I have the front suspension, trying to figure out the best angle for bump steer, then put it in the car I'm building where ever it needs to be for the correct roll center and then make the engine fit.

How I have it, the control arms are both pointing up, which doesn't seem anywhere near being correct to me but that's where it has the least amount of bump steer. My upper arm has almost 20* of angle.

The tie rod is at about 15* pointing up.

Trying to figure out what factory is at ride height for a better guideline. Or am I going about this wrong?

Thanks
Alex

 

Do these help any?
Factory drawings for the 1984....

 

Yes! That gives me a better idea for sure. If anybody has an angle gauge that would be nice too but these will help me a lot. That's about how I was expecting it to look. Now to figure out why my bump steer gauge was giving me those readings.

Thank you the pictures!

 

Street rod / custom car build or track car?

Do not set bump steer by changing the ride height.

Set the ride height to get the correct roll center.

If a street rod / custom car build attach the new frame section to the old frame in a higher or lower position to achieve correct stance.

Track car: When the roll center is correct the next thing is camber curve. A shorter upper a arm increases negative camber in bump (a loaded corner during a turn).

Next: Check & set toe steer. Slotted steering arms with heim joint rod ends in place of the stock outer rod ends are the answer for quick toe steer adjustments.

If a street vehicle toe steer may not be an issue.

If your road racing slot both steering arms & adjust both sides for 1/8 toe out in each direction during a turn. Suggest zero toe for straight ahead as a starting point.

Last: Remove spring & shocks.
Set the bump steer using spacers between the heim rod ends & the steering arms.

Good luck

BTW, the pic of the front suspension if accurate = a poor handling vehicle. The upper control arm is pointing down toward the outside of the vehicle. This causes the outside front tire when in a turn into positive camber = NFG.

 

Im building a streetable track car if that makes sense. Just planning autox, random track days and drag racing. Just a fun "all around" car thats a little different. Converting an 88 beretta to RWD.

I can literally place the front subframe anywhere, Im trying to place it in the best spot and make everything else fit around that. Im to the point on the build where I cant go farther till I have the front suspension set in stone as to where its going to sit. I havnt checked the camber curve yet, but that does make sense as to the upper control arm. So no matter what you think the upper control arm should be at least level or angled down then move the subframe up till the roll center is correct? I am just using factory steering knuckes/tie rods/tie rod ends. Is there something else on the market or are you talking 100% custom made?

this is the build thread if your curious
http://ls1tech.com/forums/conversion...t56-build.html

thanks!!
Alex

 

Nice custom stub, the stock illustration = NG for your application.

suggest setting it up in the following manner.

Measurements are taken from one side of the stub the other side automatically follows suite.
Work with actual measurements, you need a sheet of drywall on a table or a clean floor & welders soap stone sharpened to a point & a long strait edge to extrapolate the A-arm pivot points.
Draw a 90* angle 24" high, 4-5' long.
The vertical line is your hub face. To simplify matters this line will also be the tire center line.
The horizontal line is ground level.
Mark the vertical line 13.5" up from the ground line this is the spindle center line.
If the coil over is installed remove it.
Since the new stub is basically hanging in space suggest fabbing a small A frame to bolt to the wheel studs. This will keep the spindle at the correct height while you jack the cross-member up-down.


As a starting point block the stub up to position the lower A arm level.
Add or remove upper control arm shims for zero camber @ the hub.

Straight edge on the hub face measure in to the ball joint center lines.
The ball joint center lines are 5/8" up/down depending on joint orientation from the metal stud retainer.
Record measurements.
Measure up from the floor to the upper & lower ball joint center lines, record measurements.
Transfer the ball joint center line measurements to your drawing.
Measure lower A arm length & inner pivot height -- transfer to the drawing.
Measure upper A arm length & inner pivot height -- transfer to the drawing.

NOTE the pic of the stub you are using shows anti dive built into the upper A arm mount.
Measure the front of the A arm pivot shaft to the ground, measure the rear of the A arm pivot shaft to the ground. Split the difference = upper A arm pivot height.

Measure hub face (vertical line) to center of front cross member, mark the ground line on the drawing,
Straight edge across lower BJ C/L & inner pivot CL draw a line 5-6' long.
Straight edge across upper BJ C/L & inner pivot C/L draw a line that intersects with the lower A arm line.
This is the instantaneous center.
Draw a line from the instantaneous center to the vertical hub center line @ ground level.
Call it the RC line.

At the cross-member center mark on the ground line measure up to the RC line
This measurement is your front roll center.

Raising the cross-member raises the roll center etc.

If the extrapolated control arm lines do not intersect you have a parallelogram suspension. It will not change with ride height = a poor handling vehicle.
Remedy: The upper A arm inner mount is positioned to high on the frame it needs to be lowered or:
Remove some anti dive by lowering the front mounting hole of the upper A arm shaft.
Use taller spindles or modify your spindles to position the ball joint higher. If you modify your spindles change the KPI to 10*.
Taller upper ball joints are available they are aftermarket designed for oval track racers.
Do not know if they are available for your application, check @ Howe racing.
Tall ball joints or spindles may interfere with the wheel dependent on spindle KPI, wheel diameter & amount of positive offset.

Edit: Looking at pix of your stub IMHO the easy way to modify the upper A arm inner pivot height to achieve an instantaneous center is to remove a portion of the 2x4(?) tube that supports the upper
A arm mount.

BTW if the upper A arm mount is welded to 11ga flat stock it will rip out during hard braking entering a high speed corner. Happened to the first tube stub that I built with 11 ga. Suggest attaching the upper A arm mount to 1/4" flat stock + add gussets.

Good luck.

 

Wow that's a really good way of doing that! I never would have thought of that. I guess I know what I'm doing this weekend! I'll post up what the results are.

All the geometry is factory c4 as far as the mounting of the components goes. One of the fastest cars I've seen in autocross was a 700hp c4, stuck like glue somehow. Are you saying the stock geometry should be changed for the upper control arm? I have read that the c4 has positive camber gain right at first (and what you pointed out about the control arm proves that). Or is it a wait and see what it measures out to be?

Not seeing anything on the website, I'll have to give them a call to see if they have anything for it.

I believe it's 3/8" mount for the upper control arm. It's a pretty solid subframe! I couldn't imagine the suspension falling apart like that, talk about a scary situation!!

Thanks again!!
Alex

 

Scratch the idea of removing anti dive as stated in my previous post.
It occurred to me that stock upper control arms have the ball joint mount canted to match the frame anti dive.

I don't care how the stock Corvette suspension operates, factory builds are always compromised at some point for some reason. At this stage of your build you have the opportunity to modify what ever needs to be to get the front suspension geometry correct. Suspension modifications are very difficult after a car is built.

Draw out the suspension & lets see where everything is at. Doing so will determine what changes if any need to be made.

 

Here's a CAD model I've made

http://www.3dcontentcentral.com/part...&catalogid=171

 

Alex, I read your build thread on LS1tech....what is driving the engine so far rearward? Is it the crossmember?

I wonder if it would be better to use a CTS or F-body cross member, which locates the engine much further forward relative to the wheels, than a vette based one? You dealing w/a car that was FWD which puts the wheels physically far back in the front end -note the distance from the rear of the wheel opening, to the front of the door; not much. Then you're using a vette based front end, which puts the front of the engine, behind the centerline of the front wheels...the engine gets stuffed back into the car, which is what you're dealing with. I'd think a different front end the situates the engine further forward, relative to the front axle centerline, would be easier to work with in that chassis.

 

Alright, gonna go get my subframe and see what I can come up tonight

 

It won't let me see it on my phone. I'll have to check it out tonight when I get home.

Thanks!!

 

My issue is the engine is hitting the steering rack/subframe. I'm in way too deep to use something else, but I chose the c4 due to how well people can set the cars up for autocross, the fact I found an aftermarket subframe, and that it was a dual control arm setup with no struts to deal with. It took me awhile to realise that's what my issue was, it needs to be setup like a corvette... who knew??? It honestly never crossed my mind lol. Thinking with all the oil pans built for god knows how many swaps, I would never have an issue of any kind. I was sooo wrong lol. End of the day, it's actually better that way with the engine not hanging off the front end. If i would have realised that before I bought everything I wouldn't be going down this road for sure. Just makes the build more interesting I guess. Ideal would have been something from a ford due to the bolt pattern because the Rear is 5x114.3, but we all know ford's don't turn!! Actually couldn't find anything I wanted to use, I looked for 4 or 5 months. Then my friend pointed me to the company that builds the c4 stuff and I had some custom hubs built.

 

Maybe. I hear you about being so far in...but by the time you're done, you'll have cut out 50% of your floor pan, lost a ton of interior space, and IDK how you'll keep your HVAC in tact. W/the "engine behind the axle line" philosophy on a car that has the front wheels so far back, there is going to have to be major compromise (as you're now discovering). I don't know that you can meet your goal of "stereo, air, full interior" etc. that way.

I'd keep it in mind. Our CTS-V has the engine sitting dead center on the axle line, has SLA suspension, and it handles pretty darn good. Best of luck. It's a cool project!

 

Yeah I'm literally redesigning the inside of the car to make room. Kinda a pain in the ***! Planning on using an hvac setup for a street rod. The trans killed the heater core a long time ago lol. Gotta do what ya gotta do tho, I'm a little over 3k into the front end so far (not counting wheels), still have the konis to buy. The early c4 also has the correct track width and the ackerman angle is REALLY close to center of the rear diff (slightly forward). The ctsv is pretty wide. I'll keep in mind tho, they are VERY impressive cars.

Thanks!

 

So I drew it all out, ran out of garage to finish it. I'll figure something out tomorrow. Ended up drawing it on big sheets of paper, really should have just drawn a scale version lol. How it's setup, the upper control arms are level with the ground. Bottom control arms are slightly angled down at about 4.5*

The roll center looks like it will be about 3" from the ground like this.

What's ideal front roll center?

 

The lower A arm is apparently angled down toward the ball joint otherwise the vector lines would never intersect if the upper A arm is level & the lower A arm were angled down toward the center of the stub = no instantaneous center.

Your front stub does not need modification + you now know what you have.

3" above ground roll center is good I happen to like that #. I am using a 3" front RC on the car I am building now. There is NO ideal front roll center number it is vehicle dependent. Rear roll center height, roll couple & center of gravity height all play a roll per vehicle. RC height is somewhat adjustable with a chassis height change but it will not make a notable difference. Chassis height changes = camber curve changes which have the biggest effect on corner speed.

I would install the stub LCA level the upper A arm will change to negative camber in bump. A shorter upper A arm = faster/more camber change for the track. I use plate spacers with shorter upper A arms

Good job!!! I'm happy for you.

 

Figured it out the rest of the way, it's at 2 7/16" from the ground, so a bit lower. Not sure of that makes a difference or now. I'm assuming it won't just due to The whole setup of the car. The rear subframe I have will be a bolt in setup, I'll be able to adjust the height with spacers if i need to.

So with my upper control arms.. spc adjustable. I removed all the factory c4 control arm spacers and bolted it directly to the subframe but adjusted to compensate exactly to what stock would be. I didn't know a shorter arm made for quicker camber change. Is there an ideal # of change per inch of movement? I have dug into that yet, never eve crossed my mind, I thought it was going to be what it was going to be.

Thanks!!

 

The reason the vector lines are as long s they are with your set up is that there is only 4.5* of difference in control arm angles. No big deal it just means that camber change in bump will be minimal.

RC height is ok, above ground & centered under the car = good.

Check camber @ the static height of 13.5". Raise the spindle 1" take a camber reading to see where it goes. It should change to negative camber. For set up I like a 1* change in 1" of travel. Fine tune the camber setting at the track, read tire temps with a digital pyrometer.

You have an advantage using adjustable upper A arms. Shorten them up all the way, reset the camber with shims then take another reading raising the spindle 1". There should be an increase in negative camber.

Have the same digital level it is handy but I do not understand why it has a temperature function.

 

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