Go_Gators
Go_Gators Reader
8/15/19 8:39 p.m.

Rear grip throughout cornering was really holding my C5 based vette-kart (aka LS1-KART) back at autocross. It was the definition of loose, even on 315 series toyo R888r(es) . Looking at the aftermarket for ways to increase negative rear camber, I was amazed at how few options there. With the stock adjustments, I couldn’t even get to 1 degree negative. The only way I saw to get into the 3 degree range was to go with Borg offset bushings. Which appear really nice, but are $1200, currently sold out and require removal of both upper and lower control arms.

 

Not willing to pay or wait, I decided to buy a used OE upper control arm off ebay ($25) to brainstorm with. Between a fellow autocrosser, Buck (also performing the welding in this thread) and myself, we came up with excavating the existing tapered ball joint hole, weld filling and re-drilling the tapered hole inboard.

 

original:

 

after excavating the old hole (required to get full penetration weld):

 

Bye bye hole.  Weld filling using aluminum alloy ER5356 filler. After welding and grinding the surface back flat the welded area was dye penetrant tested to confirm there were no cracks, indications or significant pitting in the surround aluminum. 

 

 

After successfully producing a sample, I ordered a set on ebay and Buck set off on eliminating the hole:

 

 

I calculated based on rough measurements that the hole would have to shift ~9/16” inboard to gain the 2 degrees I was seeking.

 

 

Running the 7deg tapered bit through to seat the ball joint:

 

Though the end of the a-arm cleared the spindle, i went ahead and removed it. 

 

I ran them at our local autocross for the first time this past weekend. The kart is transformed. I only got two dry runs before the rain came, but it blew me away how much grip it has now. i am typically 2-3 full seconds behind our usual fast time of day winner in a GTR. Without even pushing the kart hard I closed the gap to just over 1/2 second, posting the second quickest run of the day. It’s a small sample size, but I think this ~$50 mod is the best thing I have done to the kart. By far.

 

Hopefully this inspired you to get out there and redrill all upper control arms!!  But, if you are interested in this service, buying a set of used-modded A-arms, Buck is willing to do the work for a very reasonable rate. He has been a professional welder, a professional weld inspector and does great work. His email address is: amgreenway at gmail.com

It appears the same process can be applied to C6 upper arms based on pictures.

JG Pasterjak
JG Pasterjak Production/Art Director
8/15/19 11:15 p.m.

Very cool mod. Just kind of riffing off the top of my head here, but I'm trying to think of any potential second-order negative effects you might run into down the road, or that might make this a no-deal for a full-bodied Corvette.

First off, we use AMT's camber adjusters. We can get over 3 degrees in the front and right at 3 in the back. They move the bottom of the lower arm out instead of moving the outside of the upper arm in, so your level arms all stay the same length as OEM. In your case, you;ve shortened the upper lever, but if you have to change the length of any of the levers involved, that's probably the place to do it. The net effect should be that you have not only more static negative camber, but a more favorable dynamic camber curve. Especially since the initial rear roll in a C5 tends to add positive camber momentarily before going to a more favorable negative gain curve with heavier roll. 

For a bodied car, I think I'd still prefer to move the lower arm out rather than the upper arm in since there's already inner fender clearance issues with wide tires. But you've got no such restrictions, so this seems like a solid option. 

So, yeah. Looks like a good solution to a problem given you had no restrictions to work within. Glad the butt-dyno confirmed everything.

Go_Gators
Go_Gators Reader
8/16/19 1:49 p.m.

In reply to JG Pasterjak :

Appreciate it.  Definitely looking forward to pushing this new configuration to the limits.

I am somewhat baffled how the AMT kit gets the rear to 3 deg. Obviously, my car was put into a ditch and totalled, but i pretty convenced my frame and cradles are fine. So why i was only able to get 0.9deg with the factory cam and the AMT kit, using the same slot, can achieve 2 more degree is a mystery to me...

albeit slight... any less wide i can make the rear axle (by not moving the lower arm out) i think is an advantage for the kart on autocross... the thing feels SO WIDE already!!  

 

 

JG Pasterjak
JG Pasterjak Production/Art Director
8/16/19 2:03 p.m.

I've actually seen a lot of car-to-car variation on both front and rear. We got like 1.9 with the stock rear adjusters, but I've seen everything from less than a degree to over 2. I think with the AMTs it's mostly a matter of the pivot bolt being so much closer to the edge of the cam than with the stock stuff. 

dclafleur
dclafleur Reader
8/16/19 2:23 p.m.

The rear cradle can shift around enough on the pins that you can be limited on one side in how much camber you can dial in.  Most of the folks I know use the AMT camber brackets because they are less prone to slipping than the stock eccentrics and they can dial in a bit more.  Anecdotal but I also have heard quite a few C5 guys indicate that going to far negative on rear camber hurt their ability to get into the throttle quickly on corner exit.  I spend more time around the road course than autocross so the experience might be different.

noddaz
noddaz SuperDork
8/16/19 6:41 p.m.

Just back from looking at cheap Corvettes on CL.  Thanks...   Thanks a lot...  lol

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