So get on with the reports on Daily Driving the thing!!!
So get on with the reports on Daily Driving the thing!!!
Sadly, I wont get to drive it until next weekend... The car is still an hour and a half away and my wife wont have an opportunity to go with me to drive it back until next weekend(12/12).
So next weekend I'll give the report.
I was able to get the car this weekend after all. I drove it 140 miles(mostly 60-75mph). The car handles okay. It still needs some work in the handling department...
There is a little bump steer in the front, taking turns under acceleration at 50+ makes the whole car act a little funny. I will recheck the if there is any toe in the rear.
Also, when accelerating hard at 40+, the whole car pulls to the left a little.
Overall it drove pretty well. No excess shaking even at 80+mph. Didn't feel any pulling while driving straight. Averaged 23mpg without the torque converter locking up(need to adjust my transmission tune).
Very cool project.
What is the rear doing under acceleration?
There is a section of I-45 in Houston where the surface is really uneven(imagine a wave with an wavelength around 10ft with a height of 1-2inches). When driving on this the whole car moved right-left in yaw. Almost like the whole car is angling left and right a little bit(did the same thing when taking turns under acceleration at 50+).
I am thinking that it may be a little bit of bump steer in the rear that I need to fix, but not positive that is the issue.
So, I moved the rear suspension through its travel and there is no bump steer. There was quite a bit of camber gain(don't have a gauge right now, but it was pretty obvious).
I did notice that my passenger front suspension has a bit of bump steer. I will get some washers to space the steering mounting point up about an inch which will eliminate it.
Still not sure why the car was shaking/wobbling in yaw while I was on the freeway...
Is there something flexing? Perhaps between the subframe and tube chassis?
How consistent are the tire pressures?
Daewoo,
I don't think anything is flexing, but i cant tell for sure. I checked all of the bolts that connect the subframe to the chassis and everything is tight. I know I don't have a diagonal brace in the back. I will add one if I cant fix it with a toe adjustment. I tried to replicate the previous experience by shaking the car side to side and cycling one side of the suspension and I couldn't see any flexing.
Warren,
I checked the tire pressure, all tires were at 35psi. I knocked them down to 32psi at 60 degrees. I'll check them again when it gets a bit warmer.
Also, I re-checked the rear toe and it looks like it is toed out a 1/4in total in the rear. I checked what the dodge vipers have(a coworker has one) and it is 1/8th in toe in per side.
Any thoughts if I should toe it in or make it toe neutral?
Toe-out will absolutely cause those symptoms. A little toe-in at the rear will help stability, assuming your roll centers are somewhat reasonable. Another thing to check: jack up the car, grab a wheel, and twist it in every direction as hard as you can. Usually if there's a problem, you'll notice by hand. Just don't knock the car off the jack stands. 
Your car is sufficiently different from most things out there that you'll want to experiment. 1/8" toe-in is probably a good place to start. I'd move in 1/4" increments to find the sweet spot, then 1/16" increments to dial it in. The sweet spot will change drastically with geometry, tire construction and size, weight distribution, ride height, camber settings, and even the front suspension settings.
Austin296 wrote: Daewoo, I don't think anything is flexing, but i cant tell for sure. I checked all of the bolts that connect the subframe to the chassis and everything is tight. I know I don't have a diagonal brace in the back. I will add one if I cant fix it with a toe adjustment. I tried to replicate the previous experience by shaking the car side to side and cycling one side of the suspension and I couldn't see any flexing. Warren, I checked the tire pressure, all tires were at 35psi. I knocked them down to 32psi at 60 degrees. I'll check them again when it gets a bit warmer. Also, I re-checked the rear toe and it looks like it is toed out a 1/4in total in the rear. I checked what the dodge vipers have(a coworker has one) and it is 1/8th in toe in per side. Any thoughts if I should toe it in or make it toe neutral?
Something I've done in the past to find suspension weirdness is to just zip tie my phone so the camera is pointed at the suspension. Then take a video while doing the things that cause weirdness.
And yes, toe out on a midengine car is probably not a good place to start.
Looking at the trailing link on the lower control arm.
Can somebody who is smarter than me describe what the motion of the wheel center is going to do as the suspension cycles past the neutral plane? I see it pulling the wheel center rearward and that is going to have an effect on the steering arm. If it does what I think, it would create a corkscrew action as you drive over a bump.
If it were me trying to sort out the chassis, I would be tempted to replace the coil-overs with a solid rod and go for a drive around a smooth lot while performing transient maneuvers. That would help separate any chassis issues from suspension geometry issues.
If the car feels solid, put the front springs on and give it a go. If that still feels good, put the rears on.
Changing that front lower arm to a wishbone instead of a trailing link.
Also aligning the LCA inner pivot with the rack pivot joint will help bump steer.
Longer arms will cause less geometry change through normal travel.
Supple springs/dampers with short arms will allow more normal deflection = more geometry change.
Tire inflation still high try 25 or even 22, maybe even 20 (not much weight in the front). (still have large tires on it?)
Warren,
Ill try that tonight.
Daewoo,
That's a good idea. I have a go pro style camera that I will mount back there before my next drive.
NOHOME,
I am not sure exactly what you mean by a corkscrew effect. I went back and dug up my front suspension numbers:
Caster: 0 Camber: -3 Toe: 1/8in in
I don't have it written down, but I believe the rear camber is either -1 or -2.
Also, not sure if it is important to note, but I don't have a front swaybar and I do have the stock riviera swaybar on the rear(I believe it is either 1.125 or 1.25in diameter).
bentwrench,
My suspension came off of a circle track race car, so the geometry is a little different from side to side. I looked at bumpsteer and I am going to move the passenger steering arm mounting point up ~1in and that removed the bumpsteer that I am currently seeing. The length of my control arms is ~8inches. I completely agree that there is a lot more geometry change with shorter arms.
Regarding the tire inflation, are you suggesting I run 22-25psi on the street or at SCCA type events? Also, would that be on all 4 tires or just the fronts?
The tires I have on the car now are 275/40R17 BFG Sport Comp2.
In reply to Austin296:
I think he is suggesting lower tire pressures. Its not a heavy car, i dont remember exactly but wasnt it like 1800lbs?, thats quite lighter than most street cars, even small ones. I don't have a tire load chart handy, and im not sure they even go down that low, but yes I think 22 front 25 rear doesn't sound unreasonable. At 32 your close to what 2200-2500lb cars run.
Between fixing your toe out, and tire pressures I think you may get rid of most of that wiggle.
java,
The car is 2200lbs. 750 front, 1450 rear. I'll knock the front down to 22 and the rear down to 26 and see how that works.
As you go over a bump, the trailing arm in your lower a-arm is going to get shorter. That is going to pull the lower ball-joint both inwards and towards the rear of the car. If the top ball joint stays in place, your upright will be tilted forward and outward. The upright will go from | to / when viewed from the rear or sidewiew as the car cycles past the neutral point both in droop and compression. Question is how will this movement affect your steering arm?
The relationship between the rack pivot points and the a-arm pivot points we covered way back on page 3 I believe.
NOHOME,
Overall it didn't seem to be that bad while driving it. I did notice there was some bumpsteer on my initial 140 mile drive.
I checked the bumpsteer in the front and the tire moves about a 1/2in inward under compression(eyeballed it while jumping up and down in the cab). I was able to get the suspension to compress about 2 inches while doing this.
I mocked up the steering arm with a spacer moving it up about an inch and it got rid of the bump steer. I just bought the longer bolt and some washers to space it up today, so I'll put it on when I get off work tonight.
I adjusted the front steering arm and eliminated bump steer. I also put about 1/8in toe in on the rear.
Took it for a test drive (50mph max) and it seemed a lot better. Wend over several large bumps and potholes and no pulling anymore.
I did uncover another problem though. If you let go of the steering wheel the car pulls to the right a bit. Any thoughts on the best way to fix that? I am not sure if it means the steering rack isnt 100% centered when the wheels are straight or if I need to change the toe on one side or what.
Pull usually means caster isn't the same side to side. Assuming there's nothing weird with your tires, that is.
I am wondering if you are usign the term bumpsteer to describe camber gain...
Bumpsteer is when the wheel turns left or right when the suspension compresses.
Camber gain is when the top of the wheel leans in or out when the compression compresses.
A pull to one side is almost always caster.
DaewooOfDeath wrote: Pull usually means caster isn't the same side to side. Assuming there's nothing weird with your tires, that is.
I'd add a boat load of caster if you can. It will add some steering effort but the wheel will snap back to zero if you let it go (good for when the back end wants to step out)
The car has quite a lot of camber gain which I could see while compressing the suspension, but I was specifically looking for side-to-side movement in the forward most section of the sidewall. There is a blue dot on the sidewall which I pointed forward and watched it for left/right movement, trying to ignore all the camber gain.
I need to borrow a camber/caster guage and recheck what the front is set to.
Took the car on another test drive today at higher speeds and with some turns. It handled much better.
No more pulling to one side when accelerating at 50+. No more shaking in yaw while turning under acceleration. Although it still does pull to the right a bit when you let go of the wheel.
I checked the tire temps with a heat gun after a 30 minute drive and the temp from the tires were within 5 degrees from outside to center to inside.
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