XR75/80 into YZ80?

I am considering a project - mainly for fun but I hope the end result
will work. I have an 11 year-old son and I would like to get a bike for
him. We live in town and he will not get to ride it often so I don't
want to spend very much. I like the power characteristics of the old
XR75/80s, but the have rubbish suspension. I can get a 1985 YZ80
rolling frame cheap. I would hope I could pick up an old XR75 cheap. I
have done a bit of research and found people like BBR that will do
really nice conversions, but this is a bit beyond what I am looking at.

I have access to welders, steel etc. and am capable of basic fabrication
work. But, I am not planning to build a whole new frame

Can anyone say if this will basically "fit", or am I off my rocker?

Thanks,

Brian

 

Yes, you are insane. Anybody who fools around with motorbikes has to be
nuts about them. Anybody who wants to get that deep into a motorbike
that he wants to swap engine should be locked into his garage and told
to "Go for it!"

Drive chain tension and antisquat properties of the drive chain
geometry are more important than the size of the XR75/80 motor, which
is probably taller than the YZ80 motor. You can get a general idea of
how tall the XR motor and YZ motor are from scaling off of drawings you
may find on the web. I prefer drawings. Photographs can be very
deceptive. If you find a photograph of a side view of the motobike in
question and the tires look oval, you can use the height of the tires
for a scaling reference instead of the wheelbase. I would rather use
the wheelbase for a scaling reference, but cameras distort the view
with their wide angle lenses.

Once you have an idea of how much taller the XR motor is, you can
decide whether to buy the roller.

Don't worry about existing motor mount brackets, you will probably
fabricate your own motor mounts.

The first thing you need to do is locate where the center of the
countershaft sprocket needs to be located vertically from the lower
frame rails and how far forward it has to be from the center of the
swing arm pivot.

The top run of chain probably lays on top of the front of the swing arm
with nobody on the bike, so you at least need to set the XR75/80 motor
in the frame to have its weight compressing the springs. Then see how
far the engine has to go forward to tension the chain to the minimum.

Once you've located that position in space within the YZ80 frame, you
can figure out where the top of the XR75/80's cylinder head will go and
you can start planning motor mount brackets. If you have to alter a
horizontal frame tube that braces the steering head to make room for
the XR's head, you can install a bent tube in its place.

(When Honda developed their 4-valve per cylinder CB-750F/900F/1100F
models, the cylinder head was so tall they needed to not only bend the
lower steering head brace, they had to design a removable lower side
rail to get the taller engine in and out of the chassis...)

Be sure that the YZ frame allows you to get at the valves to adjust
them.
And, check to see if the shifter pedal will hit the lower frame rails
when you downshift.

Otherwise, anything is possible, it's just a matter of how much
modification you need to do and whether the quality of the work is up
to motorbike standards.

 

You must mean on really tight "motocross" tracks where the big bore
bikes are too big and heavy to turn easily. Calvin Franks and Stu
Peters brought motocross to California about 1969, and I was in the
California Motocross Club, one of the first thousand members of a sport
that went insane immediately...

The whole idea of motocross is racing on natural terrain, whatever kind
of terrain that might be. The natural terrain of southern California is
deserts, rocky hills, sand dunes, cliffs(!), and dry sand washes with
sloped walls you could ride on. The AMA desert clubs were doing
something similar to what motocross was really supposed to be about
when they ran "European Scrambles" in the desert.

But Calvin and Stu had been to Europe and thought motocross should be
about MUD, and that it should be a spectator sport where everything
happened close to the onlookers, with mud, barrel turns, and unnatural
built up jumps that squashed the suspension and bent the wheels on
landing...

And so the tight tracks like Indian Dunes and Saddleback Park were
built and there were no 70 mile an hour straights where the big bikes
would come into their own. Instead, the 125's were quicker because they
didn't weigh anything and didn't dig ditches with their power.

There were so many youngsters and even females wanting to try out the
new sport of "motocross" as conceived by the entrepreneurs, they would
have to run five or six divisions of juniors in every class, they
didn't have time to run three 45-minute motos for each class, I was
lucky to run in two three-lap races and get three laps of "practice" in
on a Sunday race day...

At that time, the motorbikes that I hated to have to race against were
the CT-175 Yamahas. I was on a 250cc DT-1, and the CT-1's had shorter
wheelbases, so they could get around the tight hairpins more easily...

 

That's what I had gathered from the reading I have done, and it's
probably the level of work that is just beyond me. I'd really like it
to slot into the existing cradle. I think I'll take krusty's advice
here and see if I can find a few photos that look to be nice and square
and do some scaling. The photo's I have seen of YZs this age look to
have lots of open space up there but I understand it is a combination of
countershaft position and space that I need.

I don't want to go hot - remember this is for my 11 year old to learn
on. But I would maybe warm it up a little as he gets used to it.

My opinion there was formed by having an RM80T when all my mates had
XR75s - just no comparison!!

Yep - been there. Lots of cool little bikes!

Yep again - I can see myself putting a few kilometres on this if I ever
get around to doing it.

Thanks,

Brian