Wheels and tires for a 55 C4 conversion

[mmsalt]

I have a 55 2 door HDTP with the Newman Chassis using a 86 C4 suspension. Per CN calculator it looks as if my wheels and tires I'm leaning towards will fit. On the rear Im still undecided if I want 17 or 18 x 10 7" BS with 275 maybe 285 x 40 x 17 or 18. Will I regret going to 18 on the back and 17 on the front? Is a 285 too much for a 55 as it is I will have to remove the lower shock mount to install either way I go. Front is simpler 17 x 8 6" BS 245 45 17.

Any opinions on the NITTO NT555 G2?

Thanks
Mike

[chevynut]

If the car is mini-tubbed you could go with a 295 tire max. A 275 should be an easy fit and is as big as you can go in a stock tub. The dogbones are really what limits the inside of the tire, but the tub is just about in the same place so you need some tire clearance. You only need around an inch and a half more.

The 86 C4 rearend is 62 1/4" wide at the wheel mounting surfaces. A 10" wheel with 7" BS has 1 1/2" positive offset. That makes the tire track 59 1/4" wide.

The outside of a pair of 275 tires would then be at about 70 1/8". That leaves about 5/8" from the tire to the fenderwell lip depending on the body dimensions and centering.

A larger diameter wheel will make it easier to remove them from the car. One trick I learned is to rotate the tire UNDER the disc when installing and removing. Also, use a shock that's longer than necessary so it will extend further, if you have the room, which you may not have with a Newman frame. Every little bit helps.

My Nomad has 18s in the rear with 295s and 17s in the front with 245s. I think they look good. When I get the final custom wheels they will probably all be 18s. I might go 20s just to piss off the old schoolers. LOL

20120325_0005.JPG

[mmsalt]

CN Thanks for the response. The car is not mini tubed. Is 5/8 from tire to fender acceptable?

[Custer55]

I have 90 Vette suspension on my 55 using 90 Vette wheels, 285/40/17 on the rear, 255/40/17 on the front. no issues with rubbing at all. I do have a 3/16 spacer on the rear to gain a bit of clearance on the inside, no spacer on the front. I wouldn't go any bigger than the 285 in the rear. In the front I had the stock size Vette tires, 275/40/17 and did not have any rubbing issues so you should be fine with 245's as long as the backspace is right. Not sure if 6" would be enough on an 8" wheel. Stock wheel is 9.5" with 7 7/16" backspace so the outside of the tire will be about the same place but the tire center line would be closer to the fender. I would go with more backspace if it were me. You can always move the wheel out with a spacer and longer wheel studs until you find the sweet spot that puts the wheel farther out without any rubbing.
I have Nitto NT 555 tires on mine (older version on the G2) I like mine. I think you will be happy with them.
Brian

[chevynut]

Custer, the 86 C4 front and rear are 1" narrower than the 88-96 suspensions you have. That will affect the backspacing needed.

I typically like the tire outside width to be 67 3/4" or less if the car sits low in front. I'm not sure how Newman sets his ride height. With the 61" front end width of the early C4 and a 245 tire you need a wheel offset of 1 7/16" or more to keep the tire width at that limit. On an 8" wheel that's just under 6" backspacing. So 6" should work fine.

[55 Rescue Dog]

Depending on your goal, I think a 245/45-17 is a little undersized, since the stock tires in a lighter/smaller C4 is either a 255/45-17, or 275/40-17 both of which have a load index of 98. The 245 is 95 load index. I don't think is quite possible to make the front of a 55 lighter than a C4. For me, I want maximum cornering traction on the front so I will go as big as possible on the front, which will determine what I go to on the rear. I might go with a square setup instead of staggered too. The biggest issue for me depending on wheel/tire combinations is how many tire choices are out there. On staggered setups with matching tires you are lucky to find one or two, if any. With something like a 255/45-18 there are like 48 different brands/models to pick from. Little front tires and big rears are a recipe for an understeering car plowing through the turns, which is safer, but not as fast/fun as a balanced car.

[chevynut]

Depending on your goal, I think a 245/45-17 is a little undersized, since the stock tires in a lighter/smaller C4 is either a 255/45-17, or 275/40-17 both of which have a load index of 98. The 245 is 95 load index. .

A 95 load index tire can carry 1521 pounds. I don't think the front end of a tri5 is anywhere near 3042 pounds, even with passengers, so there goes your theory....again.

https://www.tirerack.com/tires/tiretech/techpage.jsp?techid=35

The BFG G-Force Comp-2 A/S tire in a 245/45-17 size is rated at 99 and can carry 1709 pounds EACH.

https://www.tirerack.com/tires/TireSearchResults.jsp?zip-code=80524&width=245/&ratio=45&diameter=17&rearWidth=255/&rearRatio=40&rearDiameter=17

And we all know that load ratings, like most everything, are conservative.

[NickP]

5/8 is tight. Easiest thing to do, manually run the suspension through its articulations.

[chevynut]

I don't know Nick....I think 5/8 is plenty if the body is centered and you have that on both sides. My calculations actually showed 11/16 but that's splitting hairs . And he may be able to get more depending on the actual tire size, body width, etc. Also keep in mind that camber increases as the suspension compresses, so the top of the tire tilts inboard.

[chevynut]

I forgot to mention that with the above measurements, the INSIDE of the rear tires would be about 48 7/16 wide using a nominal 275 tire. The stock wheel tubs are 46 1/2" wide inside. The dogbones are about 45 1/4" wide outside to outside in the CENTER of the dogbone between the bushing ends. That's 1" clearance from the 275 tire to the inside of the stock tubs which is about perfect since the tire leans inward at top.

My Nomad can handle a larger tire in front but only on the inboard side as the outside width is already at about the limit at 67 3/4". I think I have about 3/4" clearance from the tire to the frame there at full lock which should just barely fit a 275 tire. The narrowed rearend will handle a 345 tire but I'll likely never go that large. Now, wait for it, here comes RD's BS about how narrowing the rearend screws up the geometry, even though he doesn't know how. We've been through all of this nonsense before....more than once.

[mmsalt]

Thanks for all the good information. The wheels will be special order due to the BS so I need to nail everything down before ordering.

Mike

[Custer55]

Custer, the 86 C4 front and rear are 1" narrower than the 88-96 suspensions you have. That will affect the backspacing needed.

I typically like the tire outside width to be 67 3/4" or less if the car sits low in front. I'm not sure how Newman sets his ride height. With the 61" front end width of the early C4 and a 245 tire you need a wheel offset of 1 7/16" or more to keep the tire width at that limit. On an 8" wheel that's just under 6" backspacing. So 6" should work fine.

I missed that he has the 86 C4 suspension which is narrower so you are correct. An 8" wheel with a 6" backspace should be fine as the original wheels on an 86 were 16 x 8.5 with a 32 mm offset which works out pretty close to a 6" backspace.

[55 Rescue Dog]

This thread got me to thinking, and shopping for what setup I was going to try and make work. I finally had the chance to put one of the front 17 x 9 wheels off my C5 with a 50mm offset/7" BS on the rear of my tubbed 55 C4. They could move almost an inch outboard on the tight side of my car. There are definitely a lot of choices going with a 245/45-17 tire, but to me they look kind of like 14 inch tires. So, since I want somewhere near a 27 inch tall tire, which also increases the contact patch, I need to use 18 x 9 inch wheels front, and rear to get the biggest selection of tire sizes, and compounds. I will probably have American Racing custom make one of the 5 spoke wheels in a 18 X 9 near a 34mm offset/6.34 inch backspacing. I will use something plus, or minus around a 275/40-18 front, and rear. It's going to be quite awhile, even though I've already had fun driving the car a lot in my head. I'm pretty sure my Nomad sized Tahoe would handle like shit if I narrowed the rear, and put some 13 inch wide tires on it. No BS, narrowing a C4 IRS would never ever enter my tiny brain.

[Rick_L]

You do have a tiny brain. A narrowed suspension and bigger tires is going to result in the same width across the tire patch, just bigger tires.

And just how do bigger tires on the rear make the front tires slip? Or, for that matter, make the rear tires slip?

[55 Rescue Dog]

You do have a tiny brain. A narrowed suspension and bigger tires is going to result in the same width across the tire patch, just bigger tires.

And just how do bigger tires on the rear make the front tires slip? Or, for that matter, make the rear tires slip?

It's a very complicated subject that you don't know much about either. Vehicle dynamics is an interesting subject, as to what the effects are between track widths, wheel offsets, geometry, and tire sizes. These links are just the tip of the iceberg. I'm going to rethink my tire/wheel choices. There is more to it than just wide tires.

http://www.tuneruniversity.com/blog/2011/04/wheel-tech-part-ii-size-matters/

http://dawsengineering.com/linked_fi...lus_sizing.pdf

Art Morrison refuses to narrow an IRS
https://www.hotrod.com/articles/1984...ar-suspension/