Author Topic: CB750k wheel sizes (Read 17380 times)

Grabcon · « **on:** January 22, 2011, 06:09:45 AM »

I just recently purchase a 1974 CB750k barn bike for $400. It has sat for many years but had it running and carbs rebuilt and synced up in about 8 hour. A good buy with a lot of other work. It will end as a cafe racer, to much missing to bring it back to original.

Well at any rate. I was at the local salvage yard and found a beautiful set of alloy wheels on a 1978 CB750A. I don't think they have ever had a new set of tires on them. No marks, no pitting or corrosion, they are true and round and the spokes are clean and rust free. Oh yea it was $150 for both and that included the front brake rotor. Needs tires but that's expected.

But the rear is a 17" rim. Front is the standard 19". Any thoughts on the impact of the 17" compared to the standard 18" rear?

ekpent · « **Reply #1 on:** January 22, 2011, 06:14:44 AM »

Go for it you will be fine.Use a little taller profile rear tire if worried. 16" conversion rear wheels were used all the time.Not sure if they are a direct bolt on though,experts will chime in but the rims can be laced for sure.They have a nice look.

paddy paul · « **Reply #2 on:** January 22, 2011, 06:20:34 AM »

Grabcon, you have to tell where this salvage yard is , I have been hunting high and low for a set of those rims and I live just down the road in Pueblo west, they should be a direct bolt on, and Welcome to the forum.

MCRider · « **Reply #3 on:** January 22, 2011, 06:21:52 AM »

Yeah should be fine. Biggest concern is the rear hub. Brakes and spacing all the same? I think so, but...

It will be a little harder to lift up on the centerstand and it will stand a little straighter on the sidestand. Its not likely that you can get a 17" tire with a taller profile than the stock 18" were, so you'll have a net drop of 1/2". Could be offset with longer shocks if you plan on swapping them out. If not don't worry about it.

The 16" conversions did have much taller tire sidewalls than the 18" they replaced so the net drop was about the same as what you are looking at, 1/2+". Usually.

Kong · « **Reply #4 on:** January 22, 2011, 06:24:21 AM »

What are the widths of the rims? It matters.

Grabcon · « **Reply #5 on:** January 22, 2011, 06:35:02 AM »

Guys, thanks for the input.

The salvage yard is called Steele's Motorcycle Salvage, Littleton CO. They post stuff on eBay but you get a much better price if you go there. It is a "You Pick" yard. I spend hours there looking for stuff.

Rim width I don't know. I will need to look and see if the rims are stamped with their size.

MCRider · « **Reply #6 on:** January 22, 2011, 06:43:08 AM »

Quote from: Grabcon on January 22, 2011, 06:35:02 AM

Guys, thanks for the input.

The salvage yard is called Steele's Motorcycle Salvage, Littleton CO. They post stuff on eBay but you get a much better price if you go there. It is a "You Pick" yard. I spend hours there looking for stuff.

Rim width I don't know. I will need to look and see if the rims are stamped with their size.

The rear width of the 17 is slightly wider than the 18s were, 2.5 vs 2.15 I think. That will be fine and you'll appreciate it. It will allow a slightly wider tire and spread it slightly more. Its what it should have had to begin with IMO. The front should be the same. 1.85

Kong · « **Reply #7 on:** January 22, 2011, 07:08:20 AM »

Well, the point on width goes farther than it might at first seem. Remember how modern tires are sized? There is the overall width of the tread first, let's say 150mm, then there is the aspect ratio, let's say 70, meaning the sidewall's height above bead is 70% of the width of the tread, then there will be the hole size and the speed rating.

If you have a wider rim that means you can put on a wider tire - pretty obvious - but that in turn also means that the tire will be taller, because that aspect ratio must be maintained, so if it gets wider it also gets taller. So with that in mind we go back to your original question, of if the 17" wheel's smaller diameter means lower in the rear - well, maybe, maybe not. If the 17"wheel allows a wider tire then that tire will also be taller and so the 1" difference in hole size does not mean a 1" difference in overall height, and much of the difference is dependent on the rim width, because wider tires require wider rims.

MCRider · « **Reply #8 on:** January 22, 2011, 07:21:20 AM »

Quote from: Kong on January 22, 2011, 07:08:20 AM

Well, the point on width goes farther than it might at first seem. Remember how modern tires are sized? There is the overall width of the tread first, let's say 150mm, then there is the aspect ratio, let's say 70, meaning the sidewall's height above bead is 70% of the width of the tread, then there will be the hole size and the speed rating.

If you have a wider rim that means you can put on a wider tire - pretty obvious - but that in turn also means that the tire will be taller, because that aspect ratio must be maintained, so if it gets wider it also gets taller. So with that in mind we go back to your original question, of if the 17" wheel's smaller diameter means lower in the rear - well, maybe, maybe not. If the 17"wheel allows a wider tire then that tire will also be taller and so the 1" difference in hole size does not mean a 1" difference in overall height, and much of the difference is dependent on the rim width, because wider tires require wider rims.

I'm not sure where to dispute this, I'll just say I have a 3.5 x 17" rim on my project which has a 140/70-17 tire. The overall height is considerably lower (1.5"?) than the 110/90-18 that was on it. So relative to the stocker, virtually any width 17 will be shorter.

I think the problem is assuming a /70 on the 18. I don't think this is even available and certainly stock was a /90. The modern 17s are mostly /70s, may find a /80, never seen a /90-17".

Remembering that the overall height is halved when considering what happens to the bike.

Kong · « **Reply #9 on:** January 22, 2011, 08:10:17 AM »

You are correct in part, but the difference in wheel size you got was due to the changed aspect ratio, not the difference in hole size. You went from a 90% aspect ratio when you were using the 18" wheel to a 70% aspect ratio when you addressed the 17" wheel. Its an apples to oranges comparison.

MCRider · « **Reply #10 on:** January 22, 2011, 08:21:06 AM »

Quote from: Kong on January 22, 2011, 08:10:17 AM

You are correct in part, but the difference in wheel size you got was due to the changed aspect ratio, not the difference in hole size. You went from a 90% aspect ratio when you were using the 18" wheel to a 70% aspect ratio when you addressed the 17" wheel. Its an apples to oranges comparison.

I guess that's what I'm saying. The OP Grabcon, will be going thru the same conversion, so he will likely notice a drop in wheel size, and maybe a big drop, as /90-17 are not available (I think) and he has stock (assumed, old barn find) now.

If /90-17 s are available then he may not notice the drop so much or at all.

I did a quick Google search and /80 and /90-17s are only available in dirt bike and enduro treads. All the street bike treads are /70. My search is far from complete I'm sure.

Grabcon · « **Reply #11 on:** January 22, 2011, 08:26:54 AM »

To all Thanks for the input.

I guess I am somewhat surprised as to the responses and opinions. Tire height will play as an issue. I need to get the new wheels on and look at the stance. and then do a little math to see where it will end up sitting.

Kong · « **Reply #12 on:** January 22, 2011, 08:31:32 AM »

Good point, I hadn't considered that.

Later in the day (if it ever warms up at all) I'll go out and measure the height of two mounted tires I have. One in a stock original 18" rear wheel/tire (90 aspect ratio) from a 550 Honda and the other is a stock 17" wheel and tire from a 2002 Harley Davidson (60 aspect ratio). Want to guess which one will be taller?

MCRider · « **Reply #13 on:** January 22, 2011, 08:32:38 AM »

If I'm right and that rim is a 2.50, the widest tire appropriate is a 130. If i am also right that /70s are all that's available in the 17" diameter, you'll see a drop in ride height in the rear. But overall i think you'll either not notice or you'll like it. IMO

MCRider · « **Reply #14 on:** January 22, 2011, 08:36:05 AM »

Quote from: Kong on January 22, 2011, 08:31:32 AM

Good point, I hadn't considered that.

Later in the day (if it ever warms up at all) I'll go out and measure the height of two mounted tires I have. One in a stock original 18" rear wheel/tire (90 aspect ratio) from a 550 Honda and the other is a stock 17" wheel and tire from a 2002 Harley Davidson (60 aspect ratio). Want to guess which one will be taller?

Oh I agree it depends on the width. If that Hog has a fattie, like twice the width of the 550 I'm sure it does, it will be taller. 90% of 100% is less than 60% of 200%. Let us know, and contribute to the knowledge pool.

Grabcon · « **Reply #15 on:** January 22, 2011, 12:33:19 PM »

Well folks thanks for all of the advise, but this has turned out to be more than a simple wheel replacement.

Many things seem to be different between the CB750A and the CB750K rear wheels other than the rim size.

First all spacing and offsets are different from the center of the tire. On the sprocket side the piece that holds the sprocket and also is mounted in rubber on the CB750A has a greater offset than the stock CB750K. The spacers on the axle are different, and the nearest I can tell the rim is offset different from the hub itself than the standard 18" steel rim.

Switching parts around help, but I think that the rim to hub offset is the culprit, and that needs to be corrected before any further work on the mounting on this bike. It looks like to head done this path a wheel re-lacing is in order.

If anyone has tried to do this please let me know how you corrected the issues. Thanks again.

MCRider · « **Reply #16 on:** January 22, 2011, 01:36:50 PM »

Quote from: Grabcon on January 22, 2011, 12:33:19 PM

Well folks thanks for all of the advise, but this has turned out to be more than a simple wheel replacement.

Many things seem to be different between the CB750A and the CB750K rear wheels other than the rim size.

First all spacing and offsets are different from the center of the tire. On the sprocket side the piece that holds the sprocket and also is mounted in rubber on the CB750A has a greater offset than the stock CB750K. The spacers on the axle are different, and the nearest I can tell the rim is offset different from the hub itself than the standard 18" steel rim.

Switching parts around help, but I think that the rim to hub offset is the culprit, and that needs to be corrected before any further work on the mounting on this bike. It looks like to head done this path a wheel re-lacing is in order.

If anyone has tried to do this please let me know how you corrected the issues. Thanks again.

"Biggest concern is the rear hub. Brakes and spacing all the same? I think so, but..."
I was afraid of that, though I had thought not.

Relacing should be doable, but there you'll need to see if the spokes will work, from the A. Spokes can be bot for about $100 from Buchanans if yours won't work. They'll know what you need.

754 · « **Reply #17 on:** January 22, 2011, 04:11:44 PM »

I doubt it has a different offset..

Use the sprocket holder & INNER SPACER..greasy one), from your bike, and it should all fit in..

Grabcon · « **Reply #18 on:** January 22, 2011, 06:06:18 PM »

Well I am embarrassed. The old guy had a brain fart, which will happen to you all at some point in your life. You still must swap out the sprocket and dampener, and the original spacers. Ah you all will like this, you must put the big spacer on the appropriate side of the wheel.

As I have found out after my brain fart the spacer is on the brake side not the sprocket side.

Okay all laugh at once. But it now aligns perfect.

Grabcon · « **Reply #19 on:** January 23, 2011, 07:00:01 AM »

Ah here is one last note from me. I found a tire height calculation that you might be interested in for future reference.

Formula: Diameter in Inches = Rim Diam Inches + [ (2 x Aspect Ratio x Section Width) / 25.4]

Example # 1: 200/55R18 Tire:
Tire Height 26.66" = 18 + [(2 x 0.55 x 200)/25.4]

Example # 2: 300/30R18 Tire:
Tire Height 25.09" = 18 + [(2 x 0.30 x 300)/25.4]

News:

Author Topic: CB750k wheel sizes (Read 17380 times)