Wheel & Tire Weight and handling, difficulty choosing rim/tire combo

[64Chevelle]

Hi,

choosing the right wheel/tire combo seems to be one of the most difficult things.

I have 64Chevelle and I'm about to upgrade the front with ATS spindle and Baer 13" brake kit. So 17" is the lowest I can go with those brakes. And I don't like the look (or prices for that matter) of larger than 18" :hmm:. So it's either 17" or 18" for me. Please bear in mind that I want the best performance and handling for my money, looks are secondary.

I have some questions that I need an answer too before I decided for wheel/tire size.

- I read an interesting thread (https://www.pro-touring.com/forum/showthread.php?t=35886) about how the rim/tire weight affects acceleration, The largest amount of weight is at the outer part of the rim meaning that the Moment of Inertia will increase greatly for spinning the wheel when going to a larger diameter wheel. So acceleration and braking will be somewhat negatively affected, but the difference between a 17" and 18" will probably not be that great. But how much will the increase in unsprung weight for corner handling matter? Is it negliable?
- Let's compare a 15" to an 18". The 15" will have a higher aspect ratio tire compared to the 18" and a lot lower weight. Will the lower weight of the 15" or the higher stiffness of the 18" be better for handling? There's a guy here in Norway who run a 1966 Corvette and gives all the high performance newer cars like Audi's and Porsches a lot of trouble. He runs 15" wheels. Perhaps it will actually be better to buy smaller rims and invest more in the quality of the brakes (not size)?
- What width rims should I choose? I see it's common with 275 tires on 10" rims on american cars, but I check ebay in Germany and they usually put 245's on 10" rims. I have read a long thread about this before on this forum and the general rule was that tire width should optimally be equal or slightly less than the rim width. So what is the optimal rim width if I want to choose 245/275 front/rear?

A lot of thoughts and questions, but I hope some of you guys will chime in with some solutions and ideas.

[bbcc]

Very interesting questions. I to am in the painfull state of picking out wheels, and I've thought about exactly what you asked. Seventeens are my choice simply because of tire selection. Sixteens will do fine, but try and find good rubber for a fair price, its kinda hard. But I did some real quick and simple M.O.I. calc's just to give a decent idea of whats happening.

I found weights for some bbs cast wheels in 16, 17, and 18" sizes. The 16X6.5" are 15 lb, the 17X7.5 are 20lb, and the 18X8" are 25lb. Now I know these sizes aren't really what we would be looking at, but to tell you the trueth, i just didn't feel like researching the weights of wheels offered in that range in diameters over varying widths over 8". Anywho, the M.O.I. formula i used is for a solid cylinder over the x axis. Again, i got lazy and didn't pull up some section drawings to find out the true M.O.I. so these numbers i found will be higher than expected but still a decent comparing tool. The 16" are 2163.2 lb.in^2, the 17" are 2890 lb.in^2, and the 18" are 3969 lb.in^2. As you can see there is a substantial increase in M.O.I. of the 18" over the 17". This is due to the greater increase in circumfrence of the 18" over the 17", compared to the 17" over the 16". Mind you, a more fair representation would be the same widths, but so would knowing the exact radius of gyration, which none of the manufacturers list......at least that iv'e seen. Tires play a huge role in this aswell. But if you think about it, as wheel diameter increases the sidewall height will decrease (given the same o.d. is maintained). Therefor the M.O.I will decrease, and possibly negate the increase of M.O.I the increase in wheel diameter gives. But again, in the few brands I quickly looked at, I didn't find one that gave weights for similar o.d. but increasing wheel diameter while keeping a fairly constant section width. Again that comes down to me being lazy right now.

But to answer your question about optimal widths, it really comes down to tire manufacturer. To give you a rough idea, however, a 245 series tire will usualy need a 8.5" wide wheel, and a 275 series will need a 9.5" wide wheel.

I hope my rambling helped you out. Really, from a performance standpoint, go with the lightest wheel you can afford, and try not to go bigger than you have to.

Edit: I totaly just remembered that there is a wheel weight list in a sticky. Don't I look silly.

Cheers bbcc

[64Chevelle]

Very interesting questions. I to am in the painfull state of picking out wheels, and I've thought about exactly what you asked. Seventeens are my choice simply because of tire selection. Sixteens will do fine, but try and find good rubber for a fair price, its kinda hard. But I did some real quick and simple M.O.I. calc's just to give a decent idea of whats happening.

You are probably right about rubber for the 16's, I never checked. Anyway, fitting 13" brakes would probably be impossible so that's not really an option in my case.


I found weights for some bbs cast wheels in 16, 17, and 18" sizes. The 16X6.5" are 15 lb, the 17X7.5 are 20lb, and the 18X8" are 25lb. Now I know these sizes aren't really what we would be looking at, but to tell you the trueth, i just didn't feel like researching the weights of wheels offered in that range in diameters over varying widths over 8". Anywho, the M.O.I. formula i used is for a solid cylinder over the x axis. Again, i got lazy and didn't pull up some section drawings to find out the true M.O.I. so these numbers i found will be higher than expected but still a decent comparing tool. The 16" are 2163.2 lb.in^2, the 17" are 2890 lb.in^2, and the 18" are 3969 lb.in^2. As you can see there is a substantial increase in M.O.I. of the 18" over the 17". This is due to the greater increase in circumfrence of the 18" over the 17", compared to the 17" over the 16". Mind you, a more fair representation would be the same widths, but so would knowing the exact radius of gyration, which none of the manufacturers list......at least that iv'e seen. Tires play a huge role in this aswell. But if you think about it, as wheel diameter increases the sidewall height will decrease (given the same o.d. is maintained). Therefor the M.O.I will decrease, and possibly negate the increase of M.O.I the increase in wheel diameter gives. But again, in the few brands I quickly looked at, I didn't find one that gave weights for similar o.d. but increasing wheel diameter while keeping a fairly constant section width. Again that comes down to me being lazy right now.

Interesting calculation, but I believe you are incorrect about sidewall height decrease will negate the increase of wheel M.O.I. I was going to write the same thing in my first post, but when I thought about it, it was obvious that the small decrease in sidewall rubber is nowhere near negating moving more of the wheel mass out towards the tire contact patch.


But to answer your question about optimal widths, it really comes down to tire manufacturer. To give you a rough idea, however, a 245 series tire will usualy need a 8.5" wide wheel, and a 275 series will need a 9.5" wide wheel.

That was more like what I originally thought. I know the C5 wheels are 8.5 front and 10 rear with 245/275.


I hope my rambling helped you out. Really, from a performance standpoint, go with the lightest wheel you can afford, and try not to go bigger than you have to.

Anyone making carbon fiber wheels yet??? :cool: Too expensive, no doubt :) I was going to buy 18's, but the more I think about it, 17's will probably be the right choice. I'll have to check with Marcus at SC&C if the brakes will fit though.


Edit: I totaly just remembered that there is a wheel weight list in a sticky. Don't I look silly.

lol, that list is awesome, but I guess even more wheels should be in it. I am considering bying some cheap C5 wheels with tires off ebay and it would be nice to know the weight of those.

Thanks for the answers btw. I'm getting closer to a descision :)

[64Chevelle]

Another place to check wheel weights, really nice. (http://www.wheelweights.net/)

[bbcc]

Interesting calculation, but I believe you are incorrect about sidewall height decrease will negate the increase of wheel M.O.I. I was going to write the same thing in my first post, but when I thought about it, it was obvious that the small decrease in sidewall rubber is nowhere near negating moving more of the wheel mass out towards the tire contact patch.

Yes, you are correct. The decrease in tire M.O.I. is far less than the increase in wheel M.O.I. but there still is a slight bit of offseting that will help the overall M.O.I. So then by that convention the smallest wheel possible to do the job will be the best. But, I've been pondering the relationship between the increase in tire contact area with the increase in tire overall diameter. With a larger overall diameter the sidewall has to increase or the wheel diameter has to increase. So at what point is the increase in tire contact area offset by the increase in M.O.I. an increase in overall diameter will net. Perhaps I am just looking to far into the subject, but it does seem like there would be a measurable performance advantage to be had by optimizing all the variables.

Way to many compromises to be made. I think I will go with whats cheapest at the time and convince myself that I made the best choice.

Cheers bbcc

[64Chevelle]

I believe the biggest issue with choosing a smaller wheel while keeping the same O.D is that there is more tire sidewall. Tire sidewall is obviously softer than the wheel and will impact corner handling. Although drag racers often use a lot of tire sidewall and smaller wheels, so that combo should be better for good launches. I guess it all boils down to compromises and what you want the car to do best. Less sidewall = harsher ride, but better handling, but probably not a better launch, maybe even a bit worse.

I hope some of the experts will see this thread and give some answers :)

Anyhow, I have decided to bid on the C5 magnesium wheels off ebay, hope I'll get a good price :)

[Procharmo]

"So what is the optimal rim width if I want to choose 245/275 front/rear?"

Its easy I ran 8.5" and 10" with 235/245/40/17 and 275/40/17 for 15 years street racing against modern European cars with great results. Wider tyres on small rims means when you change direction the car becomes unstable. Essential for the street with its hazards. Too narrow and you loose grip.

I've just purchased 13" BAER brakes and its tight but they fit.

https://static1.pt-content.com/images/noimg.gif (http://imageshack.us)

Going with bigger wheels allow access to 14" and 15" brakes but better callipers at 13in is the way to go imho.
17 inch allows a modern american Muscle car look. Any larger and its too close to present day. depends on how old your ride is.

https://static1.pt-content.com/images/noimg.gif (http://imageshack.us)

[Procharmo]

My day car is a Porsche 996 C2 with +1 sized 275/30/19 rear tyres. Grip is phenomenal, ride is harsh, lowered 4 inch on Eibachs, I love it.
https://static1.pt-content.com/images/noimg.gif (http://imageshack.us)

Will be cutting the Hotkichs coils on the Camaro pretty soon. But will stick with the new 17 x 8 and 17 x 11 TTII's. If any one is interested I'm selling my 17 x8.5 and 17 x 10 AZEV Camaro wheels on Ebay now!!!!
http://cgi.ebay.co.uk/17-inch-AZEV-A-Wheels_W0QQitemZ300218852288QQihZ020QQcategoryZ988 8QQssPageNameZWDVWQQrdZ1QQcmdZViewItem
https://static1.pt-content.com/images/noimg.gif (http://imageshack.us)

[64Chevelle]

Hi,

haven't checked the thread for a few days, it's been a busy weekend. Wow, nice car's u've got there. If I were to choose a different car it would definately be a 2nd gen camaro, I just love them. And Porsche is always nice, but out of my league financially :(

Thanks for the pictures of the rims with brakes and calipers too, nice to see those 13" Baers fit inside the 17". I agree with you about the rotor size vs. getting better calipers too, but I think I'll stick with the SC&C setup for now and perhaps upgrade later. Also, 17's both seem to look and perform the best for our older cars. And bigger than 18 seems like a waste to me, but I respect that some guys want bigger for the bling factor :)