New project-reducing bumpsteer

[RebekahsZ]

My car wanted to steer off the track on braking above 170mph at the Ohio Mile last year. It was a white knuckle experience for me. The direction that it went seemed to alternate with each application of the brakes, so I'm blaming it on bumpsteer rather than a problem with my brakes. So, it is time to try to reduce bumpsteer. With the same set up that I ran (my autocross alignment-lol: 3.25 negative camber, 4.5 caster, 1/8" toe in, 225# coilovers, stock LCA pick ups), I got some baseline toe change measurements as a starting point by using toe plates and an air-powered bridge jack to lift the car from the point where the front tires were just touching the floor to the point where the airdam was on the floor (by running the coilovers seats all the way down). I am running 1st generation TTT LCAs, which I think are longer than stock LCAs. Ride height was measured from the top of the fender arch to the floor. Alignment plates were used. When coming down from a ride height of 26" to 24", the total toe in goes from 3/4" toe in to 1/8" toe in (in this range, the car toes OUT as expected). Then from 24" (my racing ride height) to 22" (low enough for the airdam to scrape the ground-which it does on the standing mile), the toe in goes from 1/8" to 3/8" toe in (in this range of travel, the car toes IN)! This was a big surprise-I expected toe out to continue as the nose dropped.

Edited September 2, 2016 by RebekahsZ

[RebekahsZ]

Over the winter, I slotted my front crossmember. I am waiting on AZC steering knuckles to allow spacing on the tie-rod side of things as well, but the last batch he had made did not pass QC, so that modification will be delayed until after the race. So, my only real options that I see are: adjust LCA pick up height, reduce camber to as close to zero as possible, maximize caster, and stiffen up the spring rate to reduce ride height changes. So, tomorrow I hope to reduce camber and start playing with the LCA pick up height. I did install a chute over the winter, so that may help with getting the car slowed a good bit before applying the brakes, IF the chute mount is well placed relative to the car's CG (if not, that could alter my attitude too).

Edited September 2, 2016 by RebekahsZ

[heavy85]

Rear toe in helps stability under braking. Just drill out the stock knuckles and use 5/8 rod end and spacers - can get bump steer well under control with this. I found the same thing measuring where it starts toeing one way then you reach a ride height where it switches and then toes the other way.

[JMortensen]

A few observations:

1. That splitter should generate some pretty good downforce. Your front springs are very soft in terms of controlling any significant amount of downforce.

2. Downforce increases exponentially with speed; the faster you go, the more you need to control that front end so that its attitude with the ground doesn't deviate and change the amount of force you get, literally from second to second. Flat splitters are pitch sensitive.

3. Lack of rear aero. I would think that the last thing you want is hundreds of lbs of downforce in the front and hundreds of lbs of lift in the rear, but I expect that might be what you have. Looking at the wind tunnel results, they had 77 lbs lift with BRE spoiler in back, and 22 lbs downforce with "MSA short spoiler" installed, although the rest of the car wasn't set up like yours, so we're really guessing at what you would have. Regardless, it doesn't look like a whole lot to me.

4. The more suspension movement you have, the more bumpsteer you will experience. You can't get rid of it altogether, but since it the amount of even minimized bumpsteer will depend on the amount of suspension movement, seems like for that particular issue you want to minimize suspension movement.

 

I like your suggested changes, minimizing camber will minimize camber thrust, and maximizing caster will tend to make it want to go straight. Minimizing bumpsteer is a no brainer. I also like Cameron's suggestion of a bit of toe in out back. What I think you're missing, just by looking at it, so more of a WAG on my part, is both rear downforce and the spring rate to handle downforce. I expect your front end makes more downforce than anything tested in the windtunnel, by a long shot. If you want aero balance without a bunch of drag, you might cut the splitter off the front to reduce front downforce. If you want it to be less pitch sensitive, you could raise the center section of the front of the splitter. In the tunnel they generated 110lbs downforce at 100 mph with what I think is a poorly designed front end on the wind tunnel testing (sorry Roddy). Since the course is likely pretty damn flat, unless you have way too much rebound damping and you're packing down along the length of the course, you can assume that you're generating in the neighborhood of 900 lbs of downforce (225 x 2 springs x 2").

Last time you went I was genuinely concerned for your safety. Anyway best of luck and I hope whatever you do it works better than last year.

[RebekahsZ]

Heavy85-care to send me a picture of your configuration and spacer set up as it sits today?  Thanks for your advice.

 

Jon-the splitter has been cut off (did it track-side last year) and the front of the trimmed airdam still hit the ground.  I think that it hit the ground due to the bumpiness of the course, not so much from down force.  I will add a picture tonight that shows the spoiler added to the back of the car.  I know lots of folks ignore advice, but I'm trying really hard to comply with you-guys' recommendations.  I'm concerned about my safety too!!

 

For both of you, the rear has 1/8" toe in.  I bet that increases under acceleration and decreases under braking.  I will check that at multiple ride heights.  I've installed o-rings on the strut rods to serve as "tell-tales" to let me know just how much actual compression I am getting.  I wish I had a way to video the wheel movement and review it in slow-motion.  More to follow.  And thank you all.

Edited September 2, 2016 by RebekahsZ

[RebekahsZ]

Ok guys. I did some experiments tonight. I took baseline measurements with -3.25 camber last night. Tonight, I removed the springs and took measures with the stock LCA pick ups, then repeated with the LCA pick ups raised an inch. The super simple graph attached tells the tale.

 

Basically, with my autocross camber setting: from &26" to& 24" (measured from front fender arch to floor) the car toes out 1/2". But from my racing static ride height of 24" to the 22" (when the airdam hits the ground), the car actually toes IN 1/8".

 

When I set the camber to zero, the car toes way in from 1/8" in to 3/4" in at static ride height of 24". When I ran the ride height from 26" to 22", this configuration produced the least change, constantly in the toe-in direction.

 

Very interestingly, raising the LCA pick up an inch made things crazy! From 26" to 24" there was 1/2" toeing in, then from 24" to 22", the car toed in an additional 1-1/2" for a total of 2-1/4"!!!

 

So, perhaps my bumpsteeringbis coming from toe IN rather than toe out? I'm open to suggestions/recommendations. Going to study where the ride height break point is for the LCA angle.

 

The bottom photo does not indicate ride height. To achieve this ride height, the LCAs look like a V! I just dropped it to check tire clearances and snapped the photo cause it looked so cool. I'm actually running at about 6.5" at the rocker pinch weld.

Edited September 9, 2016 by RebekahsZ

[74_5.0L_Z]

Do you have stock control arms?

How much caster?

Bump steer spacers?

Camber plates?

 

The stock camber curve at stock ride height starts off toeing out with bump and then at extreme bump start to toe in. When you lower the car with everything else stock, you operate closer to the hump where the curve transitions from toe out to to in on bump. The bump steer spacers help get a lowered car back in the safe toe out on bump region of the curve.

 

From the pictures, you have lowered the front substantially. Bump steer spacers will help but probably not enough. Raising the LCA pickup helps the camber curve on a lowered Z, but also moves you more toward the toe in region of the curve.

 

Your options are as follows:

1. Raise the car so that you are operating in the toe out region in bump.

2. Raise the steering rack relative to the LCA pickup points.

3. Lower the outer tie rod ends to get back in the toe out range.

 

In all of the above, you are just moving around on the same curve. To change the curve, you need to use a different length rack.

 

I wish you were close enough for me to come help you out.

 

Good luck.

[RebekahsZ]

I wish I was too!! I'm looking now at ever immersing with lowering the outer tie rod ends. I have stock knuckles. "Bumpsteer spacers" and about 4.5 degrees caster. How do you guys feel about drilling the steering knuckles for 5/8" bolts to allow me to lower the tie rod end?

[JoeK]

To install the TTT tie rod ends, you have to drill out the steering knuckles to 5/8", so at least the engineers over there think its okay.

[heavy85]

How do you guys feel about drilling the steering knuckles for 5/8" bolts to allow me to lower the tie rod end?

Been racing for 8 years that way. No issues except you have to make sure that the rod ends have enough angle articulation so they don't bottom out. Also ends us with wheel clearance issues at least with 15" diamond wheels - ended up grinding the 5/8 bolt down for clearance and shaving the 1" "bump steer spacers" down to 3/4" to get JUST enough rim clearance.

 

E-mail also sent. Feel free to post pic if you want.

[tube80z]

One thing that will help a lot is what Jon mentioned about raising spring rate.  I would start with 400 lbs/in all around and this will reduce the amount of travel you are seeing and that will help a lot to keep you from seeing as much bumpsteer.  Ideally you need to get your bumpsteer down to around a 1/16 of an inch over the range of travel.  That will make a world of difference.  

 

The last batch of AZC knuckles I saw needed a large wheel for clearance.  I can't remember if we had 15s or 16s but the wouldn't clear with the wheels and spacers we had.  We ended up using a spacer to lower the outer steering arm pickup point.  The amount will also depend on how much caster you have cranked in.  From what I recall this spacer was about 1.25 to 1.5 inches.  It had to be clearanced as Cameron mentions when running a 15 inch wheel.

 

If I still have these parts (knuckles, spacers, bolts) I'll send them to you but I'm pretty sure they were on the car when it was sold.

[JMortensen]

One thing that will help a lot is what Jon mentioned about raising spring rate.  I would start with 400 lbs/in all around and this will reduce the amount of travel you are seeing and that will help a lot to keep you from seeing as much bumpsteer.  Ideally you need to get your bumpsteer down to around a 1/16 of an inch over the range of travel.  That will make a world of difference. 

Not sure what to make of Dan's earlier comment that all you can do is change the part of the curve you're in. I've adjusted on my car twice. Once with mostly stock suspension by raising the LCA pivot and using a bumpsteer spacer, and once with custom (slightly narrower than stock) power steering by raising the rack and LCA pivot and lowering the tie rod and using a bump steer spacer. In my case I saw a much diminished amount of bumpsteer in a fairly wide range of motion.

 

I hesitate when I think that Dan is wrong and I am right, but you're indirectly backing me up here, so... ???

[clarkspeed]

All good advice here.  On the race car I had a combination of about 1/8" adjustment slot on the cross member, the big 1"+ TTT strut spacers, stock LCA, modified tie rods with 5/8" rod ends,  and I reamed out the steering knuckle to fit a Pinto style 5/8" bumpsteer adjustment bolt.  The tapers are cheap from Speedway Motors and ;you can get the pin and shim kits from any circle track store.  The pin was a little long for my 16" 4.5" backspace wheels, but a little grinding did the trick.  I made the tie rods out of tubing, 5/8 threaded tube end, and cutting the inner few inches off a stock tie rod. It is detailed on Hybrid somewhere.  On the next car I will try to do a lighter version of all that.

 

Also agree with higher spring rates for your speeds.  The car should really feel planted and solid at speed, not floaty at all. You should be able to drive it with 1 hand.

[RebekahsZ]

Thanks so much gentlemen. I'm so thankful that a friend came over to help me with the trial and error. I decided that rather than drilling out the stock knuckles, I would try to get the most out of what adjustment I have on the car while waiting for Dave at AZC to get his kit finished up. I see all the issues that come with spacers and wheel clearance issues. Lance, the photo you sent me was priceless-thank you so much. And it is such a privilege to talk to this group of mentors (we are getting scarce on this forum) and it is so nice to be talking about something other than what size wheels and tires will fit a Z!

 

What I found was this:

When raising the LCA pick up via a 1" vertical slot, it really steepened the bumpsteer curve very much more than leaving the LCA pick up in the stock location. The flattest curve that I could produce was with the stock pick up point. I do have 1" "bumpsteer spacers at the ball joint end of the TTT LCA. From a day and a half of trial and error, what finally worked the best was to raise the static ride height an inch! If I raised the car more, the curve got steep again. So, I ran thru a toe measurement sequence varying the ride height from 5.5" to 3.5" and here's what I came away with after toe was set to zero at 5" (measured from a bracket on my airdams support structure):

 

5.5" 1/8" total toe in

5.0" 0 total toe

4.5" 1/16 total in

4.0" 1/16 total in

3.5" 1/8 total in.

 

The track at Ohio has a lot of rise and fall (bumps), so with the 225# springs, the car was getting 2" of compression at the front strut. That equaled 3" of compression at the front of the airdam. I'm sure the relationships are not linear (that would be too easy!), but I'm hoping that with 450# springs I will see approx 1" of compression at the strut rod (I installed an o-ring on the strut rid to act as a tell-tale). I'm going from 250# in back to 400#, so hopefully the rear shouldn't squat (front end lift) as much during the hard-acceleration portion of the 1-mile sprint. I will take my alignment gear to the track and vary static ride height between 5" and 4.5" and see how the car feels.

 

I guess if I can't stand the high airdam, I can always add a little to the bottom! And I sure hope the chute is mounted right at CG so that it doesn't pitch the front end up or down!

 

I will try to go thru this prices again with Dave's bumpsteer kit this winter to try to do better. I will go ahead and get a bumpsteer gauge too. But for now, I have too much else to get ready on the car to chance ruining a steering knuckle, etc.

 

I will let you guys know how it turns out. Time to get the springs on it.

Edited September 3, 2016 by RebekahsZ

[JMortensen]

The flattest curve that I could produce was with the stock pick up point. I do have 1" "bumpsteer spacers at the ball joint end of the TTT LCA. From a day and a half of trial and error, what finally worked the best was to raise the static ride height an inch! If I raised the car more, the curve got steep again. So, I ran thru a toe measurement sequence varying the ride height from 5.5" to 3.5" and here's what I came away with after toe was set to zero at 5" (measured from a bracket on my airdams support structure):

 

5.5" 1/8" total toe in

5.0" 0 total toe

4.5" 1/16 total in

4.0" 1/16 total in

3.5" 1/8 total in.

I can't imagine why this would be the case. When I did mine with stockish stuff I was using a 3/4" spacer and raised the inner pivot 7/16 and that was enough to make my car stop doing a crazy emergency lane change type of maneuver over one particular bump at one track. These cars are old and modified and there will be variations from chassis to chassis, but they should be basically the same if the chassis isn't bent up. This is how Design Products is able to sell a "bumpsteered" steer knuckle for 510s, as an example.

 

The bumpsteer spacer really doesn't affect the bumpsteer, just the part of the curve you're driving in, it's going to be the angle of the LCA relative to the tie rod that will be important, and that should be fairly similar from S30 to S30, barring any front end collisions or mods.

[RebekahsZ]

I think it's gonna be bumpsteer spacers that it takes to totally flatten that curve on the chart with x's. That's the final setting I'm gonna run. I will have a report in a month.

[RebekahsZ]

When I raised the static ride height, I lost from a baseline of 4.5 caster down to 2.6 caster. Will be trying to work as much caster back in as fender clearance will allow, then bumpsteering the car again to see how much that messes things up. With a doubling of the spring rate, this thing doesn't move much! Got the rear ride height set, then set rear toe to zero from a baseline of 1/2" toe in. It is funny that with the suspension set 1" higher, the rear toe in was only 1/8", so the rear suspension is a source of bumpsteer too! I know in theory it is not, but in reality it is.

Edited September 4, 2016 by RebekahsZ

[JMortensen]

Sorry to say, but you have convinced me that your measurements are wrong with this rear toe thing. If they're that far off in the back, I'm guessing that they're similarly erroneous in the front as well.

 

Just take a step back and think geometrically about the rear bumpsteer: the inner pivots are parallel to the chassis centerline. You measured 1/2" toe in (that will wear out a new set of street tires in about a week's driving, I can say from personal experience). Let's assume you had 1/4" toe on each side. Since the inner control arm bushings are parallel and level with the chassis, the difference in length at the spindle would be the difference from taking the length of the control arm inner pivot to the front of the spindle at one angle, and the difference in length at the rear and subtracting. Then do the same at the second angle. You're saying 1" of chassis movement gave you 3/8" toe change. I don't believe this is possible. You have some rake in your car, so it wouldn't be exactly the same amount of change it would be VERY slightly different front to back, but there is no way that you're getting that much change out of 1" of change on the ride height.

 

20 something years ago I had access to a Hunter alignment rack and I tried my alignment settings at street height and race height--I used to adjust the coilovers to lower the car about 3" when I got to the track--and the rear toe change was negligible.

Edited September 4, 2016 by JMortensen

[tube80z]

With a doubling of the spring rate, this thing doesn't move much! 

 

That's the idea.  I think you'll find the car is much more stable this way.  You'll have less movement on your bumpsteer curve and when accelerating the nose won't raise so much.  And at some point you can revisit lowering the nose again.  But I wouldn't do that until you get the rest of this sorted.  And once the front is sorted then you can look at making some toe-link rear arms that will allow you to mess with rear toe much easier.  BTW, I looked for the parts from Dave's car but couldn't find anything but the drilled steering arms.  

 

Cary

[RebekahsZ]

I'm very open to the possibility that my measurements are crap. Very, and for same reasons you cite and I can't defend them. I'm sure that a set of toe plates and tape measures leaves a lot to be desired in accuracy. I see guys align super fast drag cars with tape measures, so Im not sure how perfect we truly have to be. But I'm doing the best I can. As for wearing out tires-that's what do best! I've been dead-hook, front tire-lifting launching this car for a couple of years. The rear has poly inner bushings and rubber outer bushings with early Modern Motorsport rear LCAs. Steel bushings were just too freakin' loud. I'm betting (and photos suggest) that the tires move forward in the wheel well. I think that any deflection in the bushings on a car driven this way would cause toe-in? So I wasn't surprised that the rear measured with toe-in. I also wouldn't be surprised if these LCAs aren't less rigid than the stock versions. Hoping for a test drive either tonight or in the morning.