Bump steer Kit/Torsion Bars

[zl0ca]

Hello,
I'm looking to go below the minimum ride height of the 10mm spacer bump steer kit(~24.5 inches). Any recommendations to alternatives? I saw several solutions on our hosts website and I'm wondering what most people use and the feedback they have for it. I have turbo tie rods. Also, I have replaced the shocks(Bilstein Sport) and to me the car still feels little too soft. The main use of the car is street with the periodic DE events in the future. Question is, is 22mm(front) and 29mm(rear) torsion bars a good solution for me? In previous cars this is where I would purchase coil overs and get both my questions answered, but for this car it's out of the question(who has 5k for coils and engine drop for reinforcement????....not this guy)
Thanks,

[toddu]

I have those TBs on my SC with custom valved shocks. Pretty stiff ride, but not unbearable.

Todd

[Driven97]

I'm pretty sure the best solution is a raised spindle. Lowers the front of the car and keeps closer to the original intent correct geometry (roll center, camber curve, tie rod angle.)

I run 23/33 TBs and want to go stiffer, but the 33 rear is as big as the spline diameter. Engineering wise, lager seems like a bad idea.

[Trackrash]

If you want to go much lower than stock you need to raise the spindles, as mentioned.

When you lower the front suspension, with out any other mods like raising the spindle, your a-arms will be angled up, causing poor geometry and a mushy feeling.

For spirited driving larger torsion bars are a must. With today's grippier tires firmer springing is a must.

The way to go is to have the spindles raised, larger torsions, custom valved Bilsteins, and one of these type of bump steer kits. Porsche 911 (1965-1973) - Steering System - Page 2

[will hung]

When I bought my car, it had a similar bump steer kits as yours. I then lowered the car when I rebuilt the suspension. After lowering, the front end grip just wasn't there, it would understeer very badly. Plus bumpsteer became a huge issue.

So I had my front spindles raised 30mm and got a bump steer kit from Elephant Racing. Before installing the new struts, I raised the front back up to stock ride height. So now I have a car that sets 30mm lower in front, with no bumpsteer, it's totally compliant over bumps, and has tons of front end grip.

[zl0ca]

That's exactly what I'm feeling. Are the bump steers required with a raised spindle ?


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[Trackrash]

Originally Posted by zl0ca

That's exactly what I'm feeling. Are the bump steers required with a raised spindle ?


Sent from my iPhone using Tapatalk

Yes.

Be careful. More than 25mm spindle raise needs a 17" wheel.

[chris_seven]

I have been intrigued by Bump Steer in 911s for a while and can't find any data on the subject.

I am not sure how much bump steer there is with a standard car at standard height and what effects eliminating this would produce.

If you then lower the car how much increase is there in bump steer and how much do you dial out when making changes?

I guess my real question is what happens to the turn-in behaviour of the car of you eliminate all of the bump steer.

[Bill Verburg]

Originally Posted by chris_seven

I have been intrigued by Bump Steer in 911s for a while and can't find any data on the subject.

I am not sure how much bump steer there is with a standard car at standard height and what effects eliminating this would produce.

If you then lower the car how much increase is there in bump steer and how much do you dial out when making changes?

I guess my real question is what happens to the turn-in behaviour of the car of you eliminate all of the bump steer.

Bump steer can never be eliminated, it can only be minimized at the ride height used.

Stiffer suspension is one step toward minimizing bump steer because suspension travel is reduced.

Bump steer goes hand in hand w/ scrub radius, as increasing scrub increases the magnitude of any bump steer that is present.

What you want is to minimize bump steer and at the same time have a small amount of scrub. This is what gives the 'live' feel to the steering wheel. Of course caster and camber also enter into the equation too but they are more adjustable.

bump correction falls into 2 basic categories
rack spacer
raise spindles and/or use off set tie rod ends

which to use depends on on how far below static euro ride height you are going to go, i've use the rack spacer down to 135+/- w/ good results w/ close to stock scrub and stock t-bars


static euro height is 108mm+/-5mm, the SC/RS was the lowest factory street legal height @143+/-5mm

[will hung]

Originally Posted by Trackrash

Yes.

Be careful. More than 25mm spindle raise needs a 17" wheel.

19mm max for 15" wheels, 30mm max for 16" wheels, and 40mm max for 17" wheels. I have 16x7" Fuchs at the front of my car with 30mm raised spindles. It's a tight fit though. I had to have my wheel weights moved inbound to clear the bottom of the ball joint.

Originally Posted by chris_seven

I have been intrigued by Bump Steer in 911s for a while and can't find any data on the subject.

I am not sure how much bump steer there is with a standard car at standard height and what effects eliminating this would produce.

If you then lower the car how much increase is there in bump steer and how much do you dial out when making changes?

I guess my real question is what happens to the turn-in behaviour of the car of you eliminate all of the bump steer.

I don't think bump steer effects turn in. Bump steer is caused when the tie rod end lengths are different than the control arm lengths. This causes the steering angle to change when the front suspension is compressed, such as when driving over a bump, thus the bump is steering the car.

Bump steer isn't a concern if you drive on perfectly smooth surfaces, but if you drive on bumpy roads (like southwestern PA is known for) it can be exhausting, and terrifying, driving at posted speed limits.

[Bill Verburg]

Originally Posted by will hung

19mm max for 15" wheels, 30mm max for 16" wheels, and 40mm max for 17" wheels. I have 16x7" Fuchs at the front of my car with 30mm raised spindles. It's a tight fit though. I had to have my wheel weights moved inbound to clear the bottom of the ball joint.




I don't think bump steer effects turn in. Bump steer is caused when the tie rod end lengths are different than the control arm lengths. This causes the steering angle to change when the front suspension is compressed, such as when driving over a bump, thus the bump is steering the car.

Bump steer isn't a concern if you drive on perfectly smooth surfaces, but if you drive on bumpy roads (like southwestern PA is known for) it can be exhausting, and terrifying, driving at posted speed limits.

Bump steer is caused by the different arcs that the wheel and tie rod ends move through as the suspension is exercised. This causes the relative length of the tie rod to change(the geometry changes) thus causing the wheel on that side to toe in or out. It can occur when traveling straight on bumps but it also occurs in cornering when the suspension on one side is compressed and on the other extended.

if the centers and radii of the arcs of the wheel and tie rods were the same there would be no bump steer.

here is the factory recommendation for set up


and the factory suspension curves for a 108mm ride height, vorspur is toe in w/ ride height change ie 'bump steer' vorderachsen is 'front axle'

[will hung]

Originally Posted by Bill Verburg

Bump steer is caused by the different arcs that the wheel and tie rod ends move through as the suspension is exercised. This causes the relative length of the tie rod to change(the geometry changes) thus causing the wheel on that side to toe in or out. It can occur when traveling straight on bumps but it also occurs in cornering when the suspension on one side is compressed and on the other extended.

if the centers and radii of the arcs of the wheel and tie rods were the same there would be no bump steer.

here is the factory recommendation for set up


and the factory suspension curves for a 108mm ride height, vorspur is toe in w/ ride height change ie 'bump steer' vorderachsen is 'front axle'

Cool. Thank you for the more technical explanation. Relative lengths is what I meant since metal can't change shape unless it's bent. But I still don't think it effects turn in characteristics of the car unless you are hitting a bump on turn in.

[javadog]

Originally Posted by zl0ca

That's exactly what I'm feeling. Are the bump steers required with a raised spindle ?


Sent from my iPhone using Tapatalk

Originally Posted by Trackrash

Yes.

Be careful. More than 25mm spindle raise needs a 17" wheel.

The answer is maybe. It depends on how much you lower the car. Rack spacers are good for about 10mm. Raised spindles are discussed above, relative to how much you can accomplish with a given wheel (some people have reported problems at those numbers, so I probably wouldn't use the last couple millimeters). If you lower the car beyond what you can correct with raising the spindles, then you can move the rack also.

JR

[Jesse16]

What year is your car ? I've recently gone to 22/29 TB's and lowered the car so I raised my steering rack the max. that it would go and cannot say I've ever felt or noticed any bump steer. I think the issue is possibly overblown for street and occasional DE use. My car sits at approx 24.5" ride height at the fenders which is very low, almost too low for all around use at times. My advice is to do nothing extreme for bump steer until you can honestly say you need to. I know of plenty of others at DE events that tell me the same, at least for 87/89 cars with turbo tie rods, etc..

[javadog]

Originally Posted by chris_seven

I guess my real question is what happens to the turn-in behaviour of the car of you eliminate all of the bump steer.

There's a little info on it here, if you scroll down the page towards the bottom. Not 911-specific...:

Technical Articles

[Flieger]

Originally Posted by Bill Verburg

Bump steer can never be eliminated, it can only be minimized at the ride height used.

Stiffer suspension is one step toward minimizing bump steer because suspension travel is reduced.

Bump steer goes hand in hand w/ scrub radius, as increasing scrub increases the magnitude of any bump steer that is present.

What you want is to minimize bump steer and at the same time have a small amount of scrub. This is what gives the 'live' feel to the steering wheel. Of course caster and camber also enter into the equation too but they are more adjustable.

bump correction falls into 2 basic categories
rack spacer
raise spindles and/or use off set tie rod ends

which to use depends on on how far below static euro ride height you are going to go, i've use the rack spacer down to 135+/- w/ good results w/ close to stock scrub and stock t-bars


static euro height is 108mm+/-5mm, the SC/RS was the lowest factory street legal height @143+/-5mm

I'm not sure what you mean by "bump steer goes hand in hand with scrub radius" as the change in toe is an angular parameter and this is unrelated to the scrub radius which is just a length (radius). More scrub radius (actually the spindle length which is basically the scrub radius with regard to the wheel center, the two are very similar if your camber and steering axis inclination are about equal) will increase the kickback due to bumps but this kickback would still be present even if there were zero bumpsteer (tie rod and control arm moving on same arc).

[javadog]

Basically, he said what you just said.

JR

[Flieger]

Originally Posted by javadog

Basically, he said what you just said.

JR

Not really, they are two different things. You can have zero scrub radius and horrible bump steer. You can have zero bump steer and horrible kickback.

[javadog]

Try this:

The greater the scrub radius, the more kickback you will have from a given amount of bump steer.

Clear?

JR

[javadog]

He said "increasing scrub increases the magnitude of any bump steer that is present."

You said "More scrub radius will increase the kickback due to bumps."

JR