Flyboy I understand completely.
When I drove my 70 Z28 Solo car on PA roads, with 10.5" wide autocross race rubber on it, it was a handful. The dishes worn into the asphalt are made by normal cars with average 6-7" wide tires..
You can actually see it when it rains and see the 7" wide puddle strip right where the tires track. With the 10" tires you are straddling both sides of the dished-out section by just an inch or two. Just two inches movement to either side and you have one side of the tire in the "air" of the dish. I tried that in the rain once on the PA turnpike with my 0 treadwear rated race tires, almost bald with 2/32" tread, and almost wound up into the guardrail, like every 100 feet or so. It would suddenly jump 2-3 feet either left or right. I literally could not keep it in my lane. It was the most frightening white-knuckle ride I ever had in my life!

I had 6-7 degrees of caster in that car, and good PS. And zero offset wheels. it did not help or matter. It was that the tire width was way too wide for the dish or cup shape in the asphalt. When one side of the tire had significantly more traction/pressure than the other, it was like someone grabbed the wheel and jerked it. That's why the skinnier tires feel better, they fit the cup in the road surface better. My car did not behave that badly even when I had the 245/50-15 BFGs on. And they were pretty wide, maybe 9 " It was only when I put on the 10.5" wide 245/45-16's that it jumped all over the road, right and left.

So yeah the skinner tires are the solution, not the alignment or wheels. I had really short sidewalls, and so therefore not a lot of flex.

I also did run toe-out on that car for Solo. If you add some toe-in, vettes seem to like it, that may help a lot. Experiment and try it yourself!

 

Thanks Leigh. I was hoping you would weigh in on this. My first set of tires on this car were Goodrich 255.60/R15, which have the same 10 inch wide tread on the ground as these 17's I have now......and they also tramlined. It was only when I went to the 225/70R15 that the car tracked much better. I really don't believe my alignment numbers are much off from the last alignment, and close to all recommendations, which was roughly a slight toe-in on fronts, zero camber and 2.5 caster. Rears are at zero camber, and I believe slight toe in or close to zero toe. Visually they all look straight up on camber.

I drove the car this morning 100 plus miles.....and on good roads, its solid with little tramlining. One thing that I believe about the modern W rated Nitto 555R tires is that they provide some "insurance" policy on handling and traction. JB78L82 mentions pushing the tires to 8/10's of their capability. I won't do that in my driving style....but it does give me that extra margin over the Coopers, which are NOT really performance tires. The 406 has made me a little more aggressive......because its fun. So what I don't want is to be working into the 8/10th of the capability of the Cooper 15's,.....and losing it. This is what I need to decide. Live with tramlining on bad roads, or give up safety margins and go with lesser tires. If I do put the 15's back on it, I am going to have to be more careful with the "fun" stuff with all the torque I now have. The car really launches when the pedal is pushed....even a little.....and blasting it forward is exactly what I wanted. I got it. Speed is not the goal, as I am off the gas before long, but those launches could end up spinning tire on the 15's, with possible loss of control. I guess everything has a compromise......I just need to decide.

I fly high performance jets for a living.....being careful with excess performance is part of what I do. Managing the power in this car is something I can do....I hope. I do not want to be in this car out of control.

 

I would try a little more toe-in front and rear before you do anything else.
IIRC the rear does not like zero toe, it likes toe-in. The rear has a toe-out curve under bump and that will steer the car. But too me your problems sounds like it is in the front.
Look up your specs that you have now, or just measure it. Toe is easy you need two 4 foot straightedges and two locking tape measures.
How much do you have now?

And I like your analogy to a high performance jet and a 400+HP sports car. Similar "excess" performance. LOL
I am probably one of few on here who have done 10G loops in jets and also raced at 1.25G.

 

Just one clarification to my earlier statement about pushing the tires to 8/10's of potential ( and the power of my L-82 355 which is considerable, as well). I think we are all in agreement that on public roads, I too drive very conservative, most of the time, doing what Flyboy/Leigh describe, hard acceleration in a straight line, quickly backing off the throttle, once speed begins to get too high. Most of the time I am cruising on winding country roads at 55-65 MPH....same as you.

The wider sticker 255/45/17 Ultra High Performance Summer only Kuhmos do provide insurance with the occasional high speed highway blasts, Dry/Wet braking, and very rare deserted country winding road/highway entrance/exit ramps where my C3 has on occasion been at the limits of adhesion, with the car slightly on the verge of 4 wheel drifts, pronounced understeer, throttle oversteer.etc. I would call that behavior "8/10's tire". Reminds of my days in the recent past when I drove formula cars on road courses. How often does that happen with the C3? Rarely BUT it does occur. Would I do the same with my former 255/60/15 BFG TA's or a lesser tire? NO!

99.9% of the time, I drive just like you...........

 

Good analogy. The 17-18" summer high performance tires just have a larger performance envelope, with near 1G potential vs 0.80G potential with the BFGs.
The difference in tire construction, compounds and sidewall also makes them less squirrely and much more predictable once you do reach their respective limits.
I think of it as a 10-20% extra "safety cushion" at normal speeds..

 

Flyboy - let me know how your alignment experiments go. I just installed Fiero seats and my car '71 is manual steering and with the increased side lumbar support and the big, manual steering wheel (non-tilt) it's a little more difficult to get in and out of. If I can free up a little wheel effort then maybe I can go with a smaller diameter steering wheel or my other options is to go power steering (don't want to do that) or get a tilt/telo steering column and install that, which is a viable option that I've thought about doing before.

 

I used a different solution on my 74 manual steer, std column car, 15” OMP steering wheel, Mazda Miata Waterfall seats. Reference post number 4.
https://www.corvetteforum.com/forums...n-options.html

 

So maybe instead of getting rid of the 17" wheels, slowly bump up the caster until you don't have any further adjustment or steering effort at low speeds gets to be too much?
Also in regards to the camber setting. Back in the day (early 1980's) I was a line tech at a Chev dealership and some how became the alignment guy. Due to the crown in most roads at that time the camber was different from side to side. I don't remember which. If it wasn't correctly it had a tendency to climb the center line crown.

 

For some visual perspective.....here is a picture of the front tires. On the left is the Cooper 225/70/R15 tire......and on the right, installed on the front of the car is the Nitto 555 255/50/R17 tire. You can see there is alot more tread on the ground. The Nitto's are very flat with almost square corners, where the Coopers have a larger radius corner.

 

I have similar tires and have noticed the same behavior vs the tires that came on the car.. Never even thought about the grooved in the road being the issue. I did notice what I perceived to be other improvements in handling with my new tires vs what I consider to bet trailer tires that came on the car.

 

Update.......I have put over 2000 miles on the 17 inch tires these last few months. With all the associated tramlining issues, I decided to reinstall my original YJ8 wheels, with 225/70/R15 Cooper tires, and drive it for comparison. Attempting to avoid any intentional bias / opinion, here are the notable differences I have perceived, starting with most significant to least significant. I also want to qualify that all of this is based on taking lots of curvy Pennsylvania two lanes roads at 50 - 70 mph. These are the speeds that I drive regularly and on these roads anything faster is just dangerous, and irresponsible, and 10-15 mph above the posted speed limits. Nothing I write below applies to speeds above these speeds as it is irrelevant to me.

- Tramlining and bump steering is now completely non-existent with the 15 inch tires. On many Pennsylvania roads I drive on regularly, the ruts / grooves moved the car dramatically with the wider 17 inch tires, and it caused a significant and bothersome movement of the car, requiring more driving attention and reactions. The bump steer was also very annoying and more noticeable. The tramlining and increased bump steer caused by these 17, 10 inch wide tires, made the experience of driving the car borderline unacceptable to me, and made the car feel less controllable. It is THE #1 reason I will not continue using these 17 inch / wider tires. FYI - I had the same experience with the first tires I had on the car, which were 255/60/R15 Goodrich Radial TA tires. So, IMO, this tramlining is a result of the wider 10 inch tread on the ground versus the 8 inch tread of the 225/70/R15 tires, not the diameter or speed ratings or any alignment specs.

- Ride comfort is better with the 15 inch tires. Bumps, ruts, etc. feel slightly less harsh with the 15 inch tires. But....the difference between the 15 inch and 17 inch tires is not significant, and I would be content with the ride of the 17 inch tires.

- Handling is better with the 17 inch tires. As predictable, have 10 inches of rubber on the ground, versus 8 inch, as well as the lower height sidewall, and better rubber, makes the 17 inch tires perform better in faster turns. With the same movement of the steering wheel, the 17 inch tires respond immediately, whereas there is a slight feel of sidewall flex on the 15 inch tires. The initial dive into the turn comes with a slight sense of body movement on the 15 inch tires, but then once that is passed, the 15 inch tires have no problem gripping and making the turns. With the 17 inch tires, the car is capable of faster entry and faster speeds in turns. However.......this is not to suggest that the 15 inch tires are reaching their limits, or is bordering on loss of control. I powered through many tight turns with the 15 inch tires, and the car handled it perfectly, and I never felt I was on the verge of spinning out.

- The 15 inch tires handled the torque of the 406 without spinning the tires, at the power launches that I am going to make. I had concerns about this when I installed the torquey 406, and was one main reason I decided to buy the 17 inch tires. I have no doubt I could spin the 15 inch tires in multiple gears, and I have no doubt that the 17 inch 10 inch tread tires would handle that torque better.....BUT, I simply am not interested in high power, clutch dump maneuvers. I will never do burnouts.....I consider that abuse, and I don't abuse my machines. My goal with the 406 is focused on smooth, high torque acceleration, not burning rubber......and it turns out the 15 inch tires handle that more than adequately. The other bonus is that the 15 inch tires protect the driveline from abuse....as they will spin before they apply the loads to the hardware.

- The last issue is "the look", and this is purely a personal taste choice. As much as I like the look of the American Racing Torq Thrust wheels, the factory YJ8 aluminum wheels look absolutely perfect on the car. Secondly, the white letters also add a detail that the 17 inch tires were missing, and made the car look more plain. Lastly, the fat sidewall look of the 15 inch tires look "right" for a C3......its what they came with, and that look is the look I fell in love with as a teenager. I would NEVER install 18 inch wheels because IMO, they just take away the look. The 17 inch wheels were an acceptable compromise, but still not the look of the 15's.

In summary, the word that comes to mind is "compromise". This experiment proved to me that there is no perfect solution, both 15 and 17 inch tires had good things, and bad. Ultimately, the 15 inch tires are my choice. I give up a small amount of handling capacity, which I admit is a significant factor in a sports car. But truth is, at the speeds and roads I drive, this small compromise in handling capacity does not offset the downsides of excessive tramlining. On better roads, and higher speeds in turns, the 17 inch tires do perform better, and if I lived in a place with better roads, maybe I would stick with 17's. But given the 15 inch tires provide 100% of my handling needs........this compromise is irrelevant. I think I notice it now, with the sudden switch of tires,.....but as I drive more, it will just be the handling that I get used to.

The tramlining and bump steering was really annoying and took alot of fun out of the car for me. It felt less controllable, as the car was being driving by the ruts, and that was unacceptable to me. Now, with the 15 inch tires, the car tracks perfectly on all the roads I drive on, and I can relax and enjoy it. In the end, the goal is to have an enjoyable experience driving the car, and the 15 inch tires do that better, for me. Big bonus is that it looks perfect. So.....I will be selling the 17 inches / Torq Thrust wheels.

And when I build the 69 with the big block.....it will have 15 inch Rallye wheels....decision done, no further experimentation required.

 

The wider 17's will give a wiggly feel simply because there is more effect rubber on the road and less cushion due to the lower sidewall. The narrow 15's are like a donut, where as the 17's are like a steamroller. All of this can be taken care of with camber......I know because my 72' doesn't move around at all.....and there are some pretty knarley roads around here in Texas. I do have VB&P control arms though and the caster is somewhere around 5 degrees......
My little brother has a 2018 Demon and he lives in York, PA......I went and drove it around last year and it tracked perfectly with 315 section front tires on it.....there isn't a straight flat road anywhere in York......Yes it is a much more modern car, but the same physics take place......
But it seems you are content....and that is all that matters.

Jebby

 

As far as tramlining and bump steer, the geometry of the suspension was designed around 15 inch rims so IMO that is why you feel such a big difference when switching between the two. I think your results would be the same if you had 10inch wide 15's. If someone has come up with better alignment/geometry for the 17 inch rims please share it as changing my wheel/tire combo is probably my next project now that my Fiero seats are installed.

Try this, put your 17's on back and your 15's on the front and see what that does.

 

All I know when I lived in Round Rock is the roads in Texas are like railroad tracks. There are tire ruts on most of the road made of asphalt because of the summer heat. There were a few around Round Rock that I could literally take my hands off the wheel of the Vette and it would follow the ruts.
If you have your alignment specs that you can post may help Flyboy.

 

I am not an expert, so apologize in advance if I get this wrong about the geometry. But....the 255/50/R17 Nitto tires I have mounted on the 17 inch Torq Thrust wheels have exactly the same outside diameter as the 225/70/R15 Cooper tires I have on my YJ8 15 inch wheels. The backspacing on the 17 inch wheels is also the same as the 15 inch wheels. What IS different is the tire width.....10 inch tread on the 17, 8 inch on the 15. I have NOT touched any alignment adjustments with the change in wheels, either before or after the swaps. Many have suggested wheel alignment changes, but all the recommendations are very close to what I already have. I am not particularly ready, or interested, in making any changes to the alignment specs, given the car tracks absolutely perfect with the 15 inch wheels, its not pulling left or right, and no adverse tire wear. I also am not taking the car back to any pro shops for alignments, and plan to set up the Cagotzman system and do my own in the future. As for Jebby's recommendations on increased caster to make the 17 inch tires work.......it might work....but I have manual steering, stock A-arms, and have no interest in modifying the front suspension to some radical increase in caster. Its just not that important to try, and find out it does not still fix the tramlining.

Also......in my update post I stated that I had the same tramlining problems with my first set of 15 inch tires, which also had 10 inch tread on the ground (Goodrich 255/60/R15). So, IMO, its the width that causes this problem.

I could try your 17 inch back, 15 on front suggestion. I suspect that it may tramline slightly less given that I think the primary issue is the 10 inch front tire tread on the ground. But if so....then what??? I tried to see if there was a more narrow front Nitto tire I could mount on the fronts, but nothing ends up the same or close to the diameter, etc. I would not have a problem with different size tires front to back,.....but I am not putting small diameter tires on the front that do not visually fill the wheel well, just to solve this problem.

Thanks for the input.

 

Good experiment Flyboy!

With your manual steering you are limited to 2.5 caster. I get it. That's important to you.
6 degrees caster and a new alignment would fix your nervous wandering problem.
But also agree that would be basically a no-go on a manual steer car.
I am presented with the same problem as you, but going in the opposite direction for a solution as you. I want the 10 in tires, and therefore require the 6 degree camber, and need the PS to go with it.
And I know how to put the "manual" effort / feel back into the PS with a new torsion bar.
Same problem, opposite solutions!?!

You rank the MS feel as #1 priority and are willing to settle on tire width
I rank the 10" wide tires as #1 and will settle for all the feel I can get.

Give me a year or so to finish mine and we'll meet up and compare drives one day! I'd like that.

 

Thanks Leigh.....and looking forward to it.

 

EDIT:
I missed this part.

I have some bump-steer with my manual steering 80 (also on 255/60R15s), but I didn't notice the tramlining that you describe. I have fully adjustable SPC upper control arms, which let me dial in a lot of caster.

I fully agree that 15" YJ8s (and N90) wheels with RWL tires are the perfect look for these cars.

 

Hey Jeff hope all is well , I'm curious did you consider a alternate tire size the 235/55/17 might solve your problem and there are more options in that size

 

I will look into it again.....I never considered 55 size tires,...but it does appear they have a 27 inch outside diameter. Thanks.