Wheelbase, weight bias, and overall weight vs. tire size

[TQuillen]

For a car that is going to see autocross use along with extensive street time, how should wheelbase, front/rear weight bias, and overall vehicle weight affect tire width choice?

More specifically, do those three variables influence how much difference in width the front of the car should have versus the rear?

I've got a 1974 Grand Prix that has a fairly unique set of circumstances, in that it is overly heavy (currently 4300 lbs and change), but potentially has better weight bias than other 73-77 GM midsize cars in that it has a 116" wheelbase, instead of the typical 112". This wheelbase increase is accomplished entirely between the firewall and the crossmember. in other words, the engine's harmonic damper is almost inline with steering linkage. This makes the front end sheetmetal heavier (this hood HAS to go), but effectively moves the drivetrain rearward relative to front axle centerline, which should improve front/rear bias somewhat. However, I'm not entirely sure how wheelbase effects handling (It appears at first glance that in any situation other than tracking a straight line, it's going to be hairy LOL).

All of these items are swirling around in my brain, because the time for tire width selection is rapidly approaching. I'll likely run a 17" wheel front and rear; this is dictated by financial constraints as much as anything. I've got tons of room for tires on this thing. I currently have a 275/60/15" tire on the car, with 1.25" of space to the inside, and 2" to the outside while remaining inside the factory fenderwell; a rough tape measure guess says a 315 is an easy fit, with a 335 being very possible. I can fit a 275 width in the front with ease, and possibly a 285 or even a 295 with some minor changes.

I realize that the fact that this isn't a dedicated track car will change what tire I end up with (which is one of the reasons I'm asking these questions). Should I put a 275/40/17 on it on all fours and call it a day? I've read a lot of threads on here about a general "no more than 40mm more width in the back" rule, but I'm concerned that the weight of this car and the wheelbase are really going to make it understeer more than normally expected if I get carried away with tire width on the rear. I've not changed the sway bars or spring rates on the car yet, so I know I can tune a little of that out, but I don't want to get myself painted into a corner here.

Am I overthinking this? I'm coming from a 100% drag racing background, so I'm swimming in uncharted waters here and relying on the experience of others to guide me. I don't have a ton of money to make mistakes with, so if there's a path of "less risk", I'm all for that as well.

Thoughts?

Thanks guys.

Travis

[stab6902]

Conventional wisdom is to run as wide of a front tire as possible for autocross/track performance, especially with a big heavy car. The downside to 275mm+ wide front tires in my experience is that you'll probably get some noticeable "tramlining" (the tendency to follow ruts or grooves in the road) on the street. You can tune some of this out with alignment settings, tire pressures, etc, but it never really goes away. I find it annoying to constantly have to correct the steering when I'm cruising down a back road or highway, but maybe I'm just getting old and soft.

Some other comments:
- These days 18" wheels and tires are often more common and affordable than 17"
- 275/40R17 tires will be much shorter than the 275/60/R15's you have on the car now (25.7" vs 28," check out https://tiresize.com/comparison/ or similar). I think big cars look better with bigger tires, but of course that's up to you.
- If you run the same tire size all around, you'll have the ability to rotate them, which will save you money if you autocross frequently. I always tended to wear the fronts out much more quickly than the rears.
- If you're running big power, you'd probably see benefit from bigger rear tires. I think the no more than 40mm wider guideline is sensible. I'm a fan of staggered wheels/tires for looks, but if you're on a tight budget a square setup probably makes more sense.
- Once you figure out a size and budget, ask around on here for tire make/model advice. Tires really set the direction for the rest of the suspension. I see way too many people build an otherwise high dollar car and slap on some Nankangs or Lifans or whatever they can get cheap. Be smart about where you put your money!

[TQuillen]

Thanks Ryan! My plan has been to run a steel 17" wheel and have it widened, if need be. It could well be that I can get into an 18" wheel for similar money, so I'll take a look at that. I'm definitely going to spend some money on tires....I have found a great number of performance tires (W rated 200-399-ish treadwear) in a 17" diameter that fit my budget, so I'll compare the 18" to those. In my mind, everything revolves around the tire selection. I don't know if this is the correct approach or not, but if I read your post right, I think maybe it is?

As for big power, it's going to be around 500hp at the crank for now, with room to grow to 800-900 when I get more time. So it's sorta on the low end for power for now.

This car is uncommon (not necessarily in a good way LOL). I'd be more than surprised if there are very many members that have used one to go around corners. It's far, far different than the other 73-77 A-bodys. Wonder how I can find out?


Travis

[stab6902]

In my mind, everything revolves around the tire selection. I don't know if this is the correct approach or not, but if I read your post right, I think maybe it is?


In my opinion/experience, yes, the tires definitely set the tone for the suspension, if not the whole car.

[NOT A TA]

I strongly suggest considering 18" wheels. Tire choices in 17" have been getting slimmer and if/when you go to put some real brakes on to haul that thing down you'll find you have a lot more options with 18" wheels than with 17s.

[TQuillen]

I strongly suggest considering 18" wheels. Tire choices in 17" have been getting slimmer and if/when you go to put some real brakes on to haul that thing down you'll find you have a lot more options with 18" wheels than with 17s.

I've been looking at that very option. It's I real fight with this car....Camaros and Novas are easier, because the wheel wells are limited (with the stock frame, anyway), and they are smaller cars, so the balance of proper tire/wheel geometry and filling the wheel wells up is much easier to achieve! This thing could fit a 35" mud tire on the back LOL. I'm also trying to keep the cost down somewhat. Wheels get fairly expensive when you get into the wider 18" selections. It's going to be a compromise between financial constraints and performance, but I'm OK with that. If I wanted a real deal track car, I wouldn't be building this car to begin with.

As for the brakes, I've been researching lots of options, and it appears that I can get into the bigger LS Camaro or even C5/6 brakes for not a lot more money than the LT1/B-body 12" stuff. I does help that I have a buddy who has a lot of the components I need. That's where the 18" wheels start to make a lot more sense.

The bottom line is that this car is going to be so much better than it ever was, and will handle better than a lot of new cars out there. And to say "it's going to be different" is an understatement. Thanks for the insight!

Travis

[TQuillen]

Well, Since I've been down in my back for a while, I had the day to really take a hard look at wheel and tire selections.

I'm officially between a rock and a hard place.

The tire selections in a 17" tire size aren't very good if you're looking for something 27" or so tall, and relatively wide (275-315mm). However, I can buy the soft 8s and have them widened to suit the dimensions of the car quite nicely, and for not a ton of money overall.


If I look at 18" tire sizes, the tires in the widths that package well are either unavailable in a tire that can be street driven with any regularity (drag radial or R7 types), or they are unbelievably expensive (That Pirelli 285/45/18 P Zero is a real dandy LOL), or require a wheel width that drives me into a wheel that is a good bit more than the budget can stand. My world, for an 18" Soft 8.

My brain is telling me that I may have to drop back to the 17" tire diameter, and just deal with the aesthetics of a shorter tire. It will definitely hurt my ability to cram big brakes on it, and may well force me into a corner in terms of tire availability (as if it needed to be any worse than it is now). Sean Rich's 73 Chevelle sure did make it work.

Danged, the compromises.

Advice, observations, encouragement, jeers, jokes, and name calling are all welcomed at this point. LOL

Travis

[stab6902]

How about some 275/40R18 tires of your choice on some 18x9.5 wheels like these: https://www.summitracing.com/parts/vsw-s0183/overview/

Those wheels (roughly $140/ea) would be much cheaper than widened steel wheels. Heck, they're almost the same price as un-widened soft 8's ($120/ea), and probably lighter.

[TQuillen]

I was looking at the US Mags equivalent, because I was terrified of the Vision brand wheel. And, the 275 width, while really nice on the front, sorta looks small on the back. But, I may end up going that direction out of a lack of better options. That tire can run on a 9.5" wheel, but it's the very bottom of the recommended range. Does that pose an issue, or is it not a big deal? When researching wheel and tire fitments, I've been trying to keep the tire size matched to the wider end of the wheel range that they recommend. I may be making too big of a deal about that, since I know no better.

I've not run Vision stuff, so I have zero first hand knowledge of what they're like. I know where's it's made, one of the lead engineers is a long time friend of mine, and the warehouse is about forty minutes from me. Reckon that stuff can take a 4300 lb car in the slalom at an autocross event?


Travis

[2yellow69]

Just a quick note on running Soft 8's with C5/C6 brakes.

I have ATS tall spindles with factory C5 brakes and the Soft 8 hits the caliper abutment. It's not even close!! I tried with some 1/8" spacers and it still won't allow me to even tighten the lug nuts all the way. I just ordered Rocket Racing Wheels Attack in 18x9 for $445 each. I know that's 4 times what you were going to spend but they have them up to 11" wide in stock. They have a thread going just below this one.

[stab6902]

They usually recommend 9-11" wide wheels with 275/40R18's. I most commonly see them on 9.5 and 10" wide rims; I see no reason to go any wider than that unless you're going for a certain look.

I've never owned any Vision products, but I do have a set of US Mags wheels, which are made in China like all other wheels in this price range. Many wheel brands actually come out of the same factories over there. I couldn't find any quality flaws with the US Mags wheels visually. I've never heard of any failures with the Vision wheels, and lots of people on here run them since they're so affordable. Your engineer friend would probably know the most - please ask and share what he has to say! Would you be better off with some US made forged rims? Of course, but you're looking at 5 to 10x the cost.

[TQuillen]

Just a quick note on running Soft 8's with C5/C6 brakes.

I have ATS tall spindles with factory C5 brakes and the Soft 8 hits the caliper abutment. It's not even close!! I tried with some 1/8" spacers and it still won't allow me to even tighten the lug nuts all the way. I just ordered Rocket Racing Wheels Attack in 18x9 for $445 each. I know that's 4 times what you were going to spend but they have them up to 11" wide in stock. They have a thread going just below this one.

THANK YOU for that. I had an idea that the big brakes wouldn't work with the 17s. I've read that the LS Camaro stuff clears, but who knows how accurate that info is. I do know that the Wilwood 13" stuff does clear the 17x9 Soft 8.

I was intending to spend $250 a wheel. Like everyone else here, I've got a set limit that I can spend. But I'm not willing to cheap out either, if a little more money buys me something. $450-500 a wheel is just out of my reach right now, with other things that I've got to do to this car. But, if that's absolutely what's gotta happen to make this pig do what I want it to do, then I'll let it sit a few more months until I can scrape the money up.

TQ

[TQuillen]

They usually recommend 9-11" wide wheels with 275/40R18's. I most commonly see them on 9.5 and 10" wide rims; I see no reason to go any wider than that unless you're going for a certain look.

I've never owned any Vision products, but I do have a set of US Mags wheels, which are made in China like all other wheels in this price range. Many wheel brands actually come out of the same factories over there. I couldn't find any quality flaws with the US Mags wheels visually. I've never heard of any failures with the Vision wheels, and lots of people on here run them since they're so affordable. Your engineer friend would probably know the most - please ask and share what he has to say! Would you be better off with some US made forged rims? Of course, but you're looking at 5 to 10x the cost.

I had my suspicions about US Mags, US Wheel, and a few others that I have been looking at. I'll call my buddy and ask him what the deal is...he'll be straight up with me. He's been there for a long time, and has made a LOT of trips to China. LOL

One other thing.... I really, really appreciate you guys taking the time to walk me through all of this. You have no idea how much. I've done nothing but drag race for the last 25 years, and I've been building race cars full time for the last eighteen years. The thought processes are a lot different in a lot of ways! The wheel, tire, and suspension setup is where I'm lacking. Engine building, EFI, turbos, blowers, fabrication, wiring, and plumbing are things I can handle. This stuff is new to me, so it helps to have people who have "been there, broke that" in my corner. Thanks.

Travis

[JustJohn]

I have a 119" wheel base and a Chevy B-body...
I won't be harassing cones but do want something that drives like a GT car. I currently have C6 Z51 brakes and will be mounting C6 wheels. They fit with clearance to spare and (obviously) clear the brakes. Bought them for $350 but it is a 18"x8.5/19"x10 staggered set of wheels.

[silvermonte]

Im going to go with the rest of these guys and recommend 18" wheels. I bought 17s a few years back and the tire selection is getting slim. As a way to save some money you dont really need the best tires out there off the start. If you are only going to autocross and daily drive get something cheaper for your first set, just so you can afford the rims and then on your next set of tires get something possibly better. Your rims will be a more permanent part of the car then any tire ever will be so spend the money on the right size the first go around.

[TQuillen]

I have a 119" wheel base and a Chevy B-body...
I won't be harassing cones but do want something that drives like a GT car. I currently have C6 Z51 brakes and will be mounting C6 wheels. They fit with clearance to spare and (obviously) clear the brakes. Bought them for $350 but it is a 18"x8.5/19"x10 staggered set of wheels.

Any idea what the backspace is on those? I had not even considered an OE wheel.....

[JustJohn]

6.45 front/8.1 rear.

I'm using hub centric adapters.

[TQuillen]

I had a good conversation with my friend at Vision. Here are some of the highlights.

- The wheel industry is not a situation where there are only a few factories in China that manufacture OE/aftermarket wheels. There are nearly two hundred that he is personally aware of. As with everything else in China, these manufacturers are not all the same. He made the statement that some of them are extremely nice, with modern CMM equipment, CNC machines, and on-site metallurgical testing. Others are small buildings with dirt floors, no process controls, and very little in the way of measuring equipment. One in particular that he mentioned utilized a single work light that the employees used when running specific machining operations. When they got done with a batch, they would move the light to the next operation.

- All of the material that is used in aftermarket wheel manufacture supposed to be A356. Some manufacturers use material that comes from ingot with material certs, while other wheels are made from ingot as well as recycled material in the form of metal shavings, rejected wheels, etc. This is where a lot of the issues arise; he told me that the most important thing that they watch for in random material checks of finished wheels is iron. Iron contained in an aluminum cast wheel leads to fracturing, and it is something that cannot be detected or tested for without destroying the wheel.

- My friend said that there are currently only three plants in the US that produce cast wheels, and that all three produce only OE wheels (to his knowledge). He also stated that a number of companies have tried in the past to bring that manufacturing back stateside, but two major factors have kept those attempts from being successful: corporate taxes and environmental regulations. The corporate tax rate in China is 15%, where it is now 25% here in the states. He was actually pretty optimistic that some wheel manufacturing could come back stateside, due to how trade relations are changing.


Now to the nitty gritty. He told me point blank that he would not trust a non-forged, purely cast wheel from any manufacturer on an application as heavy as mine where the car might see autocross or track use that induced big side loads on the wheel. A lighter car might get away with it, but 4300 lbs, no. He also added that the larger the wheel diameter, the worse this situation gets (which makes complete sense). He did indicate that there are a number of OE wheels that might appear to be cast, but have some post processing done to them that makes the material significantly stronger (he may be referring to hot isostatic processing, I'm not sure). He said that in an application like mine, a forged/semiforged wheel would be best. He did indicate that some cast truck wheels would work great, and still be a whole bunch lighter than a similarly sized steel wheel. He told me that Vision has cast wheels on trucks that run Baja, and have never had issues.

So I'm likely going to just put that portion of the project on hold, see if I can scrape up some money, and buy a wheel that won't come apart.

Travis

[stab6902]

Thank you for sharing Travis! That's the most legit information I've read regarding the Chinese wheel industry.

[andrewb70]

Have you looked at the new Rocket Racing 18" wheels?

Andrew

[TQuillen]

I have. Very nice wheel! They're definitely one I will be looking at when I get to that point. Although, I will say that this big car might not look as good on a spoke style wheel as a smaller car like a Mustang, Camaro, or Nova. You know it's bad when I think of a '70 Chevelle as a small car LOL!

Travis

[andrewb70]

Travis,

The other issue with using steel wheels is that they are very heavy in 17" sizes. Even on a heavy car, you want to keep the unsprung weight as low as possible, or as low as your wallet allows.

I put 18" wheels on my 1970 GTO back in 2002. I'm pretty sure my car was the first A-body, that was in a magazine, with 18" wheels. Some people here mentioned here that wide tires tend to tramp, but I have not experienced this at all. I run 275/35-18 front tires on 9.5" wheels, and it drives perfectly straight even on grooved roads. I think alignment is important, and also low wheels weight. My wheels are about 20 pounds.

Andrew

[TQuillen]

Andrew -
Steel wheels are definitely the last resort. I've got a laundry list of other items that I need to get done, and a lot of those items are things that are relatively inexpensive to do, or stuff I already have the parts for. Front end rebuild. Big Ford bearing housing end install and axle swap. Body bushings. Engine swap (keeping the Pontiac, I've got a new bullet sitting on a cradle waiting on me). Tons of general cleanup. Interior work (which could well keep me busy for years to come). Door pin/bushing install. So, I've got plenty to keep me busy until I can swing a set of wheels. I just need to figure out which ones. Looking at 18" tire selections in the widths I'm interested in running, the situation really isn't all that much better than the 17s, from what I can see. Tiresize.com shows four 18" tires that will work (275/40/18, 275/45/18, 285/40/18, 295/35/18). In the 17" tire, there are also four (315/35/17, 285/40/17, 275/50/17, 275/40/17). The 17" tires are roughly $75-$100 per tire cheaper than the 18", for a given tire. So, it reallly comes down to brakes, and I fully understand that. The necessity of 14" rotors on a 3100 lb Camaro is one thing, but on a 4300 pounder, I'm gonna need all the help I can get! LOL. But, budget will have to play a part in this. It has to. Not just on the initial cost of a wheel nd tire package, but on the recurring costs of tires. I drive my stuff a LOT on the street, and if I start running this thing at the track on Friday nights and autocrosses five or six times a year, I'm gonna be buying tires. I absolutely must keep it all in perspective.

Thanks again for all the insight!

Travis