front tire size for odd boomerang size

[wyly]

I have boomerangs from a special issue cb900 boldor

the new wheel is a 2.50x18 compared to the oem 2.15x19

visually sizing it up if I reduced the 110/90 19 by a half inch (18' wheel)the tire side wall will come very close to the stock fender mounting bolts

will I have issues running a 110/90 18? ..tire fling shouldn't be an issue, what about compression?

does anyone have a similar set up? will I need to reduce back to 100/90 ...

[daves79x]

First, you should not have a 110/90-19 on the stock front wheel. 100/90-19 is the correct size. With the 18 inch wheel, you should go with a 110/80-18. It will fit fine and the fender will look OK. If you don't like the look, you can lower the fender to suit.

Dave

[wyly]

Thanks dave
Why do you recommend the 110/80 -18 vs 110/90-18?

[steve murdoch icoa #5322]

On my '81 with the 18" Boomie i used a 100/90-18.
Bridgestone Spitfire S11 and BT45. Don't think they are making the S11 any more.
The fender has been lowered 1" in this photo.

[daves79x]

I think you have more tire choices and I think it handles better with the 110/80. I assume you have now the 17 inch rear as well? If so, a 140/80-17 matches well with the front.

Dave

[daves79x]

Steve - I see you have the vented Pro-Link rotors. A lot of unsprung weight there. I'd chuck those and get, say, 900F rotors and calipers. Those calipers and hangers will not work with the thinner and smaller diameter rotors. Nice-looking set-up though.

Dave

[wyly]

3.0 x 18 inch rear actually, it’s a unique size from a German cb 900 bol dor

I have bt45 130/80 on rear I bought it before I had purchased the new wheels and installed a gs1100 swingarm, so it’ll have wear out before I switch to a 140/80

The bt45 options for front were 110/80 or 110/90...my concern was would the smaller 18 have clearance issues with a 110 and if not 80 or 90 and why one vs the other?

I have a fender from a 84 cb750 that will fit the tire

[wyly]

I found a handy website for tire swap calculations, enter the old and new size and it shows the size difference and speedo readings with new tire

https://www.tacomaworld.com/tirecalc?ti ... -110-80r18

am I right in assuming the speedo gear in the hub determines the speed and not the speedometer? I have speedo gear from the 18" cb750 that donated the discs and calipers and seems appears to fit perfectly in the new boomie

going with 80x18 on front with the 80x18 that's on the rear keeps the bike at the same geometry as the original 90x19/90x18 but by shrinking the front wheel by an inch so I'm changing that geometry am I not? ...
so why wouldn't I stay with a 90 on the front and try stay as close to the original setup as possible?...
putting the numbers in the tire calculator a 80x18 is .87" smaller diameter than a 90x18 but 80x18 is 1.87" smaller than a 90x19...
whereas a 90x18 is 1" smaller than the original 90x19

will using a 80x18 on the front quicken the turn in? for the better or worse? ...then I need to factor in the new longer wheel base from the GS1100 swing arm and a 80x18 on the rear

all these details when put together are above my paygrade

[EMS]

am I right in assuming the speedo gear in the hub determines the speed and not the speedometer? I have speedo gear from the 18" cb750 that donated the discs and calipers and seems appears to fit perfectly in the new boomie

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Both do. The drive has to be matched to the wheel and speedo. The drive gets an input from the wheel and returns an output according to its ratio. The speedo converts the input rpm from the drive to a mph indication. That being said, a speedo drive for a 18" wheel will not necessarily give you the correct rpm for the CBX instrument. If you are lucky, it will.
Some Honda instruments show a stamp on the back of the housing stating the input/output. Like this: (Go to the 5th picture)

https://www.ebay.com/itm/183632086280?s ... 1555.l2649

If memory serves me well, the latest NOS CBX speedometers that Honda offered were originally for the CB1100R (MA3). These bikes had a 18" front wheel, so a speedo drive from any of these should work. Funny enough, that p/n is 44800-425-013 and is originally for a 1979 CB750 (DOHC)

[steve murdoch icoa #5322]

Steve - I see you have the vented Pro-Link rotors. A lot of unsprung weight there. I'd chuck those and get, say, 900F rotors and calipers. Those calipers and hangers will not work with the thinner and smaller diameter rotors. Nice-looking set-up though.

Dave

Good reminder, Dave.
I think i have everything to do that swap. Finding it is another matter.

[wyly]

Both do. The drive has to be matched to the wheel and speedo. The drive gets an input from the wheel and returns an output according to its ratio. The speedo converts the input rpm from the drive to a mph indication. That being said, a speedo drive for a 18" wheel will not necessarily give you the correct rpm for the CBX instrument. If you are lucky, it will.
Some Honda instruments show a stamp on the back of the housing stating the input/output. Like this: (Go to the 5th picture)

in that case it'll be simpler if I remove the 750 drive and put back the cbx drive, the speedo won't be exact but it'll put me just under the posted limit. As I generally ride over posted limits it'll save me encounters with the police. In the city they allow 9 over posted and on the highway 15 over before they issue a ticket and as I tend push up to those limits it'll give a margin of error.