[Q] Lowering the front end

I want to relocate my Thruxton's triple tree one inch
lower on the forks (ie, so that 1" of the forks would
project above the upper triple clamp).

Is this likely to have any adverse effects, especially
on handling?

TIA for any advice, SQ

 

It would make the handling quicker. Better to fiddle with
both the front and rear height at the same time so you
get the handling you like.

 

This will help the motorcycle turn into a corner quicker, while at the
same time making the steering feel slightly heavier.

One inch is quite a bit of change (I have actually done this rather
extreme mod on one motorcycle), and, you may find that the motorcycle
tends to "fall into" corners, necessitating pressure on the handlebars
in the direction of the turn to countersteer the chassis back to the
desired degree of lean.

 

As others say, slightly quicker steering. No bad thing, if your frame's
geometry is as I remember.

Slightly less ground clearance.

<Awaits Canuck flame>

 

You should see the set-up required to make a Ducati 800SS (or 1000SS)
steer properly on the racetrack. The adjustable ride-height Ohlins
rear shock is set to the tallest setting, raising the rear end of the
bike (static) about 3 inches over the stock shock at the factory
setting, and the fork tubes are raised almost 2.5 inches in the
triples. Looks like a cat in heat, but at least it will turn. Kind
of interesting what it makes the rear end do under braking for second
gear corners, too!

 

Grrrr..... look, will you bloody well shut up about all these lovely
handling mods you can make to old-skool Dukes? Because I'm sure that
it'll result in my having to spend money. Or would, if I weren't
giving the SS to Number One Son this year. He's waiting until his
insurance comes due for renewal (like in a few weeks) and there is
naked lust in his eyes.

 

One other thought.

If you have a centerstand, lowering either end of the
bike will make it a real backbreaker to put on the
centerstand. Plan on hacking up the stand if you want
to use it.

 

Back in the mid-1990's, when the Bostrom brothers were beginning to
make a name for themselves in road racing, they rode Harleys with
longer rear shocks and the forks pulled up through the triple clamps.

The "stink bug look" is what Kevin Cameron calls it in his book about
sportbike technology.

It started with other racers telling me that pulling my forks up 5mm
would enhance the turn in. Gradually I increased that dimension until
I'd pulled the forks up 25mm.

This adjustment is ok for a track that doesn't have fast esses, where
you need to transition from left to right and back. Lowering the front
end makes fast transitions a lot of work, as the motorcycle "wants" to
keep turning whatever direction you pointed it and doesn't "want" to
turn back the other way.

 

Well, I do have a very nice 2000 Ducati 750SS with fuel injection,
dead stock motor, brand new carbon cans, Ohlins shock, EBC full
floating front rotors, DucPond vestigal rear rotor, CFM (Woodcraft)
clip-ons, rear-sets, and foot controls, 1000SS front forks with race
internals, 1000SS alloy swingarm, and lightweight 5-spoke Marchesini
wheels, dead stock yellow bodywork with not a mark on it, and under
8,000 miles on it..... and more that I'm forgetting..... for
sale......

 

That would be consistent with less trail, from what I've found.

Moving the triple tree down on the forks effectively reduces
the fork length, which moves the front axle (and therefore
the contact patch) back towards the bike. However, it also
decreases the rake angle (more towards the vertical) which
also moves the steering axis intersection with the road
backwards.

Which one moves more? I dunno, but I suppose the steering
axis does, which reduces trail, making the steering less
self-centering.

My original query on the Thruxton was because I wanted
to create about an inch of room at the top of the forks
to install hardware for mounting conventional handlebars.

However, the dialog (thanks to all who commented) also
brought up the issue of sidecar steering stability, which
is another problem I seem to have. Perhaps lowering
the triple clamps will help with this problem. (I also suspect
the sidecar isn't aligned properly; for one thing the bike
doesn't seem to lean outward away from the chair,
and the toe-in needs checking as well).

Apparently Harley once featured a dual-position triple tree
for use both with and without a sidecar. The sidecar setting
had less trail.

SQ

 

"Any"????

**** me.

No.

None at all.

And it won't affect the rake or the trail in a negative way either.

 

Yes. There is less trail, less rake, and the center of mass moves
slightly forward.

Tony Foale questioned the effect of rake on steering, so he cobbled up
some prototypes with half the rake (or less) than that demanded by
conventional wisdom and the motorcycles were not squirrely handling at
all.

But a motorcycle with the fork raked between 28 and 32 degrees is a
lot more attractive to look at than the motorcycle with 18 degrees of
rake and the steering head sticking way out in front of the gas
tank...

I suppose you're thinking of clamping high rise handlebars to the ends
of the fork tubes?

I've done that before.

Another way to do it is to find some handlebar clamps from a Japanese
motorcycle that have threaded bolts on the bottom that go through
holes in the
upper triple clamp.

Drilling two 8mm or 10mm holes in the aluminum upper triple clamp will
not
critically reduce its strength and most of the load is borne by the
much stronger steel lower triple clamp.

Another way to reduce trail is by having some beveled shims made up to
go underneath the bearing races. The bevelled shims rake the forks out
slightly more, but, in the process, rake is actually decreased.

I mentioned this before. Doug Bingham's (of Side Strider fame)
machinist buddy Curly used to do this modification to Gold Wings when
sidecars were attached to them.

And BMW had an extra front swingarm pivot hole in their Earles forks,
in case the rider wanted to fit a side car...