Shortening front forks. - Page 10

 

On Sat, 24 Oct 2009 07:24:41 +1300, little man upon the stair  


My NZeta in the 1960s was quite stable at up to around 70mph.
(down-hill, tailwind and prayer ;-)
That's really (one of) the reasons I'm interested in rebuilding one.
PS handbook claimed 59mph top speed (95km/hr) and I regularly achieved
65mph on the speedo. (eventually)
The upside down bathtub structure is/was extremely rigid and the
long-travel suspension handled local conditions very well.

 



I think changing the rake would be more than I'm prepared to attempt,
given that it's set by and within the body structure. It shouldn't be
hard to increase the trail a fraction. (it's 75mm/3") Either making new
and extended leading links or triple plates if I go with alternative
front forks.

The "Y" fork structure might be rigid enough, but I'm only going to find
out at the worst possible moments! The standard spring/damper units are
simply CZ long travel rear spring/damper units and presumably will
be long past their prime.

Greg.P.

 

Greg.Procter wrote:

Okay, you're closer to it than I am.  I just assumed that using
non-leading forks would move the wheel rearward so the rake would have
to change.  Photos seemed to suggest same, but my eyes aren't too hot,
so maybe I was wrong to think about rake.

 



Rake seems adequate. Triple clamps are easy to mill to set the trail.
(advantages of having CNC mill in workshop)
In fact, the axle and head would have (exactly) the same relationship
whether leading link or telescopic were used. The leading link design
appears to have been used by the designers so they could use "parts-
bin" rear suspension units rather than making new telescopic forks.
As my motor design will have vastly more power and stopping distances
have become more critical in the 50 odd years since the original design,
I think the forks strength might be suspect.

Greg.P.

 



Well, an Earles fork with rear suspension units is not all that bad an
idea, if you're interested in suspension compliance.

The shock absorber shafts are small diameter, so there's little
stiction compared to a strut type fork.

Problem with an Earles fork is that there is a lot of mass in front of
the steering pivot, so the front end will waggle and hunt on rough
pavement.

I rode a friend's 250cc Greeves MX-5 in the Mojave desert a few times.
It had too much weight on the front tire and the front end waggled and
hunted in the dry sandwashes.

I couldn't go fast enough to get the front tire up on top of the sand,
so the Greeves wallowed along like a pig.

Another Earles fork-equipped machine in those days was the popular
Sachs 125.

Problem was that the front brake drum was anchored to the Earles fork
swing arm, and if the brake was applied it affected suspension
compliance.

Not good, having the brake make the front tire skid early.

The next motorcycle my friend bought bought was a pretty Ossa Stiletto
with conventional forks and much less weight on the front tire. It
handled more like a Yamaha Enduro model like I was riding.

But, by that time, the Japanese had caught onto lightweight
motocrossers with long travel suspensions and those machines could
skitter across the whoop-de-doos like they weren't even there...