Can I run 18" fr and 20" rr wheels on GTS?

[Boxer12]

I know the only tire option is Michelin. Never-the-less, is there any reason I can't run (or should be discouraged) this setup, for track events, like [******]'s Venom?

[Luke@tirerack]

using staggered diameter wheels and tires on a Gen2 is a bad idea .... the shorter sidewall tires in the rear will create different deflection rates ..... those different sidewall deflection rates which will adversely effect handling

[Boxer12]

Luke, I don't understand your response, why can't I maintain the same aspect ratio with a 20' rear? BTW, I just got off the phone with John [******] and he said it would be ok to run 295/30R-18 front, 335/30R-20 with adjustable shocks (I plan to use Motons) to reset the ride height to balance it out. Do you disagree?

[Jay Herbert]

Luke, I don't understand your response, why can't I maintain the same aspect ratio with a 20' rear? BTW, I just got off the phone with John [******] and he said it would be ok to run 295/30R-18 front, 335/30R-20 with adjustable shocks (I plan to use Motons) to reset the ride height to balance it out. Do you disagree?

Luke knows his stuff

Trying to adjust for different tires sidewall flex via adjustable shocks is a tricky game and completely unnecessary. One more variable that anyone truly racing their cars does not (need to/want to) mess with.

Sometimes all you have to do is look to the real racers to find the answer, in this case, look at the Comp Coupes.... they have adjustable shocks, and guess what, same wheel diameter front and back:

Comp Coupe

Wheels:

Type and Material
Three piece BBS, forged aluminum centers

Size, Front
18x11 inches wide

Size, Rear
18x13 inches wide

Tires:

Size, Front
P305/645ZR18

Size, Rear
P345/690ZR18

[GR8_ASP]

Ah Jay but the Gen III's come with 19" in back and 18 in front. I would assume by the comment from Luke that changing to 18's all around would be a bad idea. Not withstanding the fact that Comp Coupes use 18's all around on the same suspension sans shocks/springs.

Leaves me confused why in one case it is okay but the converse is not.

[Boxer12]

Jay, I am not privy to all the reasons that CC is built like it is, but tire supplies and other such issues come into play, like cost, etc. Sometimes, it is just a matter of simplicity. It looks to me like the CC has a taller aspect ratio tire in rear, accomplished via tire dimension rather than wheel.

What is the point of $5K shocks if you can't do some other things to try to get a competitive advantage? or even if you just like the looks of the setup with bigger tires in the rear? I respect Luke's opinion, but it is predicated on shorter sidewalls. The tires I was looking at have the same aspect ratio, both 30's.

It just so happens that my Testarossa tires, recommended by Tire Rack, have a shorter aspect ratio in rear that front. It's not a problem at all on that car, in fact, its a factory spec.

Here is the link to Venom specs: http://www.caranddriver.com/article....;page_number=3 Check out the wheel & tire combos, it is where I got the idea.

[Jay Herbert]

Jay, I am not privy to all the reasons that CC is built like it is, but tire supplies and other such issues come into play, like cost, etc. Sometimes, it is just a matter of simplicity. It looks to me like the CC has a taller aspect ratio tire in rear, accomplished via tire dimension rather than wheel.

What is the point of $5K shocks if you can't do some other things to try to get a competitive advantage? or even if you just like the looks of the setup with bigger tires in the rear? I respect Luke's opinion, but it is predicated on shorter sidewalls. The tires I was looking at have the same aspect ratio, both 30's.

It just so happens that my Testarossa tires, recommended by Tire Rack, have a shorter aspect ratio in rear that front. It's not a problem at all on that car, in fact, its a factory spec.

Here is the link to Venom specs: http://www.caranddriver.com/article....;page_number=3 Check out the wheel & tire combos, it is where I got the idea.

Any combination of bits and pieces is tunable, but why go to all the extra work, do you want to pay someone to do the chassis tuning, that's what chassis dynamometers are for.... and chassis dyno time is expensive. Archer has a shock dyno, another piece of the chassis puzzle. Are you planning on just fun weekend club racing, or competitive Viper Days racing? As you say the latter has tire/wheel rules for both cost and safety reasons.

The real question is what competitive advantage would you expect to get with this wheel tire combo? Better grip? Less sidewall flex? More tunable suspension (geometry)? Tires and wheels are only components of the total puzzle. The $5000 shocks allow you to tune both jounce and rebound, but don't forget about springs, sway bars, caster, camber, tire pressure, (the list is virtually endless)...... To get a real competitive advantage all these and more are tweaked via much testing for any given track. That is just the suspension tweaking.

Both you and GR8 hit the nail on the head, your Testarossa uses them as factory spec, the suspension was tuned for them. Gen II Vipers could likely be tuned for such a combination, but the effort would unlikely be worth it. And again, the real question here is, "What would the advantage be?"

Just my 0.02 cents worth.

I can't read Lukes mind, but I'd guess his comment is spot-on meaning it is not factory spec on the Viper, and to make it work in a real racing environment, (not some one-shot magazine test) would be a heck of a lot of work.

[Luke@tirerack]

The tires I was looking at have the same aspect ratio, both 30's.

but with the 30 aspect ratio it is easy to see that 30% of 295mm (88.5mm) is less than 30% of 335mm (100.5mm) so, the sidewall heights would be different and thus would have a different deflection rate (rear tire sidewall will flex more than the front)

[Boxer12]

Jay, What I am trying to do is reduce oversteer. Archer will be doing the tuning, and to me putting a unique puzzle together (I call it tuning) is part of the fun of racing...which initially will be against the clock. It's the added dimension beyond racing school (done that) and seat time. I want to learn more about the suspension tuning component of racing now, and am looking at what some other guys are doing like the H-Venom. You advice is good for most, but for me, 'leave well enough alone,' that is a nonstarter. I am not satisfied with the stock performance. I could have just bought an '06 Coupe and got better handling, presumably, but that takes the fun out of it for me. I like the challenge, but I also like the input of as many others as I can get, am not married to a particular plan. You know, the prototypes in racing always blow away the factory stock racers, so why would anyone say that factory spec is the best setup? It's less hassle to just go with the factory setup, but so is it less hassle to just keep your car in 3rd gear and go around the track than to shift, or using an automatic versus a manual. I like to try to think outside the box. I think that is what guys like Archer or [******] do and I admire that ingenuity. Scientists say that bees CANNOT fly, so theory has to be tempered with experience. Sometimes its trial and error to get the best setup. I guess that is my mindset, if it makes sense. I would have just bought another Porsche or Ferrari if I wanted the fasted lap times I can get right out of the box. Oh yea, drove that 911 for 13 years and the Ferrari for 5. Got boring. Bottom line for me, it gets boring just going around the track looking to shave tenths off my time with driver's input. I desire the added challenge of tuning, although I am a novice at it. Also,I just love talking cars, so be careful asking me to explain myself, unless you want an earful...LOL

[Boxer12]

Luke, I think I understand what you are saying. I note, however, that the stock tires are 275/35 and 335/30 which is not the same deflection rate either, a little more in the rear. It looks like the tires I propose would have a greater deflection disparity than stock, although same widths with a 35 sidewall in fr would reduce and reverse that diff (less than 4% and greater deflection in fr...don't think you want that). I don't understand the CC tire ratings, so can't compare that. Can't you simply account for deflection rate by adding tire pressure in rr? (Hope you don't mind me picking your brains...) Jim

[Jay Herbert]

Jay, What I am trying to do is reduce oversteer. Archer will be doing the tuning, and to me putting a unique puzzle together (I call it tuning) is part of the fun of racing...which initially will be against the clock. It's the added dimension beyond racing school (done that) and seat time. I want to learn more about the suspension tuning component of racing now, and am looking at what some other guys are doing like the H-Venom. You advice is good for most, but for me, 'leave well enough alone,' that is a nonstarter. I am not satisfied with the stock performance. I could have just bought an '06 Coupe and got better handling, presumably, but that takes the fun out of it for me. I like the challenge, but I also like the input of as many others as I can get, am not married to a particular plan. You know, the prototypes in racing always blow away the factory stock racers, so why would anyone say that factory spec is the best setup? It's less hassle to just go with the factory setup, but so is it less hassle to just keep your car in 3rd gear and go around the track than to shift, or using an automatic versus a manual. I like to try to think outside the box. I think that is what guys like Archer or [******] do and I admire that ingenuity. Scientists say that bees CANNOT fly, so theory has to be tempered with experience. Sometimes its trial and error to get the best setup. I guess that is my mindset, if it makes sense. I would have just bought another Porsche or Ferrari if I wanted the fasted lap times I can get right out of the box. Oh yea, drove that 911 for 13 years and the Ferrari for 5. Got boring. Bottom line for me, it gets boring just going around the track looking to shave tenths off my time with driver's input. I desire the added challenge of tuning, although I am a novice at it. Also,I just love talking cars, so be careful asking me to explain myself, unless you want an earful...LOL

If Archer feel he can do it, go for it. Archer has made some very fast racing Vipers. He knows racing and has the tools and technicians to properly execute. Be sure whatever wheel you buy it is safe for racing. BBS, Forgeline, and CCW have wheels that will not break (on Vipers) when raced hard on road courses.

As for the other fellow. He is a salesman, he'll price it, sell it, and historically maybe, or maybe not, be able to deliver it. Ever see that Venom Dragster? Ever see that Venom Supercharged car?? The fellow that bought the first "Automatic" is still owed a legal settlement of over $120K... He was told (paraphrasing) "We can do it, no problem"..... Sorry, I personally have another name for it, and it's not "ingenuity".

With Archer, you will be successful.

[Luke@tirerack]

Luke, I think I understand what you are saying. I note, however, that the stock tires are 275/35 and 335/30 which is not the same deflection rate either, a little more in the rear. It looks like the tires I propose would have a greater deflection disparity than stock, although same widths with a 35 sidewall in fr would reduce and reverse that diff (less than 4% and greater deflection in fr...don't think you want that).

well, let's look into the facts on the OE tires

275/35-18 = 25.6" in diameter
96.25mm sidewall which = 3.79"

335/30-18 = 25.9" in diameter
100.5mm sidewall which = 3.95"

a total sidewall height difference of 4.25mm or .16"

your proposed tire sizes

295/30-18 = 25.0" in diameter
88.5mm sidewall which = 3.48"

335/30R20 = 28.0" in diameter
100.5mm sidewall which = 3.95"

total sidewall height difference = 12.0mm or .47"

the second setup has .31" more sidewall height so the tires which will allow more deflection than the OE setup which will increase oversteer.

You stated you are trying to reduce over steer and what you are proposing will increase it so, it will put you further away from your stated goal.

and we have not even mentioned the gearing changes and possible ABS/speedo issues because of the three inch diameter difference

I would recommend looking into adjustable dampers and sway bars before changing tire sizes

[Boxer12]

Luke, that does clear it up. Not to beat a dead horse, but...is there any tire combo that will give appropriate (let's say similar to OEM) deflection rates on an 18 or 19 fr and 20 rr? BTW, its a non-ABS car. Something in rr with shorter sidewall? Maybe a 345/25YR20 (86.25) rr and 295/30ZR18 (88.50)Pirelli Assimetrico?

[GTS Bruce]

Reduce oversteer in a Viper?The poster boy for understeer or push?Dictionary picture of a viper or pu truck.I think most of us are tuning to reduce understeer and get a little oversteer or loose.Have never experienced oversteer or loose in my viper unless it was throttle related in a tight corner and on a slippery surface.I think most of us are looking to get some with eibach springs,shock,front tire section,sway bar and tire pressure adjustments.Possibly we are not speaking the same language.Do you mean oversteer under trail braking due to lack of good brake bias in non-abs cars?Then again they don't have enough rear bias so they push under braking while turning. GTS Bruce

[Boxer12]

Understeer or oversteer depends on driver style. Let's put it this way, and keep it simple, I want to reduce the tendency of the rear to spin out on corners. Jim