KZ650B with alternate tire size?

I'll try to get to the point. I've got a 1977 KZ650B which is supposed to run...

3.25-19's on the front and
4.00-18's on the back.

it has slightly larger set on now...
3.50-19 front
4.50-18 rear...

Just bought the thing and that's what came on it. There doesn't seem to be any problems with clearance on the sides or outer diameter on either wheel. Now, I know not all tires are built the same between brands. Also, conversions between standard and metric are rather confusing to me. Can anyone help with this question.

I was thinking of running 80's series profile tires on front and back. And I wanted to see if anyone had any practical experience with fitting these?

I'm wondering if a 110/80 would wedge into the front and a 130/80 would fit the back? Saw a listing somewhere that "baldy110" was running the 130/80's on the back of his 1978C model. So I'm mostly confident with that. I know the difference in profile from a 100/90 and a 110/80 is very close to the same.

I just want it to (#1) fit, (#2) be safe and (#3) be wide.

Any thoughts?
tomtom

Post edited by: secondhandtom, at: 2006/03/24 06:55

I'm not sure if the 80s and 90s series tires are much different in width, probably only in sidewall height.

I upgraded in Sept. 2004 from 15 year old cracked tires to the Bridgestones I have now. 100/90-19 57H Spitfire 11F up front, and the 130/90-18 69H Spitfire 11F out back.

Do the math.

We know 3.25" fit on the front. 3.25"= 88.25mm

We know a 4.5" fits on the rear. 4.5"= 101.6mm

Now do math.

110-89=21. 21/2= 10.5. So measure the front and look for 10.5 mm of space on each side.

130-101=29. 29/2= 14.5. So see if you can clear 14.5mm of space per side of the rear.

Oh, and the profile of difference between 100/90 and 110/80 is 2mm.

100/90 has a 90mm sidewall. 110/80 has an 88mm sidewall.

secondhandtom wrote:

I'll try to get to the point. I've got a 1977 KZ650B which is supposed to run...

3.25-19's on the front and
4.00-18's on the back.

it has slightly larger set on now...
3.50-19 front
4.50-18 rear...

Just bought the thing and that's what came on it. There doesn't seem to be any problems with clearance on the sides or outer diameter on either wheel. Now, I know not all tires are built the same between brands. Also, conversions between standard and metric are rather confusing to me. Can anyone help with this question.

I was thinking of running 80's series profile tires on front and back. And I wanted to see if anyone had any practical experience with fitting these?

I'm wondering if a 110/80 would wedge into the front and a 130/80 would fit the back? Saw a listing somewhere that "baldy110" was running the 130/80's on the back of his 1978C model. So I'm mostly confident with that. I know the difference in profile from a 100/90 and a 110/80 is very close to the same.

I just want it to (#1) fit, (#2) be safe and (#3) be wide.

Any thoughts?
tomtom<br><br>Post edited by: secondhandtom, at: 2006/03/24 06:55

I am running a 140/80/18 on the rear of my bike and there is just enough room between the chain gaurd and the trailing arm for the rear drum brake that was the widest i could go with stock swing arm and the front is a 100/90/19

Post edited by: rammy, at: 2006/03/24 10:47

Thanks Rammy, that's the kind of practical experience I had hoped to hear about.

I was concerned about the 130/80 width fitting. I'm not worried about the profile highth as I've already figured the 80 series tires would have to be at least 135mm wide to equal the profile of the 120/90's. 140/80's are only 4mm larger profile than the 120/90's.

For those of you who don't follow or are happy with stock tires... sorry to bore ya. I used to be that way but now I'm on the road again (5 years with no bike), I can't help but explore the possibilities.

Tomtom

As long as you're using the stock wheels, you're totally fine with running a 130/80 on the rear.
I've been running a Dunlop GT501 130/80-18 on mine with a little room to each side of the swingarm left over. Maybe not enough for a 140, but certainly enough for me not to worry about rubbing issues.

Be aware though, even if a tire is listed at a specific size (130 for example) other tire manufacturers of the same size may not be 100% the same.
Basically what I'm saying is, I wouldn't recommend trying to "push the envelope" by squeezing the largest tire on there that you can get your hands onto...it may just rub once mounted and inflated, and then you're out the $$$ you paid for it.