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Hello all! Any C8 owners have any experience going up in tire size in the front on their Z51 cars? I plan on fitting RE71R's. Sadly only 295's are available in the rears, but contemplating going up to 255//35/19 or... 265/35/19 in front. But want to avoid any rubbing at full lock. All feedback is most appreciated!
With the rear tires being narrower (295 is narrower than 305), do not widen the front tires--unless you happen to love understeer.
Wider fronts, narrower rears, more grip in front --> oversteer, not understeer. May be OK for tight autocross, not so much for high speed or road courses though.
Last edited by AzDave47; Apr 19, 2020 at 10:48 PM.
It might work, or it might not. Everything else being equal, you could end up shifting the balance too far towards front grip, or you might not. The only way to find out is to try it. It could be an expensive mistake, or you may become the new C8 handling guru.
1 hour Autoline intervow video, said it was his biggest
Originally Posted by MitchAlsup
With the rear tires being narrower (295 is narrower than 305), do not widen the front tires--unless you happen to love understeer.
Originally Posted by AzDave47
Wider fronts, narrower rears, more grip in front --> oversteer, not understeer. May be OK for tight autocross, not so much for high speed or road courses though.
Hmm, having driven 60% rear weight Corvairs for ~10 years, 1st swing axial pre '65s then my great BUT different handling 1st new car a modified 1967 with parallelogram rear suspension; this is my experience. With both, the average owner did not know how to manage a high speed skid IF going too fast for a turn.
Provided a list below from the Net that fits my experience on what can be done to "reduce" Oversteer.As noted, to reduce Oversteer you could install narrower front tires. Below I'll cover what I did and what Tadge said about why his biggest fear with the C8 design was would they be able to eliminate Oversteer! He, Andy Pilgrim (former Corvette Team racer,) and others say they achieved their objective!
SIDEBAR (Only for those who can deal with more than 280 character communications!)
There are many factors determining oversteer and understeer. To counter the inherent Corvair Oversteer GM specified 16 psi pressure in the front tires for all years. Few owners, gas stations (who back-in-the-day checked -and adjusted if needed- tire pressure while they pumped gas) and even Chevy dealer techs followed that requirement and often used the typical 30 psi on all 4. That caused some of the driver issues even on the '65 and later Corvairs!
First thing I did with the '67 was dump the Uniroyal "Tiger Paws!" I joked that GM must have put those on to give an early warning to the average Corvair owners who had no idea how to deal with a 60% rear heavy car! They "growled" as you made a turn at ~60% of the max lateral "g" force. Went through several tire brands including Pirelli Cinturato 72s (if I recall the name correctly.) Those skinny 13 inch tires were some of the first radials. Great in the ran, my objective in buying, but not best for dry handling. Then found Continental 714 high performance radial tires at the NY Auto Show and their published info and Show rep said "had to have those." They were low profile when that wasn't a "word used." But they only came in 14 inch minimum size so had to go to Plus 1 (another "word" not used at the time!) Took lots of measurements and found custom wheels for a Chevelle that had the right offset! Great tire in wet and dry.
To provide additional help with Oversteer GM used an unusual rear rear toe-in spec, 1/4 to 3/8 inches! I set it up with the max 3/8 inch toe-in! A cheap form of rear steer! In a high "g" turn the highly loaded outside rear wheel was trying to steer back into the center of the turn! My '67 had every HD option offered that year including quick steering, HD springs, sway bar and metallic brakes. I added headers, carb mods and finned (+ internal pins) aluminum oil pan and valve covers to help control oil temp (which helped but it still got very hot!)
Great handling car. But at the limit, when going to fast for a turn, you had a split second to steer like a dirt track driver to avoid spinning off the road, rear first! If you turned the wheel as if in a skidding FE car, even for a split second, you'd be off the road very fast, rear first! When testing the maximum turn speed at the limit, needed to counter steer. It was actually fun and you could anticipate when it would be needed! Porsche's of that era had the same issue! In fact Tadge said his dad was a military pilot and drove his Porsche at the limit! Tadge said he knew "all about oversteer" in a rear heavy car. In the ~I hour Autoline interview video he said "Oversteer Was His Biggest Fear with the C8 Design! In another written interview he said "It took Porsche decades to correct the issue and they (GM) had to get it right the first time!" He only gives a few clues of what they did or what Porsche gradually found over that long time to make a ~63% rear weight car into a great handler! He did give a clue in that Autoline Interview.
BOTTOM LINE
GM spent a lot of time and engineering effort with the C8 to achieve a slight amount of Understeer so the average Corvette driver, if going too fast for a turn, can manage to deal with a skid. They give few clues as to what they have done. However tire size (brand/model) would be a key variable, as is tire pressure, sway bar and spring selection, etc. I'd be very careful changing what they designed without some knowledgeable input. Just like for Tracking alignment GM defined can be used.
Ways to Correct Oversteer (from Internet source that fits my ~10 years experience with Corvairs)
Shows the tire sizes GM says will be on the "wider" ASD Grand Sport or Z06 Hybrid (the article doesn't say which BUT since the ICE mentioned is the standard LT2, my thought is it's the Grand Sport.) Note top Pic is one I picked up from an other magazine "guess," just the data is from the leaked article! Body width is my estimate based on the tire size the article quoted and my Grand Sport over base C7. Bottom Pic IS from the leaked article!
On these modern cars, tires are more a fashion statement than a functional item.... If you want more performance, change the tire compound.... not the size.
(And BTW, anybody who experiences "understeer" in this car on the street is likely to either end up in a tree, or in jail)
That said, a wider tire in the front may cause a lot of unintended consequences.... scrub, tram-lining.... every tire brings a different set of construction and driving characteristics, and engineers do a lot of testing to match the specific tire to the suspension tuning and alignment.
Maybe it will work, maybe it won't but without serious testing, you may be doing more harm than good... who knows
Hello all! Any C8 owners have any experience going up in tire size in the front on their Z51 cars? I plan on fitting RE71R's. Sadly only 295's are available in the rears, but contemplating going up to 255//35/19 or... 265/35/19 in front. But want to avoid any rubbing at full lock. All feedback is most appreciated!
Thank you!
By your username and the question, can I assume that you're looking to autox your car? In SCCA SS class (thus OEM sized wheel width)? Have you thought about running 18"/19" so that you're not constrained by the 295 section width rear tire size for the RE-71R?
I have considered this. On my C7 I dropped to 18 /19 rims for my RE71R's. But decided to (for the time stick to the stock wheels). Will stay with the stock size up front, and run the current max size of 295 in the rear.
I have been running in SS / AS (C7) and now BS in my current 2020 M2Competition. The RE71R rules for the stock classes IMHO.
Thank you all for your feedback and input! Most appreciated.
By your username and the question, can I assume that you're looking to autox your car? In SCCA SS class (thus OEM sized wheel width)? Have you thought about running 18"/19" so that you're not constrained by the 295 section width rear tire size for the RE-71R?
I would like to go to 18/19 on RE71r but have not been able to find a suitable combo.
I would like to go to 18/19 on RE71r but have not been able to find a suitable combo.
My intention is to run the RE-71Rs in 265/35-18 front and 305/30-19 rear. Comparing rev/mile, that gives a factor of 1.04 (823/793). The OEM tire sizing gives a rev/mile factor of 1.0566 (789/747). The <2% variance shouldn't affect the stability control/etc.
My intention is to run the RE-71Rs in 265/35-18 front and 305/30-19 rear. Comparing rev/mile, that gives a factor of 1.04 (823/793). The OEM tire sizing gives a rev/mile factor of 1.0566 (789/747). The <2% variance shouldn't affect the stability control/etc.
I have the RE-11s on my C6 in those sizes. No issues with ABS, although the C6 stock revs/mile are different then the C8.
Not the OP's question but with the "somewhat anemic" rear tire width compared to my 335 Section Width Grand Sport was looking at Tirerack and found this interesting tire issue for the C8. For the rear tried going to 315 Section Width from the OEM 305 but went to a 25 Aspect Ratio. It was smaller in diameter. So searched for tires that were 325 Section Width and 25 Aspect Ratio. This is what I found:
OEM 305/30/20 Michelin Pilot Sport 4S are (based on 11" rims) = 12.3" Section Width; 11.2" Tread Width; 26.4" diameter;
325/25/20 Michelin Pilot Sport 4S are (based on 12" rims) = 13.2" Section Width; 13.2" Tread Width 27.2" diameter;
(Note the use of a wider rim for the measurements won't affect Tread Width with but will reduce Section Width perhaps 3/8 to 1/2 inch less.
The speedometer will be off 27.2/26/4 + 3%, i.e. it will read 3% lower speed so speedometer at 60 mph you'd be going 62 mph. Looking at the room around the rear tire it should fit, helped by being only ~1/2 inch Section Width difference.
This is for the front:
OEM 245/35/19 Michelin Pilot Sport 4S are (based on a 8.5 rim) = 9.8" Section Width; 8.6" Tread Width; 25.8" diameter;
265/30/19 Michelin Pilot Sport 4S are (based on 9.5 rims) = 10.7" Section With; 9.6" Tread Width; 25.3" diameter;
(Note the wider rim won't affect Tread Width but will reduce Section Width perhaps a 3/8 to 1/2 inch Section Width difference.
The speedometer will be off 25.3/25.8 = -2%, i.e. it will read 2% higher speed so speedometer at 60 mph you'd be going 59 mph. Have to check room full lock right to full lock left. In this case the Section Width will be similar, i.e. 10.7" with a 9.5" rim should Redcue to ~10.3" on a 8.5" rim or only ~1/2" larger.
Fixed some of the data in the above post. Curious on the result if anyone tries the 265/30/19 Pilot Sport 4S Front and 325/25/20 Rear tires. Looks like the fit should be OK!
I was buying MPSC2 tires and wheels for my C7, and the issue of F/R wheel diameter came up..... could not get a definitive answer, but Bill from Dearborn, and some others who seem to be pretty knowlegeble suggested that if the delta is outside of a +/- 2-4%, it could screw up the ABS....
I played around and got it to about 1.9%, and it SEEMS to work OK, but this is the kind of stuff you need to consider...these cars are so computerized, and there is not a lot of really hard info out there.
I was buying MPSC2 tires and wheels for my C7, and the issue of F/R wheel diameter came up..... could not get a definitive answer, but Bill from Dearborn, and some others who seem to be pretty knowlegeble suggested that if the delta is outside of a +/- 2-4%, it could screw up the ABS....
I played around and got it to about 1.9%, and it SEEMS to work OK, but this is the kind of stuff you need to consider...these cars are so computerized, and there is not a lot of really hard info out there.
Having followed posts on the C6Z for many years, the range seems to be 2-7% delta front/rear. Stock on 18/19s is 4.3% when new. With my drag radials I run 6.25% and have no problems. I have also run 295/35-18 and 325/30-19 (close to the 2%) with no warnings, but the 295/35s did rub a little at full lock and during road course events.
Front tire max width
