Bumpsteer spacers................

[Vintage-TechZ]

I've searched at no avail to the question of " correct spacer thickness as wheel/ tire combinations change" ?

John C. has mentioned using different mm spacers per his roll center changes and obviously final tire height is a main concern for the final spacer height/thickness.

I'm going with a 17" wheel with a 245/45 tire ( I'll measure mounted tire height tonite).

I've got GC coil-overs and was about to re install everything when I was holding my old spacers in my hand ( block for 15" wheels) and began the questioning of whether or not I needed to mill them down.....eliminate them ....build thicker ones....or get everything installed and begin measureing for teh corrections ??

Please chime in for everyone who has crossed this bridge and laugh hard if I'm missing the obvious here !lol.

[JMortensen]

I've searched at no avail to the question of " correct spacer thickness as wheel/ tire combinations change" ?

John C. has mentioned using different mm spacers per his roll center changes and obviously final tire height is a main concern for the final spacer height/thickness.

There is no "correct" thickness, as these spacers change the roll center, and don't really fix the bumpsteer. I would say thicker is better, but at some point you have to worry about the shear strength of the bolts. I think I've seen 1.5" ones for 510s which I believe will bolt onto a Z. I'm using 3/4" ones myself.

 

Tire height will have no direct bearing on the bump steer spacer thickness. What will have an effect is your attempt to reduce the wheel gap by lowering the suspension. The taller the tire the higher the body of the car will be from the ground, and the low-pros look funky when there is a 3" gap between the tire and fenderwell. The natural inclination is to lower the car with coilovers, and fairly quickly the control arms go past level and start pointing up towards the wheel. When this happens the roll center goes underground and then you need larger sway bars or stiffer springs to control roll, or you can relocate the inner pivot up or use bump steer spacers to bring the outer part of the control arm down to compensate.

 

Since the bumpsteer spacer is below the axle height it doesn't affect the ride height of the car at all. What it really does is fix the roll center problem. Fixing the actual bumpsteer itself is more easily done on a Z by raising the inner control arm pivot. I slotted the holes in the crossmember and raised the inner pivots until the bumpsteer went away on a bumpsteer gauge. Another fairly easy way to fix bumpsteer on a Z is to drill out the steer knuckles to 5/8" and run a 5/8" rod end, then use a circle track (Pinto) bumpsteer spacer kit to move the tie rod down to get the bumpsteer eliminated.

[Vintage-TechZ]

Ahhh !I see what what you mean. I'll measure my current spacers thickness ( I have three different sized sets) and go with the thicker ones til' I get a good look of the geometry once preloaded on the ground again.

Do you happen to have any visuals/pix of what could be considered as "best geometry" ?

Vinny

[JMortensen]

It's not a clear cut thing. I think the higher I can get the roll center the better because on a Z it's not going to be that high regardless. Cary (tube80z) thinks the LCAs should be level or lower and sometimes even that the RC being below ground can be best. John hasn't really stated a firm preference that I've noticed, other than to say that the control arms should point down. John can probably give you a rocker height suggestion based on your wheel diameter.

 

The end result is that you can get the car lower with a lower RC, and you can make a variety of RC's work through spring and sway bar changes. If you want to drive on the street you probably want a higher RC so you don't have to have ultra stiff springs and sway bars to make it work.

[tube80z]

It's not a clear cut thing. I think the higher I can get the roll center the better because on a Z it's not going to be that high regardless. Cary (tube80z) thinks the LCAs should be level or lower and sometimes even that the RC being below ground can be best. John hasn't really stated a firm preference that I've noticed, other than to say that the control arms should point down. John can probably give you a rocker height suggestion based on your wheel diameter.

 

I think a lot of the difference has to do with how you use the car, tire type, weight, etc. In my case the car is an autoxer and I use really short 13 inch tires (20s or 22s). It may be that my RC isn't that much different than other cars on a lot taller tires. For the most part I have my lower control arms near level. I also make sure that the inner lower control points (TC rod and LCA) are level too. If not you'll introduct anti geometry and this will induce understeer, or at least that is what I have experimentally concluded. I also use spring rates very close to the corner weight supported. And I run the front with less than a 0.25 inches of droop and the rear around 1.25 to 1.5 inches of droop. I don't think you'd want to copy any of this for a street car.

 

For softer rates you'll need a some anlge down towards the wheel. I think the sweet spot is close to level when cornering.

 

I don't if this helps at all

 

Cary

[Vintage-TechZ]

I think a lot of the difference has to do with how you use the car' date=' tire type, weight, etc. In my case the car is an autoxer and I use really short 13 inch tires (20s or 22s). It may be that my RC isn't that much different than other cars on a lot taller tires. For the most part I have my lower control arms near level. I also make sure that the inner lower control points (TC rod and LCA) are level too. If not you'll introduct anti geometry and this will induce understeer, or at least that is what I have experimentally concluded. I also use spring rates very close to the corner weight supported. And I run the front with less than a 0.25 inches of droop and the rear around 1.25 to 1.5 inches of droop. I don't think you'd want to copy any of this for a street car.

 

For softer rates you'll need a some anlge down towards the wheel. I think the sweet spot is close to level when cornering.

 

I don't if this helps at all

 

Cary[/quote']

 

Actually it helps alot.........I'll better understand a RACE point and and know to stay away from those angles (at least on this car,for street)

I measured the total tire and wheel height last nite, comes to 25 1/4 tall on a 17x8 front,17x9 rear.

The bumpsteer spacer sets I have are 1.25,1.0,.75.

Sounds like I'll want to use the taller 1.25 spacers with a longer grade 8 bolt ? I made those spacers years ago and never used that set. Glad I kept them now !

I didn't section my strut tubes,simply did the perch removal and shaped the old weld so that the GC thred sleeves would slip onto the weld tightly ( the tube has a inside relief cut into it)

I had new Koni adjustables already for an unmodified strut tube length. The GC's came with 8" springs and the rates are 250lb and 275lb.

SO I hope I won't be TOO stiff........I need to corner weigh the car and KNOW where I'm going with this.

If its a street/touring turbo car, wouldnt I want the spring rates lower than the corner weights ? I don't have a roll bar in the car, and the the only chasis aids are full belly pan rails , front and rear strut tower braces.

I DO have intentions of putting a good roll bar in however........just haven't had time to build the final design before mock up.

.............Vinny 8)

[johnc]

I didn't section my strut tubes,simply did the perch removal and shaped the old weld so that the GC thred sleeves would slip onto the weld tightly ( the tube has a inside relief cut into it)

I had new Koni adjustables already for an unmodified strut tube length. The GC's came with 8" springs and the rates are 250lb and 275lb.

 

Before doing anything with bumpsteer spacers, assemble the suspension, install your wheels and tires and set the car on the ground. Then set the ride height where you want it keeping in mind that for a street car you should see at least 4" of shock shaft between the top of the strut tube and the bumpstop.

 

Once you've got that all set, then look at the front LCA angles. If they are level or pointing up then install the shortest bumpsteer spacer that will make the front LCAs point down.

 

But, as Jon said, relocating the inner mounting point for the front LCAs is a better solution.

[Vintage-TechZ]

Thanks John,

Sage advice and I'll definatly try this........thats not too tough to check out and reinstall as needed.

I DO like the idea of changing the LCA mounting hole. Did I read right that it was ovaled up about 3/4 " ? I would assume some type of ecentric bushing or a welded pre-drilled insert would work once the proper spot was determined?

I'm invisioning a plasma cutter being used here.Since I'm going to use an adjustable LCA and T/C rod combo, I'd be smart to get the stock position correct first, wouldn't you agree ?

I also wanted to ask if this would now be something that I should build the tools to do my own alignments too ? Or would perhaps a few interviews with alignment guru's actually enjoy correctly tweaking this system in ?lol.

.............Vinny 8)

[johnc]

Did I read right that it was ovaled up about 3/4 " ? I would assume some type of ecentric bushing or a welded pre-drilled insert would work once the proper spot was determined?

 

No! Drill a nice round hole from 3/4 to 7/8" above the existing hole and then weld up the old hole. Make absolutely sure the holes in the front and back of the crossmember are parallel. Then weld large reinforcing washers around the new holes.

 

I'm invisioning a plasma cutter being used here.Since I'm going to use an adjustable LCA and T/C rod combo, I'd be smart to get the stock position correct first, wouldn't you agree ?

 

No! Use a drill press. And yes, make sure everything is correct before removing the crossmember and drilling the holes.

 

I also wanted to ask if this would now be something that I should build the tools to do my own alignments too ?

 

I think you can buy everything you need.

 

Camber gauge

Toe plates

Tape measures

Fishing line

Jackstands

Notepad

Pen

[Forrest]

No! Drill a nice round hole from 3/4 to 7/8" above the existing hole and then weld up the old hole. Make absolutely sure the holes in the front and back of the crossmember are parallel. Then weld large reinforcing washers around the new holes.

 

John, I noticed JTR recommends moving the pivot point out a little as well for some negative camber gain. I have some bolt in camber plates installed but from what I've been reading, without adjustable front control arms, the range of adjustment will be at least 50% in the positive side (which I'd probably never use.) I was thinking this might be a good idea to increase my practical range, but is there any negative to moving the pivot point outward?

[johnc]

John, I noticed JTR recommends moving the pivot point out a little as well for some negative camber gain.

 

That's a common mod but it also affects bumpsteer so you'll need to determine how much affect it has. The inner LCA relocation is done to correct bumpsteer so focus on that. Use camber plates to gain camber.

 

I have some bolt in camber plates installed but from what I've been reading, without adjustable front control arms, the range of adjustment will be at least 50% in the positive side (which I'd probably never use.)

 

That would depend on how the camber plates are installed. Your adjustment rage is perfect if you can get from 0 to -3.5. I would tailor the installation to achieve that.

[JMortensen]

On the GC plates you only get about 2 degrees of adjustment and I seem to remember mine working from about 0º to -2º, maybe -2.25º. I used longer control arms to get the camber where I wanted.

 

Also on the front crossmember I have no idea how you would figure the bumpsteer then drill the hole in the correct spot. I slotted the hole and adjusted until the bumpsteer was 0. Drilling the hole first and welding up the old hole might be preferred, but I don't think it's practical. I rationalized my slotted hole by thinking about all of the Porsches I worked on that had a rear camber and toe slot on the rear semi-trailing arm, and by thinking of all the 510's I've been on the track with that have slotted rear crossmembers. Ran it that way for years, periodically checked to make sure everything was tight, nothing had moved, so far so good...

[Pop N Wood]

Couldn't you slot the hole to find the correct pivot point, then weld in the washer?

[JMortensen]

Yes you could. I didn't since I had planned on using a bump steer kit on the tie rod end. I want to raise the pivot farther because I only went up about 1/2" when the bumpsteer was gone and I'd like the roll center a bit higher than that.

[tube80z]

Also on the front crossmember I have no idea how you would figure the bumpsteer then drill the hole in the correct spot. I slotted the hole and adjusted until the bumpsteer was 0. Drilling the hole first and welding up the old hole might be preferred, but I don't think it's practical. I rationalized my slotted hole by thinking about all of the Porsches I worked on that had a rear camber and toe slot on the rear semi-trailing arm, and by thinking of all the 510's I've been on the track with that have slotted rear crossmembers. Ran it that way for years, periodically checked to make sure everything was tight, nothing had moved, so far so good...

 

When I did mine I made a piece that went on the rack end (inner tie-rod removed) and then I dropped a transfer punch through a hole in this piece to mark the crossmember. Seemed to work for me but you need to make some bits.

 

For Jon's setup it sounds like a u-bracket that you could slide shims under would be the ticket. This would allow for precise adjustement and it would be mechanically solid. I've not had much luck with the slotted adjustments I've played with in the past.

 

And to get really carried away with this topic why not make a new crossmember that allows the inner pivot points and steering to be moved as a unit. This would let you play with RC positions and camber change without messing up bumpsteer. Then you'd need to have the TC points adjustable and you could adjust anti-dive.

 

Cary

[johnc]

And to get really carried away with this topic why not make a new crossmember that allows the inner pivot points and steering to be moved as a unit. This would let you play with RC positions and camber change without messing up bumpsteer. Then you'd need to have the TC points adjustable and you could adjust anti-dive.

 

Yeah, you could. I even researched all of that and had a design drawn up for installation in the "Science Experiment." But, the car was fast enough without all of the extra work. Sometimes good enough is good enough.

 

Plus, in most of the racing classes where you would really benefit from this kind of engineering (ITS, EP), a modified crossmember is not allowed.

[74_5.0L_Z]

I have been toying with the idea of building a front crossmember as described by TUBE80Z, and toying with anti-dive. When I relocated my LCA pivots up 3/4" and out 1/4", I also relocated the T/C pivots by the same amount. Unfortunately, I am near the limit for relocating the T/C upward without interfering with the frame.

 

I wish that I had designed my front frame differently. If I were to do it again, I would have the frame rails below the LCA pivot until just forward of the crossmember. But alas....

[JMortensen]

My thought on my new one I'm going to do is to figure out how high I'd like to move the pivot (thinking 1") then drill the new hole and weld the old as John had mentioned. Problem then is that you have to adjust the bumpsteer at the outer end. I was also thinking of moving the TC hole up 1" for the anti-dive as mentioned.

 

Cary your idea of moving the pivot and the rack together is a great one. I'm just not that good at fabrication yet...

[tube80z]

My thought on my new one I'm going to do is to figure out how high I'd like to move the pivot (thinking 1") then drill the new hole and weld the old as John had mentioned. Problem then is that you have to adjust the bumpsteer at the outer end. I was also thinking of moving the TC hole up 1" for the anti-dive as mentioned.

 

I have an easier solution for you (well maybe). If you're using heims on the inner ends of your control arms you can turn them to be up/down orientation instead of front/back. The bolt can come down from the top. You'll need to make the top flat and create a tab on the bottom of the crossmember to make this double shear. You get an additional bonus in that this is a lower friction way to mount the heim (also do the TC the same way). Just mind the total travel so you don't hit the bearing race.

 

This would allow you to adjust the up and down a small amount for different RCs. I think if you move the inner pivot too muh you may need to move the inner tie-rod up or down as well. When you measure bumpsteer it will tell you what end needs to go up or down from the curve.

 

Cary

[JMortensen]

Hmm. That is a very interesting idea. I don't have any droop limiting device. I wonder if I could get enough travel out of the rod end to keep from hitting the race...

 

Why would you need to move the inner tie rod to adjust the bumpsteer? I thought as long as they were parallel there was no bumpsteer. Assuming you could adjust at the steer knuckle I'd think it should be fine. I get how you did yours by basically matching the height of the control arm pivot to the height of the inner tie rod, but I don't think that's the only way to do it.

 

I hope Vinny doesn't mind us taking over his thread. I guess we're still on topic...