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Unread 06-29-2015, 12:01 PM
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Default Re: Vorshlag Build Thread - Mark C's 2002 C5 Corvette Track Rat

continued from above

We had done the same type of rear upper shock mount on a C5 Corvette many years ago (it never went into production), but this time we refined it by adding easier access to the top knob from the rear hatch area. It's a simple upper mount using our oversized 3/4" spherical bearing with two thru-bolt holes that attach the flange of the assembly into existing threaded factory holes in the rear. Note: the access hole for the upper knob in the rear fiberglass recently became legal for many SCCA folks, see 13.5.F, but converting from leaf to coilover is not legal for Street, Street Prepared or Street Touring classes. It's fine for Street Mod or NASA TT1/2/3, though.


The new Vorshlag spherical top mounts are shown at the rear (left) and the front (right) shock locations.

We left the factory lower rubber shock bushing on the bottom in place for now, but there are off the shelf conversion kits to make this into a spherical bearing also (maybe we will do this on Round 3 of mods). The front MCS shocks lower bushing is already a spherical, as part of their T-bar lower mount.

The front upper shock mount was something a bit more unusual, done differently than the other coilover setups on these cars. Most of the spherical upper shock mounts we've seen for C5/C6 coilover front shocks are really just "eye" upper shocks with an adapter to bolt to the stock pin mount hole. That makes for a tall ding-dong looking device sitting above the shock, and the spherical bearing is way below the factory mounting hole - which eats up a lot of front shock stroke (but gains room for the coilover spring to the upper control arm). Our set-up is unique in that the spherical bearing is slightly above the factory upper shock mounting hole - which gains additional shock stroke. We offset and enlarged the factory upper mounting hole inboard, which is how we gain room for the spring clearance needed relative to the upper control arm. It will make sense when we publish the production pictures + instructions.



The face of the spherical bearing mount still has plenty of surface area - more so than the factory rubber bushings did - and can pass the loads into the whole upper shock mounting structure, which is thick multi-layer steel assembly welded to the frame of the chassis. Doing this offset upper hole moves the top of the shock inboard, which gained room for the newly added coilover spring. We also went with 2.25" ID vs larger 60mm ID coilover springs for more clearance to the control arm.


Common coilover spring diameter sizes are 1.75", 2", 2.25", 60mm, 2.5", and 70 mm - which are listed from smallest to largest

It sounds complicated but was actually quite easy. Again, this atypical spherical mount setup removed a rubber bushing from the suspension load path plus gave us MORE BUMP TRAVEL, both good things. Spherical bearings pivot smoothly and can carry a LOT of impact load, unlike rubber which can bind in pivot and crush in compression.

The spring rates we chose were a compromise - like everything on any race car, or any race/street car. Mark has since bought a trailer and decided to make this a more dedicated race car, but... he wants to hang onto some street manners for a while longer - like catalysts, interior and air conditioning - so we had a tough call to make on picking the rates. He was also stepping up from 200 treadwear tires to Hoosier A6 tires, which he told us the week we were installing these coilovers.

Jason wanted super stiff and I wanted just moderately stiff, so we argued, calculated, and finally settled on 650 front and 700 rear rates in 2.25" diameter and specific lengths that would fit the confines of the suspension. This is about triple the front spring rate and about double the rear of the OEM transverse springs - we think. You see, there are a couple of ways people measure the spring rates on the transverse factory parts and nobody seems to have real a good, definitive listing of C5 springs' actual "wheel rates" (spring rate at the wheel, with motion ratios and funky spring shapes taken out of the equation). The measurement styles or conflicting spring rate data doesn't really matter - all that we care about is how the springs and dampers perform.

Luckily we had a test event scheduled with Mark the next weekend, which we could use to dial in some of the settings, so we worked overtime to get it all together before he came by to load up with his new trailer on Saturday.

Making Spherical Bearing Shock Mounts

Some folks like watching the step-by-step images we show on social media of new parts as they are developed and built. Here's a few steps of the making of these C5/C6 Corvette upper shock mounts, which happened over the course of 2 days (from design, to a brand new machined part, to on the car and driving).



First step is always measurements from a car. The C5 was lifted up on one side and the stock shocks were removed. We test fit the shocks and several springs a few times and saw all of the tight spots. Some measurements and tracings of OEM panels were made, an initial design concept was made for both ends, which Jason and I reviewed together, tweaked, and then he made into a final design in CAD. Then the material was ordered...



By the time the two billets of aluminum alloy arrived, the CAM design was programmed and ready for the CNC machine. Specific length slugs were cut on our coolant-fed band saw and loaded into outer jaws in the CNC lathe.



The front and rear slugs were machined on one side, then custom soft jaws were installed and machined to grab the "tower" of the rough machined parts. Then the ID was machined to accept the press-fit spherical bearings, with several high tolerance dimensions. This was the first two "ops", completed all on the lathe.



The finish machined "rounds" were then ready for the CNC mill, to cut the perimeters of the mounting faces that attach to the chassis. To do this required three fixtures - one to hold the tower shape on op 3, and another two to hold the bases for op 4 on the mill. Lots of programming and setup but very little machine time. We made 8 pieces and only needed 4, and didn't anodize them but left them in raw aluminum - typical of a prototype part. Mark is a tester and knows all of this going into the build.



The spherical bearings were test-fit into the rounds to verify the dimensions and fit before they went into the mill, of course. We actually skipped op 4 (just a chamfer on the top edges of the base flange) since they were only for cosmetics and would be covered up once installed. The production pieces will get op 4, of course, along with a red anodized coating.



All told about 25 man hours were spent measuring, designing, programming, machining and test fitting these parts, all of which we eat in the name of R&D. Testing should prove the parts work (or need a tweek) and we would have another successful product that we can sell for years to come. We shall see...



That's a good visualization of the starting billet and the finished parts. Lots of steps taken to get from round bar to final product, and hopefully you enjoyed the little step-by-step here.

Additional Mods

Mark has a lot of future plans for this car but wanted a quick release race steering wheel to complete Round 2 of mods complete. We put together an estimate for his requested racing steering wheel and quick release and got he parts coming when he approved the choices, colors and numbers.

We suggested the models of the wheel and quick release ultimately chosen, using brands and models we have used in the past. The steering wheel is a MOMO model 88 in 350mm diameter (also available in 320mm) with a black suede covering and flat bottom design. I've used this wheel on four of my own race cars in the past and numerous customer cars. The GM hub adapter is a MOMO piece, which goes to a 70mm diameter 6-bolt circle common to Sparco, MOMO, OMP and other racing steering wheel makers. The quick release hub is from Lifeline, who also makes the Sparco branded part that looks identical. We've used all of these parts before, and sell these to racers all the time. The QR comes in gold and black, and Mark picked gold.



We have used a lot of different steering wheel Quick Release hubs over the years. My only advice is this: while there is a big price difference between the circle track QR hubs, Chinese/eBay QR hubs, and the high end hubs like this Lifeline/Sparco unit, there is an even bigger difference in feel and "slop" between the low priced and high priced units. This is another one of these "you get what you pay for" deals, all the way.



These parts are often finicky to install and took some special care to get the factory airbag out, remove the OEM steering wheel, fit the MOMO hub adapter, and trim the column surround plastics. The QR hub and MOMO wheel were the easiest bits to install. One thing to remember is that a QR hub always adds depth to the column, pushing the steering wheel about 2" closer to the driver.

This worked out perfectly for Mark yet when I drove the car (with a fixed seat placement) it put the wheel a bit close to my chest - but this car isn't built for me, it's built for Mark! And this car, fitted with high side bolster 1-piece racing seats, removing the steering wheel does make it easier to get in and out of. And the new wheel... mmm, it feels so nice. Good stuff.

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