I just cut off my front upper a-arm mounts and I could use some help with where to place the new pick up points and some design analysis. Car will be a track car. I didn't turn up any specific details with a search, so I figured this would be a good thread to start. A racer friend just offered to let me use his suspension modeling program but I'd love to spend my time fabricating if someone here already has the info.
First issue- Roll center. Ideally, approximately where vertically should the control arm pivot axis be for a ride height of say 3.75"? My datum reference point would be the top of the old mounts.
Second- Alignment. I've read a few posts about relocating the tabs aft, but no results on how the alignment turned out. How far aft and inboard will give a good range of caster (say 2.5 to 4.5 degrees) and camber (up to ~2.5 degrees)?
That should cover the three dimensions, leaving design for discussion. I like to keep things really simple so here is my ultra simple idea:
Horizontal bolt orientation. Four rectangles cut from 3/4" x 1.5" (I just happen to have some) flat bar drilled and tapped for the retaining bolts. Possibly size up to 7/16" fine thread for thread durability. These would simply be welded to the outside of the frame after grinding the old mount welds flush. Minor camber adjustments could then be made with shims. The factory mounting holes are ~1.25" out from the frame and the corvette pivot pin tabs are nearly 1/2" thick. Turned 90 degrees this puts the mounting surface 1" outside the frame. I purchased the poly bushing kit from Phadt which includes pins with a 2mm offset which they state gives a camber change of approximately 0.8 degrees when flipped. If a 4mm move of the arm gives 0.8 degrees then a 0.25" move from the new mounts gives ~1.25 degrees more negative camber.
I could easily slot the A-arm pivot pin bolt holes to give some easy caster adjustment as well. The Pfadt pins have a lot of meat on them to allow this.
I appreciate your input and constructive contribution.
I would recommend spending some time with the computer program to plan exactly where to put the mounting points. That will save you some time in fabricating. You will need to plot where all of your suspension points are now and see where they need to be moved to get where you want to be. Do you know how high you want the roll center to be?
I would recommend spending some time with the computer program...... Do you know how high you want the roll center to be?
Short answer, no.
Road racing is a passion I just acquired. I've been a drag racer since I was 16. I spent a bunch of time sorting out my 388" nitrous 4-link '67 Camaro. It was terribly inconsistent when I bought her, would sometimes lift a front tire and go straight, sometimes smoke the tires and sometimes wig-wag all the way down the 1/4.
Last time out, ran 9.52, 9.51, 9.49, 9.52, 9.50 and carries both front wheels for 40-50 feet.
I just figured out this year that it's time to learn to turn a corner! I had made almost no progress on my kit in the last year and a half due to having taken on way too many projects. Well I just cleared out all the others and I just have the GTM project and life is good! I tore my street build down to almost nothing and I'm starting over with a race car.
So I'm open to suggestions for a good starting point for roll center. I started reading about roll center and there seems to be some wildly different views on the subject.
I'll also make an effort on the modeling once I get the program. I'm not sure my slab is flat and I know my jackstands aren't precisely even.
You can also adjust your suspension geometry with drop spindles.
LG Motorsports makes drop spindles. They say..."Lower your Corvette a full inch without changing your suspension's geometry. A must for lowered track cars."
Mark,
Don't get confused by all the stuff here. It is all on paper. Paper is fun but not reality per say. Now that you have cut the tabs off, turn the triangle part around and reconnect them. Then use the c5 upper control arm and mount it on the bottom of the brackets. It works. It is race tested and successful in handling and comfort in driving. 14 competition drivers had no issues with it. For what I know about roll center. Don't be clouded by what you think about its importance. It is only another tool to "fine tune" a suspension. These cars are not refined enough to include roll center applications other than where it currently is. If you were starting with a clean sheet of paper, then you have a great chance to include it in your initial design. We have had 5 years of track success with not really knowing where the roll center is or where it is at other than the intent of Factory Five in their chassis design.
You can also adjust your suspension geometry with drop spindles.
LG Motorsports makes drop spindles. They say..."Lower your Corvette a full inch without changing your suspension's geometry. A must for lowered track cars."
Drop spindles will not change you suspension geometry. They will just lower the car without changing the geometry.
Correct. Noother was discussing a ride height of 3.75" for track purposes.
I also seem to remember early threads, large diameter wheels are not recommended due to suspension geometry issues. Tall tires with normal ride heights change the a arm angles and roll centers.
Joel, I read your whole thread before I posted, good stuff and impressive craftsmanship indeed, nice work! I can guess you raised the pivot point up to adjust from 3/4 to 1 1/2" higher from the great pictures you posted. I'm sure you had a reason for this placement. Did you go aft as well?
Mjollnir and TX, you bring up a very important points indeed. The right height information is meaningless unless a tire diameter is given which ties in the relationship of the frame height to the spindle height. I would guess a dropped spindle could be used to fix up the angle of the lower control arm and then the upper mounted to achieve the desired roll center. I'm looking at running a 285/30/18 up front which specs at 24.8" diameter but I'm sticking with the stock corvette pieces.
Richard, thanks for the reality check. I took detailed notes on just about everything you and Jim would spit out, I deeply appreciate your knowledge, experience and willingness to share information.
I guess with all that said, all I really need is a decent starting point that will allow me to easily get into the right caster and camber numbers.
Since I do get to start these mounts with a clean slate, I might as well design in some room for adjustment that I may use someday. I could weld on 3/4" thick vertical strips of flat bar with threaded holes that span from The PDG cars' position below the original plane to Joel's position above. Unused holes to be filled with a greased set screw to preserve threads. Simple, easy to fab.
If you look at my design, the upper control arms do all the adjustments...camber, caster and roll center are all adjusted with keys and shims. I can get whatever I want, within reason.
The optimum roll center location is really a function of A LOT of variables….tire dia, tire grip, tire construction, ride height, roll stiffness, among others. I agree with Richard, the roll center adjustment is not all that critical....start somewhere close to where it is and start testing. Even if you miss the optimum spot, you can adjust other things to compensate without hurting performance too much.
__________________
Ciao,
Joel
Working ever so slowly on GTM #269, Twin Turbo SBC, Ricardo, Kit arrived April 5, 2009
Thanks Joel, I did study and I do understand your design. Elegant solution.
Unfortunately, again, I have way more time on the computer than working on the car. Time for me to just get to work, I'll take some pics when I get the mounts machined and installed.
Richard & JCH Racer, hit on very a very important point.
Roll center location in itself, his overrated.
On the PDG GTM, we lowered the upper control arm mounting points, based on JIm Haussler's contention that camber gain/recovery in roll was more important than roll center location. Results/feedback from the drivers indicated that was a positive step for front grip/drivability on track.
Remember we only run about 2.5 deg caster, so don't think that large amounts of caster fix all problems with stability on these cars.
I seem to recall feeling very alone when saying "for the most part, disregard roll center" back in the earlier threads. So glad we are now, for the most part, all in agreement.
Richard & JCH Racer, hit on very a very important point.
Roll center location in itself, his overrated.
On the PDG GTM, we lowered the upper control arm mounting points, based on JIm Haussler's contention that camber gain/recovery in roll was more important than roll center location. Results/feedback from the drivers indicated that was a positive step for front grip/drivability on track.
Remember we only run about 2.5 deg caster, so don't think that large amounts of caster fix all problems with stability on these cars.
Thanks LS and everyone. I did do some rough measuring and mock up of the front and can see the camber gain with the upper pick up points lowered. I ended up with 1"x3/4" flat bar , 2" long with two holes each, 1" apart tapped for 7/16-20 thread. The lower hole will be placed at the approximate position of the PDG car at ,mid-1" bar, and I'll have an option to move up and inch. Everything is ready for welding, probably get to it after the 25 hour @ Thunderhill, see you there.
It's been a while, I finally downloaded pictures, thought I would post to complete the thread. Mounts are 1/2" aft of original location. No alignment yet, working slowly.
Mark
This is how tight the mount was to the spring before:
Excellent job. FFR should take note and do just what you have done for all kits going forward.
+1
Not sure why they didn't design it like this from the start. It is closer to how the Corvette suspension is mounted on the C5 - seems like it would be wise to keep it as close as possible.
Nice work Mark! That is exactly what I want to do. Thanks for the pics.
I have to say these shims are AWESOME! This would have been a major pita without them, what a pleasure working with these great pieces, thanks Mike!
I have my chassis level sitting on 4 1/2" blocks while I modify my roll cage so it resembles a street ride height. I used 25.3" tall front tires and started with shims 0,2 out on the front and 6,7 out (pretty much all the way out) on the rear of the lower control arm (that's what slipped in the slots without force) and achived:
Camber- -1.0 degrees
Caster- 6.5 degrees
I then put a 1/2"" spacer under the tire and kicked out the front of the a-arm with a 4,5 combo to see what kind of camber I could get for the track:
Great job, I was wondering what the magic dimension was and with 1/2" you have achieved the desired castor. What are the upper holes for, track? Also, have you driven the car yet, I'm interested in how heavy the steering is?
Upper holes just gives tuning options, upper holes will raise roll center but sacrifice camber gain in bump. The more I learn and as others with experience have pointed out, there is far more to sort out before you ever get to playing around with roll center.
I'm far from driving. If I wrap up the build I will lose the slowest build race. I'm very competative!
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