Torsion bar car - how low is too low?
#1
Burning Brakes
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My 911SC has stock Bilstien front struts, and I do not want to modify them so the car will be legal in PCA stock class. I have gotten comments that I may have the front too low, and that may account for the car pushing as I accelerate out of a corner.
Can anyone tell me it there is published data or common knowledge that will tell me the best lower-limit of ride height?
Thanks,
Can anyone tell me it there is published data or common knowledge that will tell me the best lower-limit of ride height?
Thanks,
#2
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Don, the most important consideration is the angle of your A-arms and bump steer. Bump steer kits are not legal in stock classes. However, Bilstein struts do allow you to lower your car more than Boge struts. But that being said, your front wheels look pretty slammed... I can't imagine how you don't have horrible bump steer.
#3
Burning Brakes
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The bump steer is significant. I do have the steering rack spacers. I am more concerned about the distance between c.g. and roll center. Do you have any idea what the a-arm angle should be? I know mine are angled up at rest.
#4
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Conventional wisdom is the max you want to go is where the a-arms are parallel to the ground. The issues I had with my old 914 was that as the suspension compressed, the camber change is positive which is not what you want as the car rolls (lots of caster helps some here but still not ideal), especially as these cars are camber challenged to begin with. The other problem that I had was that at very low ride heights on an otherwise stock suspension, the ball joint ran out of articulation. The Elephant Racing's decambered balljoints offer greater articulation and add some camber which is why I changed to them on the 914 but they aren't stock class legal.
#5
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The distance between the roll center and the center of gravity is called the roll couple. The roll couple is the lever arm that centrifugal force working on the CG uses to make a car lean over in a turn around the roll center. In a rear- or mid-engine car, these approximations apply to the opposite end of the car.
The longer the roll couple, the more weight is transferred to the outside wheels during cornering and the more the car will want to roll in a turn. A longer roll couple makes cars slower to respond to steering input. The resulting weight transfer from a long roll couple also uses the inside tires less effectively during cornering, thereby reducing the available grip.
The often-overlooked disadvantage to lowering is that roll center drops more radically than the center of gravity on most cars. This increases the roll couple and can cancel any weight transfer advantage. The huge roll couple created by overlowering will require an overly stiff suspension to control body movement.
And when your suspension is too stiff, it won't absorb road irregularities effectively, which will make it harder to keep the tires in contact with the ground. You can't drive fast if your tires aren't on the ground.
On most cars, the ideal location for the roll center is 2 to 5 inches above the ground for the front suspension and 4 to 10 inches above ground for the rear suspension. With the rear roll center higher than the front, the car will transfer more weight to the front, making it more likely to understeer. Most purpose-built racecars utilize this design because it allows them to be tuned for slight understeer at high speed and more oversteer at lower speeds.
The longer the roll couple, the more weight is transferred to the outside wheels during cornering and the more the car will want to roll in a turn. A longer roll couple makes cars slower to respond to steering input. The resulting weight transfer from a long roll couple also uses the inside tires less effectively during cornering, thereby reducing the available grip.
The often-overlooked disadvantage to lowering is that roll center drops more radically than the center of gravity on most cars. This increases the roll couple and can cancel any weight transfer advantage. The huge roll couple created by overlowering will require an overly stiff suspension to control body movement.
And when your suspension is too stiff, it won't absorb road irregularities effectively, which will make it harder to keep the tires in contact with the ground. You can't drive fast if your tires aren't on the ground.
On most cars, the ideal location for the roll center is 2 to 5 inches above the ground for the front suspension and 4 to 10 inches above ground for the rear suspension. With the rear roll center higher than the front, the car will transfer more weight to the front, making it more likely to understeer. Most purpose-built racecars utilize this design because it allows them to be tuned for slight understeer at high speed and more oversteer at lower speeds.
One point not mentioned in the article is that extreme bump steer will play havoc with the consistency of grip that you expect from the front end of the car. With the steering angle (and resultant level of grip) constantly changing due to the motion of the suspension, it is much harder to keep your car on the limit of grip.
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Larry Herman
2016 Ford Transit Connect Titanium LWB
2018 Tesla Model 3 - Electricity can be fun!
Retired Club Racer & National PCA Instructor
Past Flames:
1994 RS America Club Racer
2004 GT3 Track Car
1984 911 Carrera Club Racer
1974 914/4 2.0 Track Car
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Larry Herman
2016 Ford Transit Connect Titanium LWB
2018 Tesla Model 3 - Electricity can be fun!
Retired Club Racer & National PCA Instructor
Past Flames:
1994 RS America Club Racer
2004 GT3 Track Car
1984 911 Carrera Club Racer
1974 914/4 2.0 Track Car
CLICK HERE to see some of my ancient racing videos.
#6
Three Wheelin'
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I cannot tell you your car is too low or too high from pictures or even from measurements. So much depends....
You have one or more restrictions that I don't have... my car is targeted toward GT2r in PCA Club Racing.
In my case, I can put it on the deck. probably even run an upside down transmission, raise the floor, et cetera. But I don't think you can do what I can do.
Today I am lowering my already low car. Another 5/16. I run coilovers in all four corners. My spring plates have been modified so I can get 0 to -1 degrees of negative camber with a proper toe setting in the rear. My front struts have raised pickup points. My steering rack is spaced up. And I run Eisenlohr's bump steer kit on the tie rods. In my case, I am going to move the transmission up as much as possible to improve the half shaft angles. Doing that now.
The things I am doing come from the old school. Nothing new or novel really. A friend/mechanic and ex-IMSA pro (who built the car) is helping me learn what to do. He is also a racer in the PRC.
I have a copy of the Puhn book to which I refer often. When I have a question like yours, I network to someone like my friend with old school experience, since they figured all of this stuff out a long time ago. In situations in the past when I didn't have local access to answers I have sought, I have just called folks with pro racing experience.
Maybe there is someone in your area with such experience that you can work with that understands your constraints and can maximize your set up against those constraints.
You have one or more restrictions that I don't have... my car is targeted toward GT2r in PCA Club Racing.
In my case, I can put it on the deck. probably even run an upside down transmission, raise the floor, et cetera. But I don't think you can do what I can do.
Today I am lowering my already low car. Another 5/16. I run coilovers in all four corners. My spring plates have been modified so I can get 0 to -1 degrees of negative camber with a proper toe setting in the rear. My front struts have raised pickup points. My steering rack is spaced up. And I run Eisenlohr's bump steer kit on the tie rods. In my case, I am going to move the transmission up as much as possible to improve the half shaft angles. Doing that now.
The things I am doing come from the old school. Nothing new or novel really. A friend/mechanic and ex-IMSA pro (who built the car) is helping me learn what to do. He is also a racer in the PRC.
I have a copy of the Puhn book to which I refer often. When I have a question like yours, I network to someone like my friend with old school experience, since they figured all of this stuff out a long time ago. In situations in the past when I didn't have local access to answers I have sought, I have just called folks with pro racing experience.
Maybe there is someone in your area with such experience that you can work with that understands your constraints and can maximize your set up against those constraints.
#7
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Everything in the article that Larry posted is true. You can nver eliminate bump steer but you can minimize it.
The A -arm angle is not the issue. Bump steer is caused by the different arcs which the A arm and steering arms go through. Ideally the center and length of both arcs is the same but that is a physical impossibility so the best you can do is minimize it. It will be a minimum when suspension travel is minimized and when the steering arm starts out(neutral position w/ static loaded weight) horizontal to the ground. The purpose of all the bump steer kits out there is to restore the steering arm to horizontal.
something like this
The A -arm angle is not the issue. Bump steer is caused by the different arcs which the A arm and steering arms go through. Ideally the center and length of both arcs is the same but that is a physical impossibility so the best you can do is minimize it. It will be a minimum when suspension travel is minimized and when the steering arm starts out(neutral position w/ static loaded weight) horizontal to the ground. The purpose of all the bump steer kits out there is to restore the steering arm to horizontal.
something like this
![](http://forums.pelicanparts.com/uploads14/TechCorner31222215248.gif)
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#8
Three Wheelin'
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Great picture Bill.
Should note that there seem to be two types of things folks call bump steer kits at times.
Steering rack spacers... these move the rack up. They help things some in the front, the amount depends on how much the car is lowered and the specific geometry of the suspension.
The other type of kit is made by ERP (Eisenlohr Racing Products... sold by JWE/SmartRacing products). If you can learn about set up from Eisenlohr, you are tapped into the old school. There is also a version sold by Rebel Racing I think, and Elephant Racing... and perhaps others. These kits allow the tie rod to be attached to the strut with improved geometry such that there is less alignment change for change in ride height (bump and rebound).
I don't think that these kits are designed to fully restore the steering arm to horizontal. That would only be true for a specific suspension geometry. I think they are designed to provide improvement in the geometry.
The Puhn book explains all of that. And since these cars where raced for a long time in IMSA, there are plenty of folks out there that know how to set them up, likely even given the class-based constraints to which you have referred.
Should note that there seem to be two types of things folks call bump steer kits at times.
Steering rack spacers... these move the rack up. They help things some in the front, the amount depends on how much the car is lowered and the specific geometry of the suspension.
The other type of kit is made by ERP (Eisenlohr Racing Products... sold by JWE/SmartRacing products). If you can learn about set up from Eisenlohr, you are tapped into the old school. There is also a version sold by Rebel Racing I think, and Elephant Racing... and perhaps others. These kits allow the tie rod to be attached to the strut with improved geometry such that there is less alignment change for change in ride height (bump and rebound).
I don't think that these kits are designed to fully restore the steering arm to horizontal. That would only be true for a specific suspension geometry. I think they are designed to provide improvement in the geometry.
The Puhn book explains all of that. And since these cars where raced for a long time in IMSA, there are plenty of folks out there that know how to set them up, likely even given the class-based constraints to which you have referred.
#10
Mr. Excitement
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Still can't figure that one out. Excessive bump is a bad handling issue. it is OK to instal $$$ Motions but not fix a hasard with a small $ bolt on kit? Lifting the rack can help a little at best.
#12
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There's a lot that I can't figure out, but it seems like the "stock" rules are slowly moving in the right direction. Here's one - we finally get camber plates approved, but then you cannot open up the hole in the top of the strut tower to take advantage of them. Explain that one to me.
#13
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On my early cars the lowest you could go was dead level through the centerline of the torsion bar and the pivot center of the lower ball joint, the arms would be slightly negative. BUT, this was with big bars, snubbers and 20mm rack spacers. As mentioned above if you go too low the ball joint in the arm will bind and fail, not a good deal. The other thing to check is that with stock bilsteins and light bars(?) you will be on the high side of the shock travel, if you are too low and too soft the chassis will roll onto the snubbers, you may be rolling around the corner bottomed on the strut which will give you all kinds of weird handling inconsistencies when you uncompress the chassis out of the corner. The car looks too low in the pick but it's hard to tell. Take a look at the condition of the snubbers, do they look hammered? Lowering can really help the handling and feel of the car, to a point, then it quickly reverses the benefits...
#14
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the scruts have looked at us early stock cars and searched for the spacers on the ends of the steering arms, they are illegal. It does not say you can use them in the rule book. People with those were bumped up a class or sent to GT, don't remember which.
#15
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Don,
Aside from the obvious bump steer issue, my biggest concern would be adequate suspension travel at your current ride height.
Remember, when the suspension bottoms out (even momentarily), the spring rate leaps to infinity and you are now driving a park bench (or little red wagon),....neither one is good at all. If this happens in the front, its a recipe for terminal understeer.
Aside from the obvious bump steer issue, my biggest concern would be adequate suspension travel at your current ride height.
Remember, when the suspension bottoms out (even momentarily), the spring rate leaps to infinity and you are now driving a park bench (or little red wagon),....neither one is good at all. If this happens in the front, its a recipe for terminal understeer.