Solofast

There are several things going on with the front bushings that make it easier to tune if you stay with rubber.

One thing that many don't realize is that the front bar bushings are clamped tight to the bar and are also in torsion when clamped to the bar. This means that you have to not only compress the bushing but you have to twist it too. This effectively adds stiffness to the system. If you go with poly or Delrin you stiffen the mounts in the vertical direction, but you also take away the torsional component of the system. That means you stiffen the bar going to poly or Delrin, but not as much as you might think because you lose the torsional component. In some cars the bars aren't clamped that tight and the bar can turn in the bushings, but in the C4 it's clamped tightly enough that the rubber has to twist.

We've used poly in the front of our BSP car, but you're after balance and the limit is rear roll stiffness. What limits the rear roll stiffness is jacking, and once you get that to where you can live with it, you are just adjusting the front to keep the car from pushing and you can get there with rubber. The downside of poly or Delrin is that you can't fine tune it by controlling the bushing preload, do you're stuck with whatever bar you use and you don't have a way to fine tune it between runs.

MatthewMiller

I missed this thread when it was getting rolling. Oops! A lot has gone on it since the beginning, and there has been a lot of good advice given so far. So the problem right now that I see is that you've really hobbled yourself with your wheel selection. You're going to have to go with Bridgestone RE71R or BFG Rival S 1.5 in the 255/40r17 size if you want to have a competitive tire, even in street (stock) class. The Continental Sport tire is a fast street tire and has become a go-to rain tire for classes that require 200tw, but they will give up a second to the other two tires in the dry on a typical course. At this point, I'd consider sticking with Street (stock) class unless you're willing to get 18" wheels with much more width. BTW, Continental may be releasing a new 200tw competition tire in 2019. Early testing has shown it would be very competitive with the Rival S and RE71R, but I think there is still the question of whether it can pass the required tests to rate it at 200tw and get the DOT rating. Anyway, it sounds like you may be able to get a discount on those, if they come out in a size you can use. I would have recommend a set of four 18x10.5 C5Z wheels and 315/30r18 BFG Rival S tires as a fairly budget way to get competitive rolling stock. I run those tires on 18x11 Forgestars at all four corners, and they work well.

If you go Street Prepared (BSP), you should try to find offset lower control arm bushings for the front. These would legally allow a lot more negative camber, which is very helpful. Even with much stiffer springs than you're running, I use -3.5* front camber, and in photos of my car in corners you can see that the front tire is still at about 0* camber to the ground at full lean (radial tires would actually like to corner with negative camber). So anything you can do to get more negative camber front and rear is a good thing. In the rear, I don't know how much negative camber you can get with the stock adjustment cam/bolt. But I do know those have a bad tendency to move and change the camber setting during hard cornering. But you have to retain the stock arm and adjuster in Street or Street Prepared. So I'd say set both sides for max negative camber, and torque the hell out of that adjuster bolt. If you move to CAM, you can replace that camber arm with a link that uses rod ends, and that gets rid of the problem entirely. That's an important mod to do if you're in a class where it's legal. I run -2.5* camber in the rear.

More on bushings: unfortunately, the Del-a-Lum types that have more metal and less bushing material are not legal for Street Prepared. Poly bushings that replace the rubber with an equal amount of poly are legal for Street Prepared but not Street. Poly is a good choice for front control arm bushings, but not recommended in the rear because they will introduce more bind. Stock rubber or rod ends are good choices for the rear arms, but rod ends aren't legal for Street or SP. I would have recommended poly for both swaybars, but after reading what Solofast wrote, now I'm not sure. I will have to get under the car and test to see if the rubber D-bushings really twist with the bar - that never even occurred to me before. I know stock rubber control arm bushings work that way, but they are cinched by the control arm bolts. I do know that the vertical bushing preloading he is talking about is legit. I just thought that it worked by allowing less vertical movement of the bar in the D-bushings. If that's the case, then poly will allow almost no vertical movement, and thereby stiffen the effective bar rate, but eliminate the adjustability of the preload. More stuff to ponder!

For springs, I generally recommend the stiffer the better for autocross setups. The idea is to keep the suspension off the bump stops and to limit the time any suspension corner spends in the really gross part of its geometry (i.e., near the extremes of suspension travel). I run VBP Xtreme springs with rates of 1125f/550r, and could even go stiffer if I never cared about street driving. These should be legal for Street Prepared and above, but not for Street. The front spring is adjustable for height, and therefore not even technically legal for NCCC Group 3 (their rules are kind of strange to me - you can run coilovers on a C4 but not an adjustable leaf!). Street and/or track might be different, because autocross courses are usually pretty smooth and therefore compliance over bumps isn't a big deal.

Re swaybars, I currently run a 30mm OE front and 23mm (1984 Z51 only) rear bar. I generally agree that too much rear bar is not a good thing in a C4. They also seem quite sensitive to rear bar rates - a small change makes a pretty big difference in handling balance. I have a selection of rear bars (19, 22, 23, 24 and 26), and they are so easy to swap out that I just change as required. Typically I find that the grippier the tire or surface, the stiffer the rear bar needs to be to maintain the same handling balance. The 23mm is working well for asphalt and Rivals, but on new Hoosier A7s and/or grippy concrete, I might go to the 24mm rear bar now.

For shocks, Konis are a good choice. I have some, and they are a bit undervalved for my spring rates, but would probably work well with yours. But if I were buying new ones now, I'd take a hard look at the Ridetech HQs. They are about the same price as Konis, and for that you get a true monotube (the Konis are twin-tube) with Fox internals and valving set up by people who know Corvettes well. And if you ever move to much stiffer springs and grippier tires, they can probably revalve them to a stiffer range of damping rates.

I agree with those who said stick with the stock J55 brake setup. For autocrossing, you just need good pads and maybe braided brake lines and you're good to go. You'd want more front brake if you run track events, but your wheels won't allow that and it would bump you out of Street or Street Prepared.

For gearing, figure out your top speed in 2nd gear with the tire height you end up running. You'd like to be able to stay in 2nd gear for most courses, so you'd want at least 60-65mph in 2nd gear if you can get it. If you run into the rev limiter in one spot on a course, that's usually not a problem. The 255 tires are short, though (only 25.0"), so make sure a 3.07 rear gear doesn't put you too slow in 2nd gear to keep it there. Also, this certainly depends on the course designs you typically find at your locale. Some places are space-limited or just tend to design lower-speed courses than others. Maybe you only really need to plan for 55mph in your local events, or maybe you need to plan for 75mph.

jefnvk

Woah, lots of good info there @MatthewMiller! For context, at this point I'm fairly focused on SCCA STU and NCCC G1. I think I can get away with the majority of what I want to do there.

Regarding wheels/tire sizes, yeah the decision has been made, this first year is more about skills development than a tire war. There is a budget to be concerned about somewhat, a good summer street tire on a cheap set of stock wheels also is beneficial if I decide to dump the auto at some point in favor of stick. After this year, I can easily choose to reevaluate my tire setup without losing much (if any) money on wheels. I'm also trying to leave upgradability on some of the more subjective decisions, that as I learn I can change.

I am curious on these new Conti 200TW tho. I work on the component side of the automotive world, not rubber, so I'm not kept all that up to date on new tire tech. Do you have any further info on them? I was really hoping to stick a tire order in before the end of year (we get a limited number of discounted sets per calendar year, I've got one left for 2018), but if theyre going to happen I may be intrigued in holding off.

Solofast

One advantage Matthew has is that he can lower his rear ride height. That lowers the rear roll center and allows him to use a bit more rear bar/spring rate combination. You likely can't get away with that much bar/spring at stock ride height. As noted in an earlier post in a G1 or stock class car since you can't adjust height (other than what is available with the stock rear bolts) you are limited in what you can do. The proof is in the pudding so to speak and you will want to have as much rear bar for your particular rear spring that gets you to the point where the car gets squirrely on corner entry when trail braking, and then back off a tiny bit. Also as noted before when you are on the power the rear squats, lowers the rear roll center and that gives the car more rear end grip under power. In driving C4's you want to get on the power early to make the back end squat down and you leave the corners like you're shot from guns... Some folks in the stock classes used to set their C4's up high to get them to "rotate" (Tom Kotzian comes to mind), and were successful with it and others like Roger Johnson set the car up to be on a knife edge of oversteer but I didn't like that. What I found was the best setup for the later C4's was to set the car up with just a tiny amount of push (like very tiny), and then you could get the front to stick with a bit of a lift and with a pinch on the gas you would be able to pin the rear end of the car. In a slalom you'd be able to play the car with the throttle, a bit of a lift on turn in to stick the front end, and then a light squeeze on the throttle to pin the back end and once you got that coordinated and learned you could eat fast offsets and slaloms.... The biggest problem with an automatic is that unless you're really moving you don't get enough weight transfer in second because the torque converter loosens up the drive train and there isn't much engine braking in that tall second gear. It is also a reason NEVER to use a loose torque converter in a C4 automatic car.

One year we were at Topeka with Larry Diemer's C4 bsp car that had the slush box and there was a really fast offset near the finish, followed by a decent stratight. In the warm up Larry rode with me after his runs and was saying "offset" really loud, like he wanted me to slow down, and I just flicked the car thru at a really fast pace by lifting to stick the turn in and then planting the back with the throttle as we powered thru the offset and carried a bunch more speed to the finish... He was bug eyed and said "well I really learned something today".... Probably couldn't have done that at lower speed with the automatic, but at higher speeds with the higher engine braking you could.. After that Larry got rid of the looser converter and went to a much tighter converter that would let you use the throttle more for weight transfer. He had put in a looser converter to try to get more punch coming off of corners but the car was more controllable with the tighter converter.

What I'm trying to say here is that the auto trans effects how you set up the car AND how you drive it. With the stick shift cars we never left foot braked the car, even though I used to do that all the time with my RX7's so I had the technique down. Left foot braking with the stick cars just made them push like crazy. With the auto BSP car we left foot braked but that also effected the setup and the brake bias we ended up with. Roger Johnson always pulled the bulbs from his brake lights so he wouldn't let anyone know what he was doing. His car was set up on the knife edge of oversteer, but left foot braking (with some small amounts of power like you get with a high idle speed) will settle the car a lot and make it understeer. I don't know but I suspect that was part of his setup.

MatthewMiller

You can always get another full set of bigger wheels and tires later, and relegate these to your street and rain tires. So it's not an unreasonable starting point - just not an ideal ending point.

The last time I talked to anyone involved in the testing, they were comparing 315/30r18 sizes. So that size will be part of the lineup if these things ever get released. That "if" sounded like a big one, though. From what little info I have, it sounds like maybe a 50/50 chance that they are going to happen. I'm personally hoping they become an option sometime by halfway through 2019, so that I have them as a choice when my current BFGs wear out. Only time will tell.

Just to add to this, I can also adjust front ride height because my VBP spring has adjustable pads like a C5/6/7 front spring. That's pretty interesting about the automatic setups. One thing that I notice in the 88-96 C4s (with the revised suspension) is that they have a boatload of anti-dive geometry up front. This makes them less responsive to trail-braking: instead of turning in more on trailing brake they just keep understeering. With my manual, just letting off the gas without using the brakes much makes them turn in more, which is the same thing you found with the stock t/c. In my class (CAM-S), I could actually drill new front holes for the front upper control arm mounting bolts to reduce the anti-dive. Then it would respond to trail braking like most other cars, with less understeer. I agree with your general balance preference for just a touch of understeer in steady-state cornering. And yes, it hooks pretty damn hard on the gas out of corners.

I'm enjoying this discussion! Merry Christmas to you all.

Solofast

Agree with all of what Matthew said above. After you get the max rear roll stiffness that you can get away with without jacking, then you tune the car with front roll stiffness for just a touch of understeer and then drive it. On thing that we learned about the zero scrub radius front suspension (88 on C4's) is that they are really sensitive to caster. Because of the high kingpin angle you lose a ton of negative camber when you go to high steering angles (like you do in slow autocross corners). On some days you could feel right where the front end would start to wash out based on how much steering angle you put in. To address that more caster is a huge help. In the street prepared and CAM cars you can make some offset bushings that move the arms aft on the support rods. Jeff Glorioso did this to his BSP car and when I asked him about the bushing on the upper front control arms (I said did those help??) Jeff got all evasive and hemmed and hawed for about a half a minute and then kinda said "not that you would notice"... Even in a stock class car if you can push them aft 1.4 of an inch or so it makes a difference you can feel. I imagine that the shift Jeff had in his (more like an inch) give him a huge amount of caster... Just say'n…

Interesting discussion, some things don't change, I haven't run a C4 in more than a dozen years, but the cars were sweet in that they were a lot more fun to drive. The longer wheelbase lower CG and lower weight of the C5Z makes them faster, but not as easy to control the balance by shifting weight with the throttle and brakes and that is part of what makes the C4 more fun to drive.

Have a Merry Christmas!!!!

jefnvk

A Merry Christmas and a great thanks to you guys as well! This sure got in depth in a hurry! The engineer in me loves it, although the new Corvette owner in me feels I'm going to have to bookmark it and come back to the more technical points in a few months, after running for a bit and understanding the car a bit better. Definitely went beyond the original intent of "keep me from putting useless/junk parts on a car I don't know", but thats a very good thing!

A quick question regarding shocks: if you were someone who has run 8 or so AutoX weekends in their life, with a new car, trying to focus on driver development, would you take the class bump to 2 in NCCC to have adjustable, or just stick with a new set of Bilsteins? I feel that playing against dedicated AutoX tires is just going to make it not fun. I don't expect to be fast wherever I start, but I'm also trying to stay away from places I'm simply never going to compete. This has been fun trying to follow two rulebooks....

froggy47

I agree with 69, brakes rarely hold you back in autox, just get the right pads & call it good, suspension yes, had a 96 lt4 and base bars were like a school bus for lean. Stiff bars & really good shocks and I suggest cam-s, you will have more fun than you can stand. I have some videos on the c4 if you look on my channel.
Never did nccc.

Solofast

Adjustable shocks aren't allowed in NCCC G1.. I always used Bilstein's there and won a G1 class at convention in low speed in 93 (came in 2nd in high speed) and then won Class and group 1 in both low speed and high speed at convention in 94, so they work fine. In SCCA where adjustable shocks are legal I did the development driving for Koni way back in 1989. To be honest the Koni's could use a bit more rebound damping when turned to full stiff, but they're still a really good setup for the C4. If you set the rears at full soft and then don't touch them you can get a lot of adjustment and tuning ability with just the fronts. I'm a believer in getting as many tuning tools as you can and making sure they are quick and easy to adjust between runs. Lots of folks say just drive the car and get what it gives you, but I was amazed at how much time you could knock off a run by just making a few adjustments. The other thing was that you can quickly learn how much each tweak you do changes the balance and then if the car is sweet you can really fly, that is all about confidence. Adjusting the Koni's on the front is really easy between runs and after a while you'll have a pretty good idea based on the surface roughness and the course design what you'll need to dial it in even before the first run. As I noted in the post about using clamping to adjust front bar stiffness, you can dive under the car without jacking it up and loosen the front bar clamps and either insert a shim or remove a big fender washer by yanking it out from using the tie wrap that you put on it ahead of time. After you learn how much each shim is worth in terms of feel, you can make changes and then make two runs with the car that hooks and you can have confidence in.

jefnvk

I guess maybe the better way to ask, would you find shock adjustability in the near term worth buying adjustable shocks now and taking the class bump up to G2 with everything else at a G1 level, or just stick with Bilsteins for the next year or two?

MatthewMiller

I'm thinking that for the near term, with noncompetitive tires and trying to stay at the lowest NCCC class, it might pay to buy non-adjustables like Bilsteins for the first year or two. Then you can upgrade later if/when you decide to go with bigger wheels and tires, aftermarket springs, etc (i.e., when you pick classes to race in for the longer term). It would be great if you could get some used-but-still-good Bilsteins, so keep an eye on the C4 Parts Classified forum.

Okay, this is really good intel. I had never thought about camber gain/loss due to kingpin angle, or even the typical caster setting for later C4s. You now have me thinking evil thoughts about SPC adjustable upper arms, or slotted mounting shafts. I have to admit I'm struggling to understand the offset bushings that Jeff Glorioso was running: I'm not visualizing how they could move the arms rearward on the shafts. Did you mean that their holes were offset as with camber adjustment, but that he was setting them up with the front offset toward the outside and the rear offset toward the inside, so that the arm was angled backward quite a bit?

l98tpi

Solofast

It mostly depends on where you're going to run more events. I tended to run a lot more SCCA events because that was my background before getting into vettes and because that was where the higher level of competition was. In the Indy area there wasn't any competition in G1 and if you aren't pushed you won't go faster. I was running against Roger Johnson, John Carriere, Matthew Braun and Al Chan every weekend. If you want to go fast, run against the tough competition and you'll learn faster and go faster in the long run. I never found much competition in the NCCC. I ran the conventions, but other than that not much NCCC. Part of the fun was showing up and being an "unknown" and kicking butt and leave them with open mouths and shocked looks..

As far as the bushings that Jeff made, they were designed to let the arms slide aft on the shafts but they were still concentric with the shafts. BSP rules don't let you change the mounting points so you can't slot the shafts. You could most definitely get the same effect by slotting the mounting shafts but that wasn't legal. So what Jeff did was take the rubber off of the shafts and then made a new set of bushings. The rear bushing was pretty straight forward, he simply made the bushing so that the nut was countersunk some into the bushing and he may have cut the nut down in diameter or thickness or even used a different nut. The front bushing allowed the ring of the A arm to slide back over the bushing flange of the support rod. Since you couldn't see much of what he did there as it was "buried" inside the bushing I don't know if he spit the new bushing or if he just let the new forward bushing provide the stop for forward motion on the support shaft and let the rear bushing provide the limit for rearward motion. In looking at pics of C4 arms I would guess that Jeff's bushings gained him between a half an inch to 3/4 of an inch of aft displacement. But that would actually gain you a heck of a lot of caster. I wasn't aware of an aftermarket slotted support rod, but that would for sure get you more caster if your class rules let you use it. .

You don't want too much static negative camber as after you get past 2 or 2.5 degrees you're going to start to lose braking capability. Still, with the high kingpin angle of the C4 you loose negative camber as the steering angle goes up. It is better in my mind do use more caster so that you can have good braking and not lose negative camber at higher steering angles. Big caster also provides increases the vertical load on the inside front tire at higher steering angles and unloads the outside tire, which also reduced understeer. In a C4 in stock class you take out all of the shims to maximize negative camber, but you don't end up with much caster. I'll look up in the archives my alignment sheets but I was thinking that you typically got about 5 degrees of caster, it could be more or less, I just don't recall. I know it wasn't any higher than 7 degrees, but I was thinking it was more like 4 or 5. Anyway, I pushed my stock arms back on the stock bushings as much as I could and that made a noticeable improvement in front end grip. If I were running a later C4 in a class that allowed offset bushings I'd be getting big offset in the lower bushings and then dial in a good bit of caster with shims or if allowed a slotted support rods. Also remember you can use bigger offset in the lower aft bushing to push gain caster. I would try to push it up to closer to 10 degrees and see if that gains you some front end bite. I'll try to find my old C4 setup sheets, they're somewhere in the basement in old notebooks, but I haven't seen them in years. I can then tell you what caster we used to get from a stock C4 with all the shims out.

MatthewMiller

All but one came with the car when I bought it. They are all OE rear bars from C4s. The 23mm that I bought myself is probably a harder one to find, since it only ever came on the 84 Z51 cars. But I think I put a WTB ad up in the classified parts forum. Username Fauxari08 had two or three available, and I bought one of his. He was reasonable on price and easy to work with, and I bet he has any size they ever made. Same for the fronts. The 30mm I run up front is also an OE bar. I do have one Addco 1.25" bar as well, but all of my front bars came with the car when I bought it.Oh well, easy come easy go.

Solofast, thanks for the additional info. Those "backspaced" control arm bushings sound like what I was trying to envision. In CAM, the slotted support rod would be the easy way to go. I'm not sure if one that fits is made. SPC has a generic slotted one with 6" spacing - I don't know what spacing the later C4s use, though. OTOH, it seems like their upper control arms that adjust with rod ends and clevises could get the same effect by just lengthening the front link of the arm and shortening the rear link. That would keep the overall length from pivot to ball joint the same, but move the ball joint backward. I do have eccentric lower control arm bushings*, so I also have some room to work with on caster with the stock shims. I'll have to see where my caster is right now - I don't even know.

*I think these are also VBP parts, and therefore no longer available. Seems like something that could be made in small batches if one could buy "blanks" with no inner hole drilled from Energy or Prothane.

jefnvk

NCCC for me would be more to fill the June-August summer months. Detroit SCCA tends to have a few day events in area in the early and late months, but the summer moves up to the Air Force base in the northern part of the state for weekend events that I'd be less able to attend.

Solofast

In CAM you can mod the cone shaped spacers that hold the support rods and get more caster too. Obviously in stock you can't do that, and that was also illegal in BSP, but it would be an easy mod in CAM. What you can do is shave down the thickness of the aft spacer till the big end of the control arm hits the frame mount for the rod at the aft end. Then you can shim to get what you want from there.

vettehardt

If you can, try to come down to The Tire Rack for some of our events. My Michiana Corvette Club puts on events on Aug 10 & 11 and then a Michigan club and Elkhart Vette Set puts on events on all 3 days of Labor day weekend.

jefnvk

Will do! Northern Indiana is a bit more convenient than Northern Michigan, and I like seeing the various automotive testing grounds.

Regarding the Corvette Clubs, I get the distinct impression as I dig through those local to me that they all have varying focuses, some lean more towards competition while others lean towards social events or shows. Is this a correct understanding, as I am trying to find one to join?

vettehardt

Yes you are correct. I belong to a club more shows/parade/dine and drive oriented. But my dad and I are racers so we go and do our thing while rest of club might be doing something else. It's not a big deal as you don't have to do everything the club does. You can pick and choose what you want to participate in. If you want ones more competition oriented, look into Corvette Club of Michigan (CCM) or Michigan Autocross Group (MAG). I know people in both groups. CCM does more around Detroit. 69autoXr on here is part of CCM.

jefnvk

Yeah, CCM was what I was looking at. Just figured if I had a few options in my area, and I was joining for competition, then I should pick one more competition oriented.