SCCAForums.com

I'm playing with stack height and am wondering if anyone uses only one race with this setup?

 

I've got a really nice upper spring perch, but the spring being round allows it to have a little more play than i'd like.  I think it could use less thickness between the perch and the spring, so what pitfalls are there to using the upper perch, bearing, then race, spring?

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Rob P.

The new--05' Subaru WRX
The old--04' VW GTI
->the damage: http://i206.photobucket.com/albums/bb198/penderperson/WRX/pics005.jpg

The spring perch being of a softer material than the bearings will soon make for a ruined bearing.

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Jay Storm
96 Impreza L FSP
98 98 Impreza 2.5RS- co-driver DSP
Wives are like SCCA rules...
If she doesn't say you can...you can't

Torrington bearings don't like debris.  Since the two races encapsulate the bearing it is protected from most of the sand and gravel thrown up by the sticky tires.   If you remove one race and leave the bearing exposed it will quickly foul and cease working like a bearing.

If you need a thinner bearing, look around some.  You can find them pretty darned thin. 

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Steve Hoelscher
#27 DP - Toyota MR2
2006 DP National Champion
'98, '99, '00, '02 DSP National Champion
http://www.terriehoward.com

both excellent points, went ahead with the two races and if need be, i can have a custom perch made to combine a tapered washer and the standard koni perch for a shorter height.

 

Any mcphearson genius' available?  How do i interpret and set ride height--i've got a bunch of plots and graphs available, but have no idea what i need to do it.  I've got NF's determined and if ground control will send me the right rates i'll be set. 

 

here's the link of what i've got, 20 pages:  http://forums.vwvortex.com/zerothread?id=1363022&page=2 

i'd prefer to understand the concepts so i can interpret all that; Tune to win and How to make your car handle aren't getting me there(prob me more than authors).

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Rob P.

The new--05' Subaru WRX
The old--04' VW GTI
->the damage: http://i206.photobucket.com/albums/bb198/penderperson/WRX/pics005.jpg

VW_rob_85:

Any mcphearson genius' available?  How do i interpret and set ride height--i've got a bunch of plots and graphs available, but have no idea what i need to do it.  I've got NF's determined and if ground control will send me the right rates i'll be set. 

here's the link of what i've got, 20 pages:  http://forums.vwvortex.com/zerothread?id=1363022&page=2 

i'd prefer to understand the concepts so i can interpret all that; Tune to win and How to make your car handle aren't getting me there(prob me more than authors).

Well, the concepts are pretty simple.  And strut suspension is easier to visualize than double a-arms. 

There are a few suspension design/evaluation software packages out there that will do this scientifically, but you do need to understand the concepts. 

Setting ride height is a function of roll axis height/inclination and camber curve.   I glanced at the link and read a post or two.  For the most part they are on point.  You want to plot the camber curve and set the ride height so that the car doesn't roll so far that the camber curve gets into the positive side of the plot.  The camber curve starts going negative when angle of the control arm (or more accurately, the line between the inner control arm pivot and the center of the lower ball joint) and the line between the lower ball joint and the top strut mount is greater than 90 degrees.  As long as that angle is less than 90 degrees, the camber curve is in the negative part of the curve.  That is, negative camber increases as you compress the suspension.  When the control arm/strut angle passes 90 degrees, it begins to go positive. 

How much the car rolls is a function of roll stiffeness (spring and bar rate) and the roll center height.  Typically, if you lower the car enough to have the control arm/strut angle greater than 90 degrees, the roll center is below ground.  You dont' want that because it creates too much body roll for a given amount of roll stiffness in addition to the camber issues. 

Admittedly, this is an over-simplification of the situation.  Entire books have been written on this subject so there its impractical to fully explore the subject here.  So I recommend you do some more reading.

To give you a very basic idea of the factors involved.  Proper ride height is determined by the amount of body roll you have because body roll affects the camber curve due to control arm angle.  Control arm angle affects the amount of body roll because it affects roll center height.  Roll center height affects the amount of body roll because of the length of the "moment arm" (the distance between the CG and the roll center).  The longer the moment arm, the more body roll for a given amount of roll stiffness.  The roll center height (and the length of the moment arm) are determinded by the control arm angle and therefore ride height. 

So you see that all of these factors interact.  Most strut chassis have a "sweet spot" where the control arms are still in the negative part of the camber curve and the roll center height is near the ideal height (above ground level). 

 

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Steve Hoelscher
#27 DP - Toyota MR2
2006 DP National Champion
'98, '99, '00, '02 DSP National Champion
http://www.terriehoward.com

Ok, lets see how i'm doing.  I want to set my ride height so that in a turn my camber reaches its peak and the LCA doesn't pass 90*, would it be acceptable to exceed 90 in any case or is it pretty solid?

So using camber curve, roll camber curve and steering camber curve will all affect this.  Roll-induced camber is caused by the center of roll being lower than CG in my case and higher spring rates/NF's will minimize this as well as limit the LCA to passing 90*. 

Assuming the graphs are correct, it appears i have the largest camber gain 1/2" below stock ride height, so how do i determine the spring rates and how high i must set the rideheight for optimum camber?  Also what's a good reference for rideheight, seems like this could be confused easily?

 

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Rob P.

The new--05' Subaru WRX
The old--04' VW GTI
->the damage: http://i206.photobucket.com/albums/bb198/penderperson/WRX/pics005.jpg

VW_rob_85:

Ok, lets see how i'm doing.  I want to set my ride height so that in a turn my camber reaches its peak and the LCA doesn't pass 90*, would it be acceptable to exceed 90 in any case or is it pretty solid?

In theory, you never want to exceed 90 deg.  In practice, its not practical because we don't always run on a billiard table.  So you want enough travel to accomodate bumps which will likely run the travel past the ideal '90 deg.

VW_rob_85:

So using camber curve, roll camber curve and steering camber curve will all affect this.  Roll-induced camber is caused by the center of roll being lower than CG in my case and higher spring rates/NF's will minimize this as well as limit the LCA to passing 90*. 

NF's ???  not sure what NF's are but you are otherwise correct.  Ideally, you will have enough static camber than when the car rolls and the camber curves all work, you end up with an ideal resulting camber.  That is the tire's contact patch is at its optimum angle to produce maximum grip.

VW_rob_85:

Assuming the graphs are correct, it appears i have the largest camber gain 1/2" below stock ride height, so how do i determine the spring rates and how high i must set the rideheight for optimum camber?  Also what's a good reference for rideheight, seems like this could be confused easily?

Well, there are two ways to determine spring rates.  You can use one of the suspension modeling packages to do all of this on your computer.  I use Susprog3d.  I use it to determine roll center and roll axis locations and figure roll ratios, load transfer, total roll, etc...

Or you can do it the old fashioned way and test.  Each is effective.  To test, find a smooth site where you can drive the car through a turn at maximum g.  Put a zip tie tightly on the strut rod right at the top of the strut body on both struts on one side of the car.  Then drive the car through a turn at maximum g (typical autocross turn/speed) to compress the struts you put the zip ties one.  Measure the hight of the zip tie above the top of the strut body.  This is the amount the suspension compressed in the turn.  Now you can place the car on stands, remove the tire and jack the control arm up until the strut is at the zip tie.  Now measure the control arm angle and determine where on the camber curve the wheel is at that point. 

Now with this data you can determine how much the car is rolling and how much resulting camber you have.  If you are past the ideal range you can either stiffen the springs/bar or raise the ride height, or both.  Ideally, you want strut cars to stay pretty flat to keep the geometry from going south and reduce the amount of static camber you need. 

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Steve Hoelscher
#27 DP - Toyota MR2
2006 DP National Champion
'98, '99, '00, '02 DSP National Champion
http://www.terriehoward.com

Ok, all that makes sense, i think i'll go with testing and see how things turn out, then i'll work towards better accuracy.

 

For the extreme short term, what would you say is more important, cornerweighting or getting ride height and camber curves 'close'? 

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Rob P.

The new--05' Subaru WRX
The old--04' VW GTI
->the damage: http://i206.photobucket.com/albums/bb198/penderperson/WRX/pics005.jpg

VW_rob_85:

For the extreme short term, what would you say is more important, cornerweighting or getting ride height and camber curves 'close'? 

 

Well, if the ride height, camber curves and roll centers are screwed up, cross weight won't matter that much.  Besides, you will likely be making lots of ride height adjustments while dailing in those variables so your corner weights are going to change every time you make an adjustment.   In pactice, cross weight is the last thing to be done when setting up the car.

I should also note that I use the term "cross weight" instead of corner weight as getting the cross weight ( LF/RR vs RF/LR) set is more important than actual corner weights. 

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Steve Hoelscher
#27 DP - Toyota MR2
2006 DP National Champion
'98, '99, '00, '02 DSP National Champion
http://www.terriehoward.com

I was planning on following fred puhn's method of cornerweighting with front to rear ratios on each side the same.  It seems that this would be most advantageous for a front engine, offset driver, ST car.  What differences are there with doing crossweights?

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Rob P.

The new--05' Subaru WRX
The old--04' VW GTI
->the damage: http://i206.photobucket.com/albums/bb198/penderperson/WRX/pics005.jpg

VW_rob_85:

I was planning on following fred puhn's method of cornerweighting with front to rear ratios on each side the same.  It seems that this would be most advantageous for a front engine, offset driver, ST car.  What differences are there with doing crossweights?

Fred Puhn's book is a valuable tool with great information in it but I dissagree with him on this point.  Ideally you want the same weight carried on each side of the car, with the same weight distribution front to rear, on each side.  If this can be accomplished by moving the vehicle's components to improve their location and where the weight is carried, that's a good thing.   However, if the you can't move the balast around to equalize the weights (as in STS) and you try to accomplish equalizing the loads on the tires by jacking weights with the coil over adjusters, that isn't a good thing.

Think of the car as an "X" and compare it to a bar stool that has one leg slightly shorter than the other three.  The stool rocks across the X on the two opposing legs that are of equal length.   If you were to place four scale pads under the legs, the two legs that the stool rocks on will carry much more of the weight than the other two.  Also, the weight carried on the axis that has the short leg would shift front to rear depending on whether you were leaning forward or backward. 

Now transfer that principle to a car and lets assume that the axis with the most weight is the LR/RR.  When turning right the car would tend to have more understeer than turning left.  So the car would be inconsistant when turning one direction then the other.  This is exactly how NASCAR teams tune their stock cars.  The term "wedge" refers to the early days when a crew member would actually drive a wedge shaped block of wood into the coils of a spring to add pre-load to the spring.  Now days wedge is changed by use of a screw jack that raises and lowered the upper spring perch of the rear springs.  The front is done with spring rubbers.   These two methods change the cross weight of the car and therefore its handling balance.  This works nicely on stock cars because they only turn left.  On an autocross car, or road race car, that has to turn both directions, its important that the car have the same balance turning both directions.  This is done by equalizing the cross weight. 

 

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Steve Hoelscher
#27 DP - Toyota MR2
2006 DP National Champion
'98, '99, '00, '02 DSP National Champion
http://www.terriehoward.com

What references do you use for that method? 

 

From what i can tell on an average st car, the difference between zero cross weight and equal front/rear ratios will be something like 20lbs higher on one cross than the other.  Or am i missing something?  How much difference does this make in the balance?  Would it be obvious given that we're riding on pliable tires?

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Rob P.

The new--05' Subaru WRX
The old--04' VW GTI
->the damage: http://i206.photobucket.com/albums/bb198/penderperson/WRX/pics005.jpg

VW_rob_85:

What references do you use for that method? 

I am not sure what you are asking here.

 

VW_rob_85:

From what i can tell on an average st car, the difference between zero cross weight and equal front/rear ratios will be something like 20lbs higher on one cross than the other.  Or am i missing something?  How much difference does this make in the balance?  Would it be obvious given that we're riding on pliable tires?

You want the cross weights to be equal.  Then accept the difference in front and rear corner weights.  Noticeable?  That depends on the driver's sensitivity and ability, how stiffly sprung the car is and how consistant the surface is. 

I have a fair amount of experience setting up short track stock cars.  There, 50 lbs of cross weight is VERY noticable.  Less so in an autocross car.   Can you tell if on a car that has the cross weight off or close?   Hard to say.  It has been my experience that its a subtle adjustment but more noticeable than corner weight. 

My DP car is 200 lbs heavy on the left side with the driver.  I have it balasted to the right as much as is practical and that's as close as I can get.  So the car does turn better left than right even with the cross weights set spot on.  But the balance is the same and that's what's important. 

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Steve Hoelscher
#27 DP - Toyota MR2
2006 DP National Champion
'98, '99, '00, '02 DSP National Champion
http://www.terriehoward.com

Steve Hoelscher:

VW_rob_85:

What references do you use for that method? 

I am not sure what you are asking here.

 

I'm just wondering what other books you find useful for these topics.  Which of carroll smiths books are good for this topic? 

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Rob P.

The new--05' Subaru WRX
The old--04' VW GTI
->the damage: http://i206.photobucket.com/albums/bb198/penderperson/WRX/pics005.jpg

VW_rob_85:

I'm just wondering what other books you find useful for these topics.   

 

25 years of experience preparing my own cars and crewing for other teams including pro racing teams in both SCCA World Challenge and NASCAR? 

I have read most of the books on the subject over the years.  What I found most useful was playing around with the software packages.  I first started with a simple spread sheet back in the early 90s.  Now I use Susprod3d.  It enables you to see the effects of setup changes. 

From the Carroll Smith series, the first one , "prepare to win", is probably your best bet. 

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Steve Hoelscher
#27 DP - Toyota MR2
2006 DP National Champion
'98, '99, '00, '02 DSP National Champion
http://www.terriehoward.com

Ok, new problem...my control arms cannot reach 90* with complete contact to bumpstops, i doubt they will reach without them either due to coil bind.  It appears that fully compressed, i lack 1.3" to reach 90*.

For sanity sake, i measured the angle from the LCA mount to the ball joint to the camber plate bearing; this sounds logical? 

 

From what i can tell i have a few options to get closer:

 1.  Reduce stack height, i have anywhere from .3" to 1" that can come out of my stack height with custom spring perches on top.  This will lower the car and make the lca's closer to 90*.

2.  Shorten the konis...how expensive is this?  They do shorten the bottoms of the shocks correct?  This will also lower the car and bring the lca's closer to 90*, as well as increasing available shock travel.  How much shorter do i need to make them?  Can i buy a shortened version? (i'm using 8710's now)

3.  cut stock spring perches from konis and move threaded sleeves down?  (i think this will shorten the stroke even more...)   

4.  cut/remove  bumpstops, i don't think this is safe/practical/smart.

 

Appreciate the help! 

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Rob P.

The new--05' Subaru WRX
The old--04' VW GTI
->the damage: http://i206.photobucket.com/albums/bb198/penderperson/WRX/pics005.jpg

VW_rob_85:

3.  cut stock spring perches from konis and move threaded sleeves down?  (i think this will shorten the stroke even more...)   

4.  cut/remove  bumpstops, i don't think this is safe/practical/smart.

Well, these are the two options most people choose.  Which is ideal depends on the particular car.  Most OEM bump stops are quite long, soft and progressive.  Typically the best option is to cut the lower ends off leaving only the stiff, thick upper portion.  The idea here is to prevent the strut from bottoming internally and damaging the foot valve. 

The first thing you do on most coil over kit installs is cut away the OE lower perch.  But, you may be able to use a shorter coil over spring to prevent having to lower the threaded sleeves.  Before you do check the springs specs to determine the spring's "stroke".  This is the amount of travel the spring has and will limit the available travel to the stroke length.  If the spring coil binds before you use up most of the struts travel you haven't achieved anything.

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Steve Hoelscher
#27 DP - Toyota MR2
2006 DP National Champion
'98, '99, '00, '02 DSP National Champion
http://www.terriehoward.com

I don't think i need to cut the stock mounts, i am able to lower the car adequately, i'm limited now by shock stroke i believe.   

With konis at stock ride height, on stock mounts, there is about 1.7" of shock stroke, about the same as stock shocks had .  How do i determine the minimum i need?  So after cutting the bumpstops, i can lower even more until i reach the min shock stroke?

 

Also, what's more important attaining 90* lca's or keeping the roll center above ground/as close to the cg as possible? 

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Rob P.

The new--05' Subaru WRX
The old--04' VW GTI
->the damage: http://i206.photobucket.com/albums/bb198/penderperson/WRX/pics005.jpg

The reason for keeping the lca over 90* IS to keep the roll center above ground.  That is the basic problem with the setup on the front of VW's A4 chassis, poor roll center control.  The lower riding versions, such as the 337 and 20th anniversary editions came from the factory lowered with the roll center lower than idea.  The only way to get the roll center reasonable with a low ride height is to use the upright from the Audi TT or one of the aftermarket versions.  Ball joint extenders work too but I've seen them break which makes things interesting when it happens.  I kind of lost track of what class you are running in as to if any of the geometry fixing methods are legal for you.  If it's STS, they only allow a change in control arms for camber if I recall correctly.