C2 KO back set is what?

[Alan Drake]

What is the back set on a GM KO wheel??

Plan to take KO off car and run after market 60 style Al rims, 7" X 15", however noticed the back set is 3 1/2" while the stock steel rim was 3 1/4". When I had the rear tires mounted on the KO they fit ( 225-70 ), but no extra space. So before I mount the 225-70 on the new Al rims would like to verify what the back spacing on the KO are.

YES, the 225-70 is a bit large and if I replace them the size would be lowered to the 205-65 which is what I run in the front. (Street Use)
Thanks
Alan

[Duke W.]

Re: C2 KO back set is what?

The offset of the KO wheels (including the adapter) is +.060" - same as the '67 Rally and optional bolt-on Al. wheels.

A number off owners are running 215/70s with no significant clearance issues, but it's tight; 225/70s is pushing the envelope.

Duke

[Alan Drake]

Re: Pushing Envelope - Yes

Well Duke your're right about pushing the envelope. Had these tires (rear only) on the GM KO before the National when I went to the high performance 6.70 4 ply, good thing I needed the points.
Now the 225/70 were alright, but the new Al rims I just got have a 3 1/2 (really 3 5/8) back set compared to the 67 rally of 3 1/8. YES, I'm not talking correctly about the back set since it should be around .060 (1/16" ?)
SOoo first I need to be told how to measure the rims!

Just tried this, measured back side of 67 rally rim where rubber bead sets down to the bolt hole surface which would be against the drum, got 3 3/16. Then did same on front and got 3 1/16 a difference of 1/8 or .13 Must be off a little since I should be getting 1/16 or a little less for the .060

Using the same method on the new Al wheel I get 3 5/8 on back and 4 on front for a difference of 3/8. Assume this will shift the tire towards the outside nearer to the fender. Also ordered the 7" rim as opposed to the 6" rally, that I assume will lower the tire hieght by .0000??

If my new measurements are correct and using an already larger tire than Duke feels comfortable with; Should I mount one 225/70 and lookover (still no bumps factored in) or just purchase two more smaller tires?

Sorry for all the endless questions on this!

[Duke W.]

Re: Pushing Envelope - Yes

Offset is easy to measure with or without a tire mounted on the wheel.

Lay the tire, outside face down, on a flat surface. Lay a 3' piece of 2x4 across and measure from the ground to the bottom of the 2x4 and call this dimension A. This is the OA wheel - or tire width if a tire is mounted.

Now measure from the bottom of the 2x4 to the wheel mounting plane and call this dimension B.

Offset = B-A/2 and this will yield the correct algebaric sign.

Offset is negative if more of the width is outboard of the wheel mounting plan. Drawing a sketch can help you visualize.

1968 Corvette Rally Wheels have -0.28" offset to provide adequate inside clearance to the end of the spring. There were zero offset 7" Rallys used on other Chevolet models, but they provide inadequate inside clearance to the spring end.

You cannot mount a '68 Corette 7" Rally wheel to a C2 with "normal" size tires without shaving off the horizontal fender lips.

Duke

[Alan Drake]

Re: Another Rim Size

Thanks Duke for all the information and keeping me from using the wrong rims.
Will go with the 6" after market rims. (unless you see a problem)

There is a difference however as follows;

After 6" +.175 3.625
67 Rally 6" +.060 3.56 for a total difference of 3/16 closer to the spring

It appears I have 1" of extra space now with the KO's on the car, so hope the additional 3/16 will not be a problem.

As a side note, I have a set of G60-15 mounted on 68 Rally Wheels which I've used for racing (1/4mile) and have never had a problem with them. (Tire is a Pro Trac 60, Street Pro) I assume using the 60 profile keeps the tire below the rear fender so cutting is not necessary.

[Duke W.]

Re: Another Rim Size

As you concluded the change in offset will move the sidewalls laterally an equal amount, and an increase in offset in the positive direction will move the sidewall inboard that much. I think you will be okay, but carefully check for spring end interference. I think the worst case is with the suspension at full rebound, so check immediately when you bolt the wheel to the hub.

It's possible that the G60 is short enough not to interfere, but a 215/70 or 225/70 on the '68 Corvette Rally wheels definitely won't work on a C2 without shaving the fender lips.

Measure the offset of the 7" Rallys to be sure what version they are. The Corvette version has -0.28" offset. I'm not sure of the exact offset of the pass. car version, but I think it is close to zero.

Duke

[Alan Drake]

Re: Another Rim Size

Thanks again Duke.

Just to close the loop, the 7" rally with the G60-15 race tire was measured at -0.25 - so I'm off a little when reading the scale, but its a corvette rally.

[Duke W.]

Re: Another Rim Size

Yes, measuring with a tape measure won't give you micrometer accuracy, but your offset measurement is reasonably close enough to conclude that it's a 7" Corvette Rally wheel. You were only 1/32" off, which is about as close as you can get with a tape measure.

The G60-15 must be short enough to not cause the interference that would certainly be there with a 70 profile tire.

Duke