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The above makes sense, but the problem when lowering a 911 is usually seen as the control arm going outer end UP when you lower the car. the inner end gets too low and the control arm is no longer parallel to the ground. A raised spindle will lower the outer end, making it more parallel to the ground, which is what I thought we wanted?
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Old 05-11-2006, 06:09 PM
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Quote:
Originally posted by analogmike
The above makes sense, but the problem when lowering a 911 is usually seen as the control arm going outer end UP when you lower the car. the inner end gets too low and the control arm is no longer parallel to the ground. A raised spindle will lower the outer end, making it more parallel to the ground, which is what I thought we wanted?

Read my previous post again.


It has nothing to do with being parallel to the ground.


It's ALL about parallelism between the tie-rod, and the lower control arm.


Ride height only affects where on the bump-steer curve you end up.
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Old 05-11-2006, 07:00 PM
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Mike, I'll try to get to the root of most people's misunderstanding.



Since a stock 911 has intentionally built-in toe-change with bump, the further the suspension compresses, the more it toes-out.

At stock ride height, the change is small. But the further you lower the car the greater the amount of toe-out you get for a given amount of travel.

What people usually do is install rack spacers. This doesn't change where you are in the bump-steer curve, but instead reduces the over-all amount of toe-change by bringing the tie-rods and lower control arms closer to parallel. The result is a similar amount of toe change as you had before at stock ride height.


If you raise the spindles, you are really essentially just lowering the outer end of the control arm. The spindle and steering arms are still in the same place in relation to the car, so the tie-rod is also. Now the lower control arm is even less parallel to the tie-rod, requiring either raising the rack further, or using drop-links for the outer tie-rod to lower it down to match the angle.



All the hoopla about the angle of the lower control arm in relation to the ground is just a quick way to tell how far past the normal bump-steer curve you are from stock ride height.



The reason it is so confusing is that both the amount of toe change in the bump steer curve, and where you are in the curve at your given ride height, both can have an equal effect on the amount of bump-steer experienced by the driver, and measured on a bump-steer gauge.



In other words, a car with a lot of built-in toe-change, but set very close to the zero point in the bump-steer curve at ride height, will likely feel better than a car with less built in toe-change, but way past the zero-point in the curve at ride height.


You can improve a cars manners either way. You can also play around with the handling manners by manipulating both. The factory did it for a number of reasons. They cured a few ills on the original cars with these settings. Some of these ills don't exist anymore due to aero improvements, tire technology, and wheelbase and track width changes.
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Old 05-11-2006, 07:35 PM
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Nicely done!
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Old 05-11-2006, 09:11 PM
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yea, LOL, right after I posted, and was driving to a friends, I thought..."Uh oh...that's not quite right...the inner control arm point is key...I should have kept my mouth shut, LOL"

Good catch, nicely explained, esp without pictures and a powerpoint presentation!
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Old 05-12-2006, 02:36 AM
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But back to the original poster...he hasn't *yet* raised the spindles...and is running 1/2" below Euro ( agreed...whatever that means) height.

So.... to get *at least* the amount of bump steer "goodness" that we had stock....I think 1/2" ( 13mm) rack spacers are an economical compromise. Is there a flaw in this reasoning? ....when we recognize bump steer can't be completely eliminated ???

Tyson...your explanation is archival quality.... I need to review your text carefully with diagrams I had made earlier in my life... to help me "visualize" what's happening.

Maybe a cliff note version of the spindle height change effect is the fact the strut body steering arm goes up with it. Would be much less of a problem if the spindle ( only) could be raised, and the "end-point" of the attached steering arm on the strut can be placed where it was originally. ( ???). This is the reason to heat and bend the arms, but it seems in their new location they would be a "tad" too short...and ideally should be a bit longer to compensate. True?... or is the change small enough to be only academic?

Wil
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Old 05-12-2006, 04:35 AM
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Quote:
Originally posted by Wil Ferch
But back to the original poster...he hasn't *yet* raised the spindles...and is running 1/2" below Euro ( agreed...whatever that means) height.

So.... to get *at least* the amount of bump steer "goodness" that we had stock....I think 1/2" ( 13mm) rack spacers are an economical compromise. Is there a flaw in this reasoning? ....when we recognize bump steer can't be completely eliminated ???

Tyson...your explanation is archival quality.... I need to review your text carefully with diagrams I had made earlier in my life... to help me "visualize" what's happening.

Maybe a cliff note version of the spindle height change effect is the fact the strut body steering arm goes up with it. Would be much less of a problem if the spindle ( only) could be raised, and the "end-point" of the attached steering arm on the strut can be placed where it was originally. ( ???). This is the reason to heat and bend the arms, but it seems in their new location they would be a "tad" too short...and ideally should be a bit longer to compensate. True?... or is the change small enough to be only academic?

Wil

Yes, the rack spacers are fine for simple lowering like he's proposing. I agree.


You're also spot-on in regard to the ideal situation with raising the spindles. Too bad they aren't more modular in their construction.

But bending the steering arms, as I mentioned before, does have some positive effects besides improving the bump-steer. It reintroduces some Ackerman (toe-out when the wheels are turned) into the geometry. This is lost when you remove the stock bump-steer that also doubled as Ackerman in the stock configuration.

The other side benefit is a marginally quicker steering ratio, but it's no more of a change than using a 930 steering rack.
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Old 05-12-2006, 08:44 AM
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Thanks for the info, I always thought the raised spindles would HELP bump steer, or Porsche would not have used them on the RSRs.

It seems odd that when you lower a car and the lower control arm is angled up on the wheel side, the tie rod is even MORE angled up and needs to be levelled more to reduce bump steer (and make it more parallel to the control arm).

Maybe I'll stick to the cup car

have fun!
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Old 05-12-2006, 09:03 AM
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Quote:
Originally posted by analogmike
Thanks for the info, I always thought the raised spindles would HELP bump steer, or Porsche would not have used them on the RSRs.

It seems odd that when you lower a car and the lower control arm is angled up on the wheel side, the tie rod is even MORE angled up and needs to be levelled more to reduce bump steer (and make it more parallel to the control arm).

Maybe I'll stick to the cup car

have fun!

The spindles were raised to raise the front roll-center height, and restore lost strut travel with lowering.

Those issues are more important to lap times than bump-steer.


The beauty of raised spindles, is that you raise the roll center height, while lowering the center of gravity. If you do that, you end up with a car that has more built-in anti-roll. This means you don't need as much suspension stiffness in the form of sway bars and stiffer torsions/springs to achieve a given amount of cornering flatness, and the resulting increase in cornering force.

But you have to balance that front to rear like anything else.

Fine tuning of the front to rear roll-center heights can be achieved by simply altering the cars rake. That has an effect on aero though as well, so that has to be considered.

Lots of things at play. It's rather mind boggling when you start looking at everything. That's why this stuff takes years of practice and tons of R&D to find the right solution.
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Old 05-12-2006, 09:20 AM
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Very instructive and intersting, even if I don't understand all the above.

Can you outline the "right solution" for a few scenarios?

1. Street only with not much lowering?
(rack spacers only? or also bend the steering arms)

2. Combination street/track car -- maybe a lot of DEs?

3. Autocross car?

4. Track only - on high speed tracks?

5. Track only - on tracks with lots of corners; i.e. low to med. speed tracks?

Also, can you post a pic of the curve you discussed above? I think it was a curve of toe-in/out vs. 'bump' (displacement).
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Old 05-12-2006, 09:45 AM
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O.K., I'll take a crack at it. But keep in mind, it's just my oipinion, since there are a thousand different ways to skin this cat, and no right answer. It's all about individual set-ups, and driver preference.

These are my suggestions for the scenarios you gave:


1.Street/lowered: rack spacers

2.Street/track: raised spindles, drop-link style bump-steer kit. (ERP, Elephant Racing, etc.)

3.Autocross: raised spindles, possibly leave the bump-steer for the added Ackerman,

or-raised spindles, bent steering arms 935-style

4.Track only-high speed: raised spindles, remove any and all bump-steer with drop-links, or combo of drop-links and bent steering arms.

5.Track only- technical track: raised spindles, bent steering arms 935-style, with slight amount of toe-out in bump left behind to improve manners. (Slight Ackerman from shorter bent arms and the slight toe-out on bump left in.)
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Old 05-12-2006, 10:24 AM
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Let me build upon randywebb's question...

---Tyson... can you relate your answer to *amount* of lowering..instead of using "street" and "track" description?

Like....if you lower from stock, but no more than 1/2".... use rack spacers.

Lower a total of 1/2" to 1".....do "X"

lower a total of 1" to 1.5"....do "Y"...

Etc...

????

- Wil
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Old 05-12-2006, 01:49 PM
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Ray the orginal poster is back, I had to go soak my head in ice water to keep it from exploding trying to absorb this information! I'm going to dumb this conversation down again by asking another question:

Using the measurement from the ground to the lip of the fender is there a rule of thumb of how low you can go with rack spacers and when you should switch to a drop link?
ex: rack spacers no lower than 24.75"F and 24"R
Same as Wil's question - only different

It is also mentioned that the Bilstein RSR has poor valving and should be redone. Does the Sport Bilstein insert also have similar valving to the RSR or is it ok outta the box. For this reason too, I assume you shouldn't mix an RSR front with a regular sport shock in the rear. Only reason I thought about this is I may have a bent strut and Bilstein does not sell a strut with a sport insert, only a HD so I was thinking of a RSR in front with no springs with sport shocks in back.

thanks again
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Old 05-13-2006, 01:24 AM
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Great stuff, Guys.

I the past, when bending streering arms, I have used the following process: I'd appreciate critical comment..

Set up a rough jig to check the bend shape against.

Set up firebricks to sheild the arm.

Heat thoroughtly with a big oxye acteylene flame until dull red.

Bend, check, reheat , bend etc.

Allow to cool slowly playing the flame over it.

When cold, hammer all the accessible parts to restore some temper..and for fine adjustment of shape..

Kind regards
David
Old 05-13-2006, 02:52 AM
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RayM:

Until Tyson or other gurus's answer...let me start off by saying this.

- common convention puts front fender lip measurement at 25.5" for "Euro"..although we understand that isn't the official factory meaurement using suspension hard-points for reference.
- the amount a steering rack can be "spaced" is only 1/2" until things start to hit...no more room.
- if you can accept the amount of "stock" bump-steer as "Acceptable"..and use this as a target..then you can't lower more than the thickness of rack spacer you can use... the same 1/2" or about 13mm. So from a fender measurement pespective, the front fender lip can't be more than 25" off the ground ( vs 25.5" stock for "Euro").
- due to the different shape of the rear fender cut-out, this measurement is another 1/2" lower in the rear..to get to stock ( or 1 degree nose-down slope). So.... stock is 25.5" fr / 25" rear..... max lowering using spacers would be 25" fr/ 24.5" rear and still have "Stock" bump steer..

- I would guess any further lowering beyond 25/24.5 would require a fine adjustment drop link ( for bump steer) and ...depending upon "how" low...a combo of doing this PLUS raised spindles to regain most ( or all) of the original suspension travel and camber "curve" during the suspension's motion. I need to re-look my diagrams....but common convention also says the lower "A"-arms of the suspension should angle down toward the outer ball joint...or at most be "flat"..but not angle the other way ( inside pivot lower..ball joint higher). I need to think if this is equally necessary with raised spindles or not... I don't have my diagrams in front of me. Perhaps Tyson (and others) can chime -in on this point too.

Wil
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Old 05-13-2006, 04:57 AM
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Well, there's a lot to go into here.

First off, there has to be a really accurate way to measure ride height. The only proper way, is to measure the differential in height between the center of the wheel, and the center of the torsion bar. That's how the factory measures ride height, since it eliminates the tire diameter variable.

From there, one could bump-steer a car for different scenarios, and give someone a guideline. But there are still more variables.


Consider:


1. Tie-rod length will affect bump-steer, so alignment settings are a variable.

2.manufacturing tolerances affect the position of all the components

3.accident damage. I can't tell you how many bent aluminum front crossmember's I've seen.

4. not everyone likes the same feel. Bump-steer settings are more about behavior than they are about performance. So whatever behavior the driver feels the most comfortable with will ultimately yield the most performance in the car, since the drive is the biggest variable.

So I really shy away from giving out blanket recommendations.




Having said that, the rack spacers are fine on any car, even if they are stock ride height. (Remember, you're still just removing some of the built-in toe-change)

The off-the-shelf drop-link style kits will work even with stock spindle heights, as it still won't over-compensate for bump-steer, so long as they are around 1 1/4 to 1 1/2" long. That's about the max I'd run with stock spindle heights. (Don't use these in conjunction with rack spacers if you have stock spindles) Doing this will make the car feel very secure and predictable over uneven pavement, and there will be less steering correction needed. It will increase steering sensitivity, since the loaded wheel will be turning more directly with the steering wheel, since it won't be toe-ing out as the body rolls. It will feel like it has a quicker steering ratio.




Now, I hope nobody takes this post out of context. It was meant to be an add-on to my previous posts.


Maybe in the future, I can start a thread where I can post bump-steer charts, and show some graphics that will illustrate the mechaics involved in tuning bump-steer. This would be a lot easier with pictures!
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Old 05-16-2006, 08:15 PM
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We need guys like Tyson, Bruce Anderson, Steve Weiner, Steve Wong, John Walker, etc on the East coast !!!!........

That way these guys can conduct "seminars" and we can go to class !!

...bit of a trek if the distance is 3000 miles away ...

- Wil
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Old 05-17-2006, 05:29 AM
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I had my spindles raised and ran my GT1R really .. really low.... so I had a very large spacer to compensate but I also had the bracket you see attached to my strut to stabilize everything and never had any problems...

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Old 05-18-2006, 12:26 AM
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Dang VZ, I had to stare at that pic for a while to figure out what I was looking at. Not a factory Porsche part in sight. ERP front suspension?

Looks like custom front struts as well. Looks like someone already thought about bump-steer, since the strut has the steering arm mounted very low. Well below the spindle. You may not even need rack spacers with that set-up.

Have you put a bump-steer gauge on it yet?
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Old 05-18-2006, 08:43 AM
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Quote:
Originally posted by Tyson Schmidt
Dang VZ, I had to stare at that pic for a while to figure out what I was looking at. Not a factory Porsche part in sight. ERP front suspension?

Heeeeyyyy .. those fuel pumps on the cross member are factory !
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Old 05-18-2006, 08:47 AM
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