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View Full Version : How difficult to add a 7.5" wheel width to the Wheel Calculators?



BamaNomad
05-03-2017, 07:58 AM
I like the wheel calculators provided, and I'm using them, but I'm looking at some 17x7.5" wheels and having to interpolate/extrapolate between the 7" and 8" wheel widths in the calculators. Would it be difficult to add a 7.5" wheel width?

chevynut
05-03-2017, 08:19 AM
I don't know who put the calculator together on this site but I don't like the drop-down menus that limit your inputs. The rear tire calculator allows you to select from the menu OR enter your own number, which is fine. But it would be great if the numbers were correct too (I may have provided some at the time :eek:).

I would like to see the 84-87 Corvette rear width set to 62.25" and the 88-96 rear set to 63.25".
In front, the 84-87 width should be 61" and the 88-96 should be 62".

The calculator should have wheel offset for an input and calculate backspacing, or vice versa.

It would also be nice to be able to navigate from one tire calculator to the other without going back to "home".

You can get the Excel version of the calculator here, which is what I gave the guy to work from on that site:

http://tri-5.chevyrides.com/front_tire.php

BamaNomad
05-03-2017, 08:46 AM
CN: I agree with your comments. Is there any chance to get a form of this table (or spreadsheet) that I could modify with those changes??

chevynut
05-03-2017, 08:59 AM
Bama, like I said you can get the original Excel spreadsheet I put together on the site I linked.

WagonWonder
05-03-2017, 10:43 AM
It's protected and you can't change the 7 to 7.5 or at least I can't. But, you can click on the word "Spreadsheet" and save to your file. To open it in Excel you will have to "allow editing".

chevynut
05-03-2017, 11:44 AM
I personally think it would be better NOT having the wheel width in a drop-down menu. It's not that hard to input 7 or 8...or 8.5. ;)

On my spreadsheet you input offset, not backspacing. The backspacing is calculated for you, so if you don't know the offset and you only have backspacing, you just keep changing it until the backspacing is right. I don't think wheel manufacturers should even talk about backspacing, because it's more confusing imo than offset.

rockytopper R.I.P 5-13-2017
05-03-2017, 11:57 AM
Why care? the tire and offset is all that matters wheel width is mute point as CN has stated many times. Same tire with 7 inch wheel and zero offset is same as 8 inch wheel with same tire and zero offset. More or less.... I'm not sure how CN derived the data to create the calculator and determine if the tire will rub or not. But as noted a 55 has taller fender opening than 56-57. What it really needs is that data as an input and the tire height also. This is never discussed and makes a big difference in determining clearance on the front. Brian's 55 is a prime example. He is running a 275 with no issues. He also running C4 and not stock trifive suspension which changes the game also.
Rocky

chevynut
05-03-2017, 12:29 PM
Rocky, the formulas are all simple math. Once you know the body dimensions, mounting surface width, tire width, and wheel offset you can easily calculate the clearances. In the rear its pretty straightforward because the wheels don't turn.

In the front, it's a different story as you said, and as I've tried to explain several times. It depends on the mounting surface width, wheel offset, year of car, the amount the car is lowered, tire diameter, and how far the wheels turn when at full lock. There's no issue when going straight, just like the rear.

It's a lot more complicated to add tire diameter and try to put a limit on the width...the larger diameter tire the sooner it will rub, but I don't know how sensitive it is to diameter, and most tires are in the 26-28" range. It would require a lot more testing and more complicated calculations to factor in both width and diameter.

I got the recommended 67.5" maximum outside width many years ago from many guys on the other site who either did or did not have rubbing issues. After getting their information and compiling it I was able to say with fairly high confidence that if you stay at 67.5" or under overall tire width you probably won't get any rubbing no matter how low the car is in front. So far I don't think anyone has been able to contradict that. As you approach or especially go over 68", rubbing issues seem to start popping up. Lots of guys reported rubbing problems over 68".

And remember, it's not only static height that matters. EVERY car has the potential to rub if the tires are too wide, AND you're at full lock when the suspension bottoms out on the stops. Dropped spindles make it worse, as do dropped springs.

Some guys claim that their tires never rub, and that's fine....they just haven't hit the conditions to make them rub yet and they may never do so. But the potential is there is you're at around 68" or more width with stock suspension and steering.

Here's an example of a guy having problems with a front end that's too wide.... with Chevelle rotors that we KNOW push the tires out 7/8" per side ;)

https://www.trifive.com/forums/showthread.php?t=172693

I believe the reason you can go wider with the C4 suspension is because of a couple of things. First, the steering angle at full lock is less than stock. I posted once an estimate of the difference between C4 and stock but I don't remember what it is. Next, the high SAI, especially on the late C4 suspensions, causes the wheel to gain significant negative camber when turned. I think the reduced steering angle is the bigger factor.

As I posted recently, my Nomad wheels and tires are 68" wide based on an actual measurement and they don't rub at full lock and full compression with a 3" front suspension drop, but they're really close. I had always assumed the tires were close to the nominal 9.6" but they're actually 9.94" wide. That makes the width wider than the 67.7" I had calculated. I believe I also calculated Custer55's width at 68" with his 275 tires. So there's proof that 68" works with a LATE C4 suspension. I assume it would work with an early C4 also, but have not proved it.

If a guy is willing to limit his steering angle a bit with a stock suspension, he could probably get by with a wider overall front tire width, but I don't know how much. One of these days I'd love to take the spring out of my 56 parts car and see exactly how wide you can go at full lock and full compression without rubbing a given tire. But I'm too busy for that. :)

rockytopper R.I.P 5-13-2017
05-03-2017, 12:46 PM
"Here's an example of a guy having problems with a front end that's too wide.... with Chevelle rotors that we KNOW push the tires out 7/8" per side ;-)"

LOl and BAMA has the same issues with a C4 suspension that pushes the wheels out so why so much hate on the chevelle rotors LOL. And FYI the guy mentioned has late model Camaro rotors according to the CPP website in there 12 inch drop spindle kit he has LOL......

chevynut
05-03-2017, 01:21 PM
LOL The Chevelle rotors will work with the right wheel offset LOL. So will the C4 suspension LOL. What's so hard to understand? It's all about overall tire width. LOL.....

My position on the Chevelle rotors has always been that there are better choices for disc brakes that don't move your wheel mounting width out by nearly 2". LOL....

Apparently the late-model Camaro rotors are just as bad of a choice. :D

55 Rescue Dog
05-03-2017, 05:34 PM
I've always wondered where are all of the disc brake choices are, that are 2 inches narrower than the Chevelle set-up that are such a better deal for the money?

Rick_L
05-03-2017, 07:31 PM
They are only a better deal if you want or need them.

The want and need is there for a lowered car. It doesn't take much lowering for a Chevelle disc setup to have problems with rubbing.

There's only so much you can do with wheel offset unless you spend big $$ for billet wheels, and there's a practical limit there too. For instances most off the shelf 7" wheels have 4" backspacing (0 offset). 4.5" backspacing (0.5" negative offset) is special order or not available on many. 5" backspacing on a 7" wheel (1" negative offset) is achievable on a custom billet wheel if it has a "fat lip".

If you don't care or don't know, then you don't need those choices either.

567chevys
05-03-2017, 08:18 PM
I don't know who put the calculator together on this site but I don't like the drop-down menus that limit your inputs. The rear tire calculator allows you to select from the menu OR enter your own number, which is fine. But it would be great if the numbers were correct too (I may have provided some at the time :eek:).

I would like to see the 84-87 Corvette rear width set to 62.25" and the 88-96 rear set to 63.25".
In front, the 84-87 width should be 61" and the 88-96 should be 62".

The calculator should have wheel offset for an input and calculate backspacing, or vice versa.

It would also be nice to be able to navigate from one tire calculator to the other without going back to "home".

You can get the Excel version of the calculator here, which is what I gave the guy to work from on that site:

http://tri-5.chevyrides.com/front_tire.php


I thought it was yours ?
I will have to look back at my emails

Sid