A couple weeks ago in the middle of driving cross country I had a puncture and had to replace the rear tires. I changed tire size on the rear and immediately started having really bad tramlining.

It's a 94 vert with C5 wheels. It had C5 stock sizes (245-45-17 / 275-40-18) on it before. Changed to 275-35-18 to get back close to the original C4 diameter (285-40-17).

Old tires were Eagle F1 GS, new rear tires are Pilot Sport A/S.

I figured at least some of the problem was from the fact that this essentially lowered the rear by a little more than 1/2". Took it in for an alignment and Firestone told me no way would changing tires affect the alignment. But when I went back they said it was way out of alignment.

After the alignment the tramlining is improved from frightening to merely uncomfortable. They didn't have the before-after printout for me, so I don't know where they set it. Past experience with Firestone is that most stores just adjust it until it's barely within spec.



Some thoughts and questions:

- I will see if another Firestone can work on the alignment to improve things. I've read that negative camber and toe-in will help. Should I have both done at once or is there one that should be tried first?

- I've read that I need to increase pressure with the lower profile tires since their load rating is lower. With the rears at 35psi the tramlining is a lot worse than at 30psi. The load rating of the stock size is 1653lb. The new size is rated 1512lb. Do I really need to raise the pressure?

- Does the relative diameter of front and rear tires affect tramlining? I've gone from having rears 1" larger than fronts to rears slightly smaller than fronts.

- Could part of it be having different types of tires front and rear?

- Does the Pilot Sport A/S tramline worse than other tires?

 

i think by decreasing the aspect ratio of the rear, the rear becomes a little more responsive to road imperfections. at the same time you probably will get much better cornering response.

if you are most concerned about tramlining or wandering, in the rear i'd go with a smidge of negative camber (no more than a degree) and zero or slight (1/32-1/16") toe-in. excessive negative camber will actually make tramlining worse.

in the front you can go with a smidge of negative camber (i would not go zero camber on the fronts) and about 1/16" of toe in.

the small or zero amount of camber and the slight toe in should stabilize your vehicle a little. realize that camber and toe will accelerate wear. also realize that with aligning for stability you sacrifice responsiveness.

tire pressures - the higher tire pressure in general will make tramlining worse, but i would change the alignment before you fool with tire pressures. set all 4 the same the frist time around (try 32 and work from there).

i'm not sure if this is a huge factor, but by lowering your rear you've also increased your caster angle, which actually should help the tramlining.

as for the tires themselves, i don't like having different front/rear tires (brand/model) since key vehicle dynamic behavior is governed mainly by the "balance" in response between front/rear. having the same tires (but maybe different sizes) usually removes this unknown. - but others may have differing opinions on this.

another piece of friendly advice - ALWAYS request a before-after printout. although it doesn't guarantee a good job, it gives you some assurance.

 

oh, here's some good reading:

http://www.tirerack.com/tires/tirete...tramlining.jsp

 

My plan was to replace the fronts once I got here. But now I'm concerned that it's throwing good money after bad. If I end up having to change the rears back to 40 series then I will probably go with cheaper tires since I'm already out a bundle on the Pilot Sports. Cost me about $900 for the rears. Not including the extra night in a hotel and extra food costs while waiting for the tires.

(Am I wishing I'd just shipped the vette? Yep.)


I always get one, and have never had to ask ahead of time, but this time they didn't have it. Which is one reason I'm not going back to that Firestone.


Going to see if alignment can help first, since that doesn't cost anything but time.

Thanks for the help.

 

the problem was probably always there and now that the tires are mis-matched it is being exaggerated .... another thing to consider is air pressure ... lower front pressure could reduce the tramlining you are experiencing ... the tire diameter would not effect the alignment or tramlining .... you may want to get matching front tires to even things out abit

 

Luke, are you suggesting to run the front air pressure LOWER than the rear tire pressure OR lowering the pressure on ALL four?

Do you have a recommendation on how much?

Thanks,

Jake

 

what pressure are you running now ???

 

Never had any significant tramlining before.

Running 28/30 currently. Rear pressure affects the tramlining significantly. Front pressure doesn't seem to affect it much.

Replacing the tires definitely affected the alignment. Car gets aligned regularly, and felt great before the tires were replaced. 1500 miles later it was "way out" according to Firestone. As soon as it was aligned the tramlining got a lot better. C4s have to be aligned after lowering the rear, don't they?

 

the alignment is not effected by tire diameter but, you do need to re-align after any suspension changes .... tires do not alter suspension geometry .... maybe some worn bushing or something??? I do all of my alignments without the wheels even on the car ...

the reason the rear tires are effected more by air pressure changes is that the tread is deeper and you are getting more tread squirm on them than the front tires. I bet they will feel alot better in a couple thousand miles

 

I'm running 28 in the front and 30 in the rear too.

Jake

 

that sounds pretty close to right .... I would put the front to 30 as well as the rear

 

that sounds pretty close to right ....but, I would put the front to 30 as well as the rear

 

Okay, I'll give 30/30 a try.

Jake

 

Problem is solved.

Finally got it to another Firestone the other day, and discovered that I had about -1/4o camber on each side.

When I reinstalled my camber brace several weeks ago (had left it off after WP/opti change) I forgot to shim it, so it was giving me the negative camber.

Took the brace off, since it makes the car too stiff for Pittsburgh roads anyway, and about 90% of the tramlining went away.

Replaced the now-unevenly-worn front tires, and now it's about 99% gone. There is still a little grabbiness on seams, but nothing out of the ordinary for a sports car with fat tires.

Interesting that the negative camber produced no noticeable tramlining with the old 275-40-18 tires. It was the combination of the camber and lower profile tires giving me the problem.

That first Firestone (Bridgeville PA) was about as incompetent as they get. They tried to tell me the tramlining was just the way Pittsburgh roads are (even though it started in Colorado...) They told me it was those darn Michelins - that Corvettes have to have Goodyears on them to work right. They aligned the car with toe out on one side and toe in on the other. They inflated the tires to 38F/50R.

The second Firestone (Bethel Park PA) was great. Clean shop, nice people, matched Costco price on some tires for my other car, didn't try to tell me my tramlining was normal, etc.