Front Suspension Crutch 84-87

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Re: Front Suspension Crutch 84-87

Post by neophile_17 »

The first race of the year is the 13-14 of August. We've been racing it since 2010 but up until now I've focused on other areas of the car. The only thing that I'll be able to implement before the August race is increased front camber. Changes don't happen very fast when the car is stored 2 hours away and everyone has other projects. But everything gets thought out a little harder.

I'd like to keep the spring rate on the lower side. Both of the tracks we're close to, NHMS and Thompson, have lots of transitions in both pavement type and track angle. My impression is that for these conditions stiffer springs will make it more of a handful. We also get the hand on the fender test during tech. They have almost moved us up a class for stiff springs on several occasions already. It's going to happen eventually but I'd like to get another race or two in before that happens.

So I guess what I want to do is fix the geometry so I can run a softer spring and as it compresses I get more camber not less. My initial plan to accomplish this was to lower the upper mount but if I can swap in a taller spindle, it could do the same thing.
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Re: Front Suspension Crutch 84-87

Post by Series8217 »

neophile_17 wrote: I'd like to keep the spring rate on the lower side. Both of the tracks we're close to, NHMS and Thompson, have lots of transitions in both pavement type and track angle. My impression is that for these conditions stiffer springs will make it more of a handful.
I don't think you realize how soft your suspension is... Your assumption that "stiffer springs are worse for these tracks" is incorrect. Your front ride frequency is in the range of luxury SUV / economy car suspension frequencies.

Common ride frequencies (src):

Code: Select all

                              Suspension Ride Frequency
Vehicle                     Front (Hz)        Rear (Hz)
99 Volvo V70 XC                1.12              1.32
2001 MB E320 4-Matic           1.13              1.29
Jeep KJ Liberty                1.14              1.46
97 Chrysler Town & Country     1.15              1.34
Chrysler Pacifica              1.16              1.23
02 Jeep Grand Cherokee         1.33              1.40
2000 VW Golf                   1.21              1.43
2000 Dodge Neon                1.31              1.67
99 Audi A6 Quattro             1.24              1.48
95 BMW M3                      1.26              1.48
Since you don't know how much your car weighs, I calculated some representative suspension frequencies for you based on the assumption that its 2600 lbs with a 45%F / 55%R weight distribution, and motion ratios of 0.6 (front) and 0.86 (rear), using your given numbers of 200 lb/in for front springs and 250 lb/in for the rear. With these parameters, your front suspension frequency is 1.17 Hz and your rear is 1.64 Hz. If your car weighs more than that or your motion ratios are less, the frequencies are lower.

Race Car Vehicle Dynamics (Milliken and Milliken) suggests "1.6 Hz to 2.0 Hz (with front higher)" for non-aero based racing sports sedans. Assuming the numbers I used for the calculations are accurate, your rear frequency is in that range, but your front is way too soft. This will lead to some handling problems over bumps and surface transitions. It's OK to have the rear higher than the front but aim for ~0.2 Hz higher, not 0.5 Hz.

Have you driven a properly damped suspension with a frequency of around 1.8 to 2 Hz? Handling on rough surfaces is not a problem. My Fiero is set up with 1.75 Hz front and 2 Hz rear. It performs very well on rough tracks with big curbs (almost jumps) and on rough dirt rallycross courses.

On rougher tracks and with lower relative ride frequencies, having the rear frequency higher than the front is more important. This allows the rear to "catch up" to the front so less body pitch is induced going over bumps. The higher the ride frequency and the smoother the track, the less important it is to have the rear frequency higher than the front. At some point it doesn't matter, and some race cars with high suspension frequencies will have the front frequency higher than the rear (as suggested in RCVD).

I suggest the following approach to getting your setup in the ballpark:
1) Get corner weights in race-ready condition with the driver in the car
2) Select spring rates to get your front in the range of 1.6 Hz
3) Adjust static camber until tire wear is optimal

If the car feels too bouncy, use stiffer shocks and struts. You could look into a revalving service, if anyone offers it for the shocks that you have. It's easy to get good front shocks for the Fiero (you can get custom-valved Bilsteins for less than $200 per corner), but the rear is tricky. Koni reds seem to be the best way to go right now. Run them at or near full stiff, or have them revalved.

So I guess what I want to do is fix the geometry so I can run a softer spring and as it compresses I get more camber not less.
What is going to happen to your low static camber while you're braking with those soft springs and high camber gain?
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Re: Front Suspension Crutch 84-87

Post by neophile_17 »

It was a very hot and wet weekend to be racing but at least it was fun. We did have an axle seal go on Saturday. I had intended to replace it Friday night but with the rain and the heat my teammates preferred to go home to the AC instead. It did make the rest of the weekend a little more relaxing. There is also something not right about the brakes. I think there is probably a little air in the rear line but I also don't like the blazer MC. The new setup will use the Fiero MC and C4 brake calipers.

I managed to get -2 for camber all around. I also maxed out the caster adjustment which got us 5, and 7. We did have problems getting the adjustable upper ball joint to bite the control arm. We'll probably tack it in or set up a cam-head bolt to keep it in position. Tire wear was much better but we had problems with heating the rears up so wider is in the cards.

The car was heavier than I had hoped. All numbers are with a 165lb driver, full tank of fuel, and 40lbs of ice/water where a passenger would be.
Front Left 650lbs
Front Right 540lbs
Rear Left 786lbs
Rear Right 726lbs
Total 2702lbs

I'll be making up some shims so we can get the corners a little closer. I'm also thinking some speed holes are in order. I'll probably target the area in the sail panel first since the cage should take the load there.

I'll probably go for higher rate springs up front. I'm going to start with 4th gen Camaro springs which are a little long and 290lbs/in. I'll cut them until we get a sane ride height. Back of the envelope calculations indicate this should give me 325 - 400 lbs/in.

I'm also going forward with the early C4 spindle swap. It looks promising at this point.
What is going to happen to your low static camber while you're braking with those soft springs and high camber gain?
My impression is that camber will change but that braking causes less compression than turning. I'm also looking at Will's anti-dive idea.

I have a question about this. As best as I can tell the gains come because the front mounting point of the LCA is moved down. Was the tilt just to minimize binding as the LCA rotates?

Thanks guys!
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Re: Front Suspension Crutch 84-87

Post by The Dark Side of Will »

neophile_17 wrote:I'm also looking at Will's anti-dive idea.

I have a question about this. As best as I can tell the gains come because the front mounting point of the LCA is moved down. Was the tilt just to minimize binding as the LCA rotates?
The tilt blocks rotate the entire suspension around the *rear* inner pivot of the lower control arms.
The UCA and LCA inner pivots are parallel in side view. My anti-dive mod preserves that. My mod just rotates the side view axes such that they are horizontal and the instant center is at infinity. As such it's only slightly lower than the vehicle center of mass (~19" high), so the dive moment is tiny and the car is essentially dive neutral.
The height of the wedges should be 1" at the axle centerline. For me, using 1/4" backing plates and spherical washers, this resulted in zero shim thickness at the rear lower crossmember bolt hole on either side (IOW, just two backing plates and a spherical washer, whereas the other three attachments on each side have two backing plates, a spherical washer and shims of varying thickness.

As delivered by GM, the axes are parallel and the instant center is at infinity, *BUT* because the axes angle down at the rear, the instant center is several degrees down, which contributes to pro-dive geometry.
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Re: Front Suspension Crutch 84-87

Post by mender »

I have sufficient camber gain with my stock '87 front end. I slotted the upper control arms to make the upper ball joints movable and can get about -2.5 camber but usually run -1.8 to -2.0 with 225/45/17 front tires on 17 x 8 rims. Dive with cut stock springs was a major issue, causing excessive negative camber and bumpsteer under heavy braking.

I slotted the front lower control arm mounts about 1/2" each in the appropriate directions to tilt just the lower control arm, took very little time and seems to be working well. Before the mod the tire would tuck up into the wheel well so far that it wore through the plastic inner liner and was rubbing on the steel rail underneath. Braking suffered as did tire life. After the mod the nose drops about half as much as before and braking is much better even with front spring rates of only 200# (checked) because of the more constant camber and toe.

Here's a video that shows the amount of dive after the mod; at 3:30, I appear on the right to pass for the lead.
https://www.youtube.com/watch?v=lNZFrgtm5Fo

These are the first few laps of the race and I'm being cautious under braking, still setting up the brake bias but the car was much better under braking. Later the video I'm leading with the camera car in second and the white BMW E36 I'm running with in third. My rental driver takes over and is getting used to the car at the end of the video but he picked up the pace and we were running nicely in first still when the right rear tire was gored by a huge bolt. Rears are 255/40/17s on 17 x 9 rims.
Last edited by mender on Mon Sep 05, 2016 2:22 pm, edited 5 times in total.
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Re: Front Suspension Crutch 84-87

Post by mender »

neophile_17 wrote:It was a very hot and wet weekend to be racing but at least it was fun. We did have an axle seal go on Saturday. I had intended to replace it Friday night but with the rain and the heat my teammates preferred to go home to the AC instead. It did make the rest of the weekend a little more relaxing. There is also something not right about the brakes. I think there is probably a little air in the rear line but I also don't like the blazer MC. The new setup will use the Fiero MC and C4 brake calipers.

I managed to get -2 for camber all around. I also maxed out the caster adjustment which got us 5, and 7. We did have problems getting the adjustable upper ball joint to bite the control arm. We'll probably tack it in or set up a cam-head bolt to keep it in position. Tire wear was much better but we had problems with heating the rears up so wider is in the cards.

The car was heavier than I had hoped. All numbers are with a 165lb driver, full tank of fuel, and 40lbs of ice/water where a passenger would be.
Front Left 650lbs
Front Right 540lbs
Rear Left 786lbs
Rear Right 726lbs
Total 2702lbs

I'll be making up some shims so we can get the corners a little closer.
I had trouble with the C4 calipers and Blazer MC initially, checked the line pressures at the calipers and was getting no pressure at the front. Took apart the master (new) and there was a spring at the end of the bore that was out of place and blocking the piston; fixed that and all was good. I'm using the stock booster and the pedal effort is almost too light even with the bigger MC; using the smaller Fiero MC would make that worse. Replaced the stock proportioning valve with an adjustable one.

I tack weld the ball joints after setting the camber. Rear tire wear is about twice as much as the fronts with 225/255 split, probably partly driving style and set-up (a little loose for most drivers). Will likely increase that to 215/275 for next year.

Your corner weights are actually quite close, only out by 16 lbs from what each corner should carry. By your weight splits your corner weights should be
LF = 632 lbs
RF = 557 lbs
LR = 802 lbs
RR = 708 lbs.
Cross = 49.1%

Just need to put in a little crossweight (wedge) and you're good.
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Re: Front Suspension Crutch 84-87

Post by The Dark Side of Will »

mender wrote: I slotted the front lower control arm mounts about 1/2" each in the appropriate directions to tilt just the lower control arm, took very little time and seems to be working well. Before the mod the tire would tuck up into the wheel well so far that it wore through the plastic inner liner and was rubbing on the steel rail underneath. Braking suffered as did tire life. After the mod the nose drops about half as much as before and braking is much better even with front spring rates of only 200# (checked) because of the more constant camber and toe.
So you slotted the mounting holes down 1/2" and that helped? I can see geometrically that it would... I guess you haven't had bump steer related problems? Did you take before and after bump steer curves?
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Re: Front Suspension Crutch 84-87

Post by mender »

Front holes slotted down, rear holes slotted up. I have the before bumpsteer, haven't bothered with the after because it needed serious correction in the before. 2001 TA power rack, great feel and quick ratio, nasty bumpsteer. Didn't check as I was mounting it but essentially followed what others appear to have done. I know better but didn't.
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Re: Front Suspension Crutch 84-87

Post by neophile_17 »

I have sufficient camber gain with my stock '87 front end. I slotted the upper control arms to make the upper ball joints movable and can get about -2.5 camber but usually run -1.8 to -2.0 with 225/45/17 front tires on 17 x 8 rims. Dive with cut stock springs was a major issue, causing excessive negative camber and bumpsteer under heavy braking.

I slotted the front lower control arm mounts about 1/2" each in the appropriate directions to tilt just the lower control arm, took very little time and seems to be working well. Before the mod the tire would tuck up into the wheel well so far that it wore through the plastic inner liner and was rubbing on the steel rail underneath. Braking suffered as did tire life. After the mod the nose drops about half as much as before and braking is much better even with front spring rates of only 200# (checked) because of the more constant camber and toe.
Did you change the control arm mounting points or the knuckle height. The angles of the 84-87 front control arms cause the camber to go positive (bad) when compressed. I can see how anti-dive would definitely be a positive thing and I'm planning to implement that. I was wondering of I could get enough anti-dive by slotting the mount points and welding a washer in to ensure they stay in the new position. I'll probably do a combination of adding a wedge and slotting the arms. The extra drop from the wedge will help keep C4 shocks from bottoming out.
I had trouble with the C4 calipers and Blazer MC initially, checked the line pressures at the calipers and was getting no pressure at the front. Took apart the master (new) and there was a spring at the end of the bore that was out of place and blocking the piston; fixed that and all was good. I'm using the stock booster and the pedal effort is almost too light even with the bigger MC; using the smaller Fiero MC would make that worse. Replaced the stock proportioning valve with an adjustable one.
From what I could find the C4 master it has 7/8" bore. The Fiero master has a 24mm bore which is larger. It may not be perfect but it'll be a lot better than what I have now. I have even wear but 3 distinct brake behaviors based on how fast you hit the pedal. My best guess is that this is a result of the residual pressure valve intended for drum brakes:

Condition 1: Slow and easy application of brakes - even stopping with no lock up
Condition 2: Medium quick application - relatively even F to R but back end dances and robs confidence
Condition 3: Quick on the brakes - fronts lock immediately with little or no rear

I'll do an autopsy of the blazer master cylinder afterwards.
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Re: Front Suspension Crutch 84-87

Post by The Dark Side of Will »

mender wrote:Front holes slotted down, rear holes slotted up. I have the before bumpsteer, haven't bothered with the after because it needed serious correction in the before. 2001 TA power rack, great feel and quick ratio, nasty bumpsteer. Didn't check as I was mounting it but essentially followed what others appear to have done. I know better but didn't.
Lol... "I know better, but I just didn't give a $@#%" :-D
It's good to get confirmation that what we think happens with regard to changes in bump steer actually happens.
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Re: Front Suspension Crutch 84-87

Post by mender »

More of "I know it's out and the new rack is in the same place", but ...
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Re: Front Suspension Crutch 84-87

Post by mender »

neophile_17 wrote: Did you change the control arm mounting points or the knuckle height. The angles of the 84-87 front control arms cause the camber to go positive (bad) when compressed. I can see how anti-dive would definitely be a positive thing and I'm planning to implement that. I was wondering of I could get enough anti-dive by slotting the mount points and welding a washer in to ensure they stay in the new position. I'll probably do a combination of adding a wedge and slotting the arms. The extra drop from the wedge will help keep C4 shocks from bottoming out.
Interesting; mine has a negative camber curve and the only thing I've done other than the slotted upper ball joints for camber adjustment is lower it. It cambers hard in the last bit of travel as the tires tuck up into the wheel wells. If you looked at the video you can see that my tires are about even with the fender lip. Before the anti-dive mod the top of the tire would move in enough to miss the lip and wear the inner fender liner.

I moved the rear lower control arm pivot points as well to add anti-squat and a better camber curve before fixing the rear bumpsteer. I also moved the upper strut mounting points in to angle the strut to compensate for the lowering (more camber gain) and for tire clearance. Front and rear camber curves are about the same, I gain slightly more camber than I lose in roll and I usually set static at around -2 degrees all around. It stays pretty flat anyway, I run the Herb Adams sway bars. A little loose for some of my drivers but I can steer it with the steering wheel and/or the pedals.
neophile_17 wrote: From what I could find the C4 master it has 7/8" bore. The Fiero master has a 24mm bore which is larger. It may not be perfect but it'll be a lot better than what I have now. I have even wear but 3 distinct brake behaviors based on how fast you hit the pedal. My best guess is that this is a result of the residual pressure valve intended for drum brakes:

Condition 1: Slow and easy application of brakes - even stopping with no lock up
Condition 2: Medium quick application - relatively even F to R but back end dances and robs confidence
Condition 3: Quick on the brakes - fronts lock immediately with little or no rear

I'll do an autopsy of the blazer master cylinder afterwards.
Do you still have the stock proportioning valve?

Mine was more related to the decel rate, too much and the nose dropped, the geometry went nuts and the front end washed out. The anti-dive made a big difference for me, and I'll likely add some more before sorting out the bumpsteer.

With softer springs you have to make sure you get the weight transfer (squeeze) before getting to max braking, so hitting the brakes hard will almost always result in front lock-up.
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Re: Front Suspension Crutch 84-87

Post by neophile_17 »

Interesting; mine has a negative camber curve and the only thing I've done other than the slotted upper ball joints for camber adjustment is lower it. It cambers hard in the last bit of travel as the tires tuck up into the wheel wells. If you looked at the video you can see that my tires are about even with the fender lip. Before the anti-dive mod the top of the tire would move in enough to miss the lip and wear the inner fender liner.
The camber will go negative again at the end of travel as the upper control arm moves past horizontal and brings the top in again. I would think the go positive then back to negative camber change would be unnerving. Maybe you cut enough off the spring to get past the bad part of the curve?
Do you still have the stock proportioning valve?
I removed the stock valve and put in an aftermarket valve to the rear only.

I've also accepted the inevitable due to time and $. I won't not be playing with the C4 spindles until after the October race. But I'm going for the anti-dive, corvette shocks, and if there's time springs.
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Re: Front Suspension Crutch 84-87

Post by mender »

As mentioned, I lowered mine and am in the negative part of the curve. Next time I have it on the ground I'll measure from the hub centre to the fender lip.
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Re: Front Suspension Crutch 84-87

Post by neophile_17 »

I took the front end apart last weekend to start on the anti-dive modifications. As part of this the shocks are out and being replaced anyway. What I discovered yesterday before they went in the spares pile is that the old shocks are different. The compressed length differs by a couple inches and they have apparently different damping. This seemed very odd to me but I got them lightly used from another racer. I just looked in the KYB catalog and there is no other shock with a similar extended length/ends but a different compressed length. Did KYB change the shock design at some point? I guess it doesn't matter going forward but it seems very odd.
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Re: Front Suspension Crutch 84-87

Post by neophile_17 »

The latest round of modifications produced a couple surprises. Not exactly out of the ordinary but it took me a couple days to figure out what happened. First off the changes:
I implemented something close to Will's anti-dive modification along with slotting the mounting points a little, and cutting half a coil out of the spring to compensate for the height change. The suspension geometry changed for the better to a larger extent than the 1:1 relationship I was expecting. Mender's experience absolutely predicts this but I like models that follow reality. With both our experiences matching It was obvious that I was missing something. Since the upper control arm is much shorter than the lower and the ride height was maintained the angle change of the upper control arm was more drastic than I expected. And that is a good thing! I may not go any further depending on how this race goes.

There is a one thing that I would like to point out to anyone thinking about this. This change pushes the knuckles out enough that mine had dangerously little thread engagement with the stock inner tie rods! I bought EV192 inner tie rods and they are more than adequate now. Please don't perform the anti-dive mod without checking that you have enough tie rod thread engagement. This could go very badly!

Good luck! Have fun!

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Re: Front Suspension Crutch 84-87

Post by The Dark Side of Will »

neophile_17 wrote: There is a one thing that I would like to point out to anyone thinking about this. This change pushes the knuckles out enough that mine had dangerously little thread engagement with the stock inner tie rods! I bought EV192 inner tie rods and they are more than adequate now. Please don't perform the anti-dive mod without checking that you have enough tie rod thread engagement. This could go very badly!
That's a very interesting observation. Are you sure you had the right ones in there to begin with?

My way of making the mod does not change the relationship of the rack, control arms or knuckles... so no change of inner tie rod ends should be required. I left the rear pivots of the lower arms exactly where they are stock. I slotted the bolt holes *slightly*, the front flange I slotted down and the rear flange up in order to make sure that the center of the bushing stayed where it was. The result was an almost total elimination of brake dive with no other geometric impacts. I have 2" lowering knuckles to overcompensate for the 1" of ride height increase. I have stock front springs (for now).
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Re: Front Suspension Crutch 84-87

Post by mender »

Found my set-up notes, my front end height is 27" almost exactly to the highest part of the fender lip with 225/45/17 tires (25" tall).
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Re: Front Suspension Crutch 84-87

Post by neophile_17 »

That's a very interesting observation. Are you sure you had the right ones in there to begin with?
I have a little more information after doing a follow-up alignment. Because I cut the springs to maintain ride height then the spindles got pushed out a little. This made the thread engagement insufficient but just barely. I feel uncomfortable if the length of thread engagement is less than the thread diameter. This is a rule of thumb I've grown to trust. It is probably a little conservative but the cost of failure is pretty high. I'm 90% sure that the tie rods I had were the originals provided by GM. Aftermarket replacements tend to vary widely and could make this worse. It is definitely worth checking if you're making a change like this.

I wish I could say I got to put the suspension through a full test this weekend but it just didn't work out that way. There was very limited time where the track was dry and we weren't handicapped by other factors. So far the reaction is very positive but the test sample is small. Hopefully I'll have more conclusive results next season.

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