'80 XL250s switching from a 23" to a 21" wheel

[Jenya79]

Hi, I am trying to build a 21 inch wheel off of the original hub that was on my 23" rim. I have a 21 inch rim, I can't quite figure out the correct size spoke set. Since it's a drum brake the wheel uses 2 sizes of spokes for the 2 sides of the hub (one longer, one shorter). Could anyone help with this information? Thank you most kindly in advance.

[stinkwheel]

Nobody can answer this for you because we don't know the effective rim diameter of your new rim.

https://www.wheelpro.co.uk/spokecalc/ This is for bicycle wheels but the concept is the same, it should work for motorcycle wheels too.

Hopefully the result will be within a rough ballpark 1" shorter than what you currently have. The effective rim diameter of your new rim is not necessarily 2" smaller than the original and the spokes meet the wheel at an angle so it's unlikely to be exactly an inch. It would only be an inch if the rims were otherwise identical and you had a radial spoke pattern.

Don't forget to measure the rim offset/dish before you take it apart! Also remember to take any difference in the width of your new rim into account when calculating the offset for the new one.
____________________
“Rule one: Always stick around for one more drink. That's when things happen. That's when you find out everything you want to know.”
I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles.

[Fizzer Thou]

I decided to do just this with my 'S' registered XL250SA many years ago,mainly because there were no decent front tyres available,other than OEM...and they were quite expensive.
A friend had a spare 21" front rim that I bought from him and I bought a good condition front hub from a buckled 23" wheel.I took the cleaned up hub and rim to a wheel builder and got them to do the work.There is no offset to take in to consideration.It just needs to be central.

Are you considering doing the work yourself?
____________________
Just talk bikes.What else is there?

Always have a 'Plan B'

[Jenya79]

[Fizzer Thou]

If you still have a set of good condition front spokes you could effectively determine the required length of spoke yourself.Just thread a few spokes though the rim from the hub and have a measure.If anything,if the spokes are not corroded,you could thread the ends of the spokes after trimming them to length.Not something that I would want to consider as it is a ball ache.

As for fitting a tyre,some dirt bike knobblies have quite soft sidewalls so are a lot easier to fit than radials.I do all of my dirt bike tyres myself,using rim protectors and plenty of tyre soap.

Back in the day I fitted a Michelin T15 knobblie,which was the go-to 21" at that time.So much better than the 23" which Honda tried to pioneer but failed miserably.

Be wary of EBC front drum shoes.Absolutely lethal when I tried a set,which even after 200 miles would still not bed in.I bought the more expensive OEM shoes and straight away had decent braking power (such as it was....)
____________________
Just talk bikes.What else is there?

Always have a 'Plan B'