do feel is common sense was that the wheelie bar showed the largest gains since it puts the chassis' pivot point so far back.

Agreed, it was nice to have the "best case" scenario as the constant (sort of) to compare things to. Deff gave me a clearer picture of what differences the various changes were making and why Brian chose to do what he did.

[turboshad]

The different angles of the LCA's keeps messing with my brain. (trying to picture what's happening through the components) I think it's easier for me to think of the spring becoming more active as a result of the changed geometry and therefore becoming "softer" as far as suspension movement which would translate into more dive during braking as you guys have pointed out.

The acceleration and braking forces will create a moment or torque around a rotation center (instant center) made by drawing a line perpendicular to the strut and through the hard points of the control arm which is proportional to the vertical distance from the force to this center. This creates a reaction force (normal force) at the wheel which opposes the torque and is proportional to the horizontal distance from the normal to the IC. This force will be reacted in the body with a positive normal force producing body lift and a negative normal force producing body drop.



1) The IC is inline with the axle vertically. The acc force travels directly through the IC so there is nothing produced here but the braking creates a CCW moment resulting in a positive normal making the body want to rise so anti-dive in braking.

2) The IC is below axle but above the ground plane. The acc force produces a CCW moment resulting an a positive normal making the body rise so pro-lift in acc. The braking forces do the same thing so anti-dive in braking again.

3) The IC is below the axle and ground plane. Same as above but the braking forces will cause a CW moment and a negative normal making the body drop so pro-dive in braking.

4) The IC is above the axle and ground plane. The acc force produces a CW moment resulting in a negative normal making the body want to drop so anti-lift in acc. The braking forces produce a CCW moment resulting in a positive normal making the body want to rise so anti-dive in braking.

The larger the vertical distance the larger the resultant force and body movement so even though 2) gives pro-lift and anti-dive both these forces will be relatively small. Also in 3) with pro-lift and pro-dive, the pro-dive forces will be less do to the smaller vertical distance to the IC. I didn't do this one but you can also see that if the IC lands on the ground plane you would have no reaction from braking.

A properly tuned suspension does not need or have a use for a swaybar in Any type of motorsport. This is because the proper spring rates and tuned suspension are doing their job for whatever conditions they were ment for. That being said, a full on road race car is going to be pretty stiff to ride in and you certainly wouldn't want to take it on long road trips!

So, from my limited understanding, a swaybar is an "in-between". A way to soften up the spring rates to give a more comfortable ride under normal driving conditions while limiting body roll under hard cornering the way stiffer spring normaly would.

This is untrue. The suspension's job is to keep the wheel in contact with the ground. The spring rates and damping coefficients needed to do this optimally may not be what is needed to keep the body roll where you want. The anti-roll is nice b/c it stiffens the roll characteristics without really adversely changing the suspension characteristics. There is some cross over but it helps get the best of both worlds.

Here are a couple pics of finely tuned suspension with anti-roll. (from this thread http://www.f1technical.net/forum/vie...=8737&p=188342)

[Reaper1]

Yes, as I speculate the spring and damper rates WILL have to change with the modified goemetry. The anti-roll bar *is* a compromise, but a necessary one. If you were to run spring rates that were high enough to provide the roll stiffeness desired in a hard cornering situation where a certain amount of body roll is the *goal*, then the wheel rates would be WAY too high on anything but a glass plate causing loss of traction on less than ideal surfaces, even with proper damping. Not only that, but the spine shattering ride that would produce. So, in order to dial in the optimal wheel rate while obtaining the desired body roll angle in a corner, an anti-roll bar is used. In drag racing and anti-roll bar in a FWD application will do absolutley nothing but add weight, given that the rest of the chassis and suspensino is set-up correctly.

I will note that I've been driving my car around for about a year without a front anti-roll bar and have noticed little difference in the handling characteristics of the car. I will say that the limits of this car with the tires that are on it are WAY beyond what I dare try on the street, so if I were to take it on a road course or auto-cross, then I might notice the difference.

Actually, om to think of it, it might actually HELP right now! I had to take out my good transmission that had the OBX in it, so I'm running an open diff right now. Without the anti-roll bar to try to equalize the wheel rate on opposite sides of the car, the inside tire might not come unloaded as badly in a hard corner allowing more power to be transferred before loss of traction. I know my friend who used to run ministock came to the same conclusion after trial and error. Hmmm...makes ya think! LOL

[shadow88]

Am I the only one who thinks this is getting a little out of hand for a bunch of guys messing with K cars?

[Shadow]

Am I the only one who thinks this is getting a little out of hand for a bunch of guys messing with K cars?

Not when those K cars are looking to run 9's one wknd, then win the local road race the next!

---------- Post added at 02:40 PM ---------- Previous post was at 02:35 PM ----------

This is untrue. The suspension's job is to keep the wheel in contact with the ground. The spring rates and damping coefficients needed to do this optimally may not be what is needed to keep the body roll where you want. The anti-roll is nice b/c it stiffens the roll characteristics without really adversely changing the suspension characteristics. There is some cross over but it helps get the best of both worlds.

Ya, was kind of thinking out loud as I'm pretty new to this stuff. Was picturing the "extream" and then pondering where the practical application falls, pass car, or pass car as well as full race suspensions. You answered that.