Motorcycle Repair: Changing the rake on a 1973 Honda CB350G, triple clamps, stock tires

Question
I am converting my '73 CB350G into a hard-tailed bobber.
The stock tires are: Front - 3.00"/18" ; Rear - 3.50"/18
The tires I will be running are: Front - 100/90-18 ; Rear - 130/90-18.
The forks will be stock. I am unsure of the stock rake on it.
The stock ride-height is 6.1 inches.
Now that all of the info is there, I am looking at having the new ride height at 4.25"

What would the formula for deciding new rake be?

Thanks in advance,
---Douglas

Answer
Douglas, the stock caster angle on the 350s is 63 degrees.

I don't know where you are taking your "ride height" measurements from.

I am assuming that you are measuring the ground clearance figures, which would run about 6" on a CB chassis.

You can't "change the rake" of the bike unless you cut into the steering head of the frame and reweld it again.

You can't lower the bike on stock wheels, unless you put shorter struts on the back end. When you do that, you will change all the geometry figures for the center stand, side stand and footpeg clearances. When you drop the rear end, the angle at the front will increase.

You can drop the forks down in the front triple clamps to lower the bike on both ends, keeping the steering geometry somewhat close to the same figures, however. Changing either end will change the caster angle to some degree, but once you start making these kinds of changes, there is no "adjustment" in the frame to compensate for lowering the bike on the suspension w/o welding on the steering head, which generally is not a good idea.

If you are deeply into the math of this bike, get some graph paper, a protractor and measuring tools and lay it out, plotting the changes as you go.

The charm of a hard-tailed bobber, will probably last about 15 minutes, then you will want to change it back again to something that gives on both ends of the chassis..

Good luck..

Bill Silver