Wheel Size

Hey guys, I've got a CB750 k8 with a 17" rear wheel and want to match front and rear wheel/tire sizes at 19" x 4.0. Is it viable to buy a front 19" rim and just respoke it to fit the rear brake hub? The picture is what I have in mind for wheel setup. Obviously I'm going to keep in mind that I'll be adjusting chain length/rear shock height etc., just want some input on the best way to accomplish this. Thanks

It's not quite as simple as you describe but close.

Ignoring for the moment all the geometry and clearance issues (you want to check stock outer diameter measurements for the stock rear wheel/tire setup and what you'll end up with when you go to a larger rim / smaller tire setup), the key issue is spoke angle.

The typical 19" rim will be drilled for a small front hub which has disc brakes. Rear hubs are usually much larger, housing drum brakes, holding sprockets etc.

So, the angles of the spokes are different, and the holes drilled in the rims for the spoke nipples are done at different angles.

Cognito might be able to sort you out quickly:

https://cognitomoto.com/collections/hubs-wheelgoods

Alternatively, you can use a front hub in the rear, with the necessary adapters to hold a sprocket and brake rotor. This will also mean adding a rear disc brake, which the 1978 K bike did not have. Perhaps the swingarm etc from a CB750F SuperSport might be a good starting point.

But a custom drilled rim / spokes will be cheaper likely than going through all that trouble and be a better investment IMHO in the long run.

A Harley 40 spoke 19" 2.15 rim can be laced to your hub. You will need spokes from Buchanan. I have not done this, but a friend has one his Honda 750 chopper. I am running the same 19" rim on the rear of my Evo chopper. If you watch EBay you can probably find a new, never ran 19" rim for about $35.00. Check seller Barce in Florida. 4.00 x 19 street tires are hard to find and I believe only Duro currently makes one. A 4.00 X 19 will give you an approximate total of 27" in height depending on the type tire. I believe your current 17" rear wheel tire combo will be 25-26" depending on the tire you are running and your front is probably right at 26". I attached a picture of my Evo when I was first building it to give you an idea of what a rear 19 looks like. It is paired with a 3.50 x 21 on the front and the height for both wheels is 27".

Regards
John

Rider52 said:

A Harley 40 spoke 19" 2.15 rim can be laced to your hub. You will need spokes from Buchanan. I have not done this, but a friend has one his Honda 750 chopper. I am running the same 19" rim on the rear of my Evo chopper. If you watch EBay you can probably find a new, never ran 19" rim for about $35.00. Check seller Barce in Florida. 4.00 x 19 street tires are hard to find and I believe only Duro currently makes one. A 4.00 X 19 will give you an approximate total of 27" in height depending on the type tire. I believe your current 17" rear wheel tire combo will be 25-26" depending on the tire you are running and your front is probably right at 26". I attached a picture of my Evo when I was first building it to give you an idea of what a rear 19 looks like. It is paired with a 3.50 x 21 on the front and the height for both wheels is 27".

Regards
John

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John, is your friend using the drum brake that comes on the standard 750 or the hub that incorporates the disc brake? And if that is the case do you know what size (length and angle) the spokes that he is using are? Thanks for the input guys your advice is appreciated!!

jeffthompson55 said:

John, is your friend using the drum brake that comes on the standard 750 or the hub that incorporates the disc brake? And if that is the case do you know what size (length and angle) the spokes that he is using are? Thanks for the input guys your advice is appreciated!!

Click to expand...

you won't be able to use the front disc hub as you'll have nowhere to mount the sprocket. in theory you could get a sprocket made to bolt onto the disc holes but you'd lose the damper rubbers that are built into the drum hub and the sprocket would be offset a long way out from the hub. you'd also have to change the swingarm to accommodate the disc on the other side and find someway of setting up the brake m/cyl in place of the stock brake pedal pivot stuff

jeffthompson55 said:

John, is your friend using the drum brake that comes on the standard 750 or the hub that incorporates the disc brake? And if that is the case do you know what size (length and angle) the spokes that he is using are? Thanks for the input guys your advice is appreciated!!

Click to expand...

you would add 10mm + 5 degrees to the spokes you have now

"John, is your friend using the drum brake that comes on the standard 750 or the hub that incorporates the disc brake? And if that is the case do you know what size (length and angle) the spokes that he is using are? Thanks for the input guys your advice is appreciated!!"

Yes he is. Here's a picture. He ordered the spokes from Buchanan. They were custom made @ about $125.00. If you contact Buchanan's tell them you are using a 78 Honda 750 hub and a Harley 19" drop center rim.
"

spotty said:

you won't be able to use the front disc hub as you'll have nowhere to mount the sprocket. in theory you could get a sprocket made to bolt onto the disc holes but you'd lose the damper rubbers that are built into the drum hub and the sprocket would be offset a long way out from the hub. you'd also have to change the swingarm to accommodate the disc on the other side and find someway of setting up the brake m/cyl in place of the stock brake pedal pivot stuff

Click to expand...

You can use a front disc hub but you have to make adapters. They will space the hub to the width of the drum hub and add mounting points for a disc brake and a sprocket. This is commonly done on Evo Sportsters to mount a 19" mag wheel on the rear when building a tracker.

Back in the 70s a company called C & G marketed a rear disc brake hub specifically for Honda 750s. The design mimicked the Honda front hub but added the width needed to properly space the brake rotor and sprocket. I attached a picture of the one I have.