Front suspension

So I've been fixing up this 93' Yamaha Xt 350, and I have heard alot
about the problems with the front suspension but I have never heard
anything about a solution for it. Is there any upgrades that I can
buy? Or is there any home jobs that I could do?

 

Cheapest suspension "upgrade" on the planet is fresh oil. Lots of people
neglect it, but it makes a _huge_ difference in handling. I'm not
familiar with this particular bike, so can't really help much beyond
this, but HTH.

--
Later,
John



'indiana' is a 'nolnn' and 'hoosier' is a 'solkk'. Indiana doesn't solkk.

 

Check www. racetech.com to see if they make a Gold Valve Emulator for
the XT350. GVE's are a compression blowoff valve that overcomes the
fixed orifice stroking speed limitations of the stock damper rods.

Say what?

If that sounds like technojargonese to you, what happens is that fork
oil can only flow so fast through the holes in the damper rod, and then
the fork locks up hydraulically and won't compress over sharp bumps.

Riders who try to improve operation of cheap damper rod forks have two
ways to go. The first is to use a good quality locally made fork oil
that may or may not be *lighter* in weight than the stock oil.

Gawd only knows what kind of oil the Japanese manufacturers use.

Some writers have claimed that original equipment fork oil is made from
fish oil. So a known quality locally manufactured fork oil of about 10
weight is indicated for a fast-stroking competition fork. 10 weight to
15 weight is good for small amplitude, high frequency "stutter bumps"
such as are found on graded roads.

But the rider finds that he has less control over larger bumps that he
encounters at much lower frequencies, such as "whoop de doos". So he
tries a heavier weight fork oil (like 30 weight) and discovers that
riding his motorbike has become like trying to control a pneumatic
jackhammer over stutter bumps.

What the heck is going on?

Since damper rod forks are *orifice limited*, oil has to flow four
times as fast through the damper rod fixed diameter hole when the fork
is stroking up and down only twice as fast, so experimenting with
heavier weight fork oils only makes the hydraulic lock up problem
worse.

The GVE is an adjustable spring loaded valve that sits on top of the
damper rod and opens to relieve excess pressure over sharp bumps and
then closes again when the pressure is relieved. Race Tech also
specifies drilling one of the holes in the damper rod larger to allow
more oil to flow through the rod...

You could use 20 or 30 weight oil with GVE's installed and a bigger
hole in the damper rod.

The stock forks have an air valve on the fork caps. I remember back in
the late 1960's, when my Yamaha 250cc Single Enduro was equipped with
check valves on the fork caps, and the experts said that air would get
trapped in the forks at sea level and then when we got to the high
desert, we were supposed to "burp" the excess air pressure to get the
fork to work well over the stutter bumps.

The other claim was that the forks sucked air in as they stroked up and
down and that air pressure built up, and we should burp the forks as we
rode.

The truth of the matter was that we had damper rod forks, and damper
rod forks can either be tuned for stutter bumps or for whoops, but not
both...

Then, in the late 1970's, street bikes began showing up with "air
assist" forks. We were supposed to inflate them with as much as 7 psi
air for a special hand pump. And the fork springs were a lot lighter
spring rate, since they depended on air to help them.

Problem was, the air assist would cause the fork seals to grip the fork
tubes too tightly, and the forks had "stiction" which described the
sticky friction of unresponsive forks.

So the factories abandoned air assisted forks in favor of forks without
air fittings and the forks had stiffer springs.

And, my 1986 GSXR750 had an externally-adjustable compression blowoff
valve with four settings. I can feel the difference in settings as I
ride over frost heaves on mountain roads.

The 1986 GSXR1100 had an electrically controlled compression blow off
valve that would react in about 1/100th of a second.

Then Yamaha and Honda began supplying cartridge forks with shim stacks
inside the internal car-ytpe shock absorbed, and the many flexible
shims in the cartridge did the compression blow offfunction.

Most riders wouldn't want to buy a whole new fork to get the
compression blowoff valves...

Race Tech's Gold Valve Emulator is an infinitely adjustable aftermarket
blowoff valve, but you do have to remove the fork spring to get at the
adjuster bolt.

If Race Tech doesn't make a Gold Valve Emulator specifically for the
XT-350, ask them what diameter GVE would fit. They probably only make
about half a dozen diameter GVE's.

If Race Tech doesn't make Gold valve Emulators for the XT-350, you
could make your own in a junior college machine shop. It should only
take you about 3 months to learn to make your first set...

 

Cheapest and easiest solution, as John Johnson says, is to change the
damping oil. Next easiest thing is to shove in a couple of decent
aftermarket springs. Google for them.

Beyond that, you're getting into the realms of complicated rebuilds and
engineering - not exactly a home job.

 

B-12 schreef:

<Snip the lot, sorry B-12>

An absolutely devastately correct story.

I did change my front fork oil from SAE 10 to SEA 30. It made
like I used to ride on an punctured front tire into a tire
that was correctly inflated.
It totally changed the behaviour of my bike.

For your info;
I have two identical bikes (as matter of fact, they were on
the Honda assembly line one after the other --frame
numbers differ one last digit, as do licence plate numbers--) .
And I rode MG87NN with grade 10, and MG88ZN with
grade 30 in the front forks..

You wouldn't believe wat difference that makes. It feels
as if I have two totally different bikes.

When I bought the (newer) MG88ZN, I swapped springs from my MG87ZN
because I thought : 'something is weird, this new bike must have
been in an accident, or springs are deteriorated', because it gave me
the deflated front tire feeling, like I was driving an HD in the
Sahara.
But it only was the grade of the front fork oil that made a totally
different feeling. --Hard to describe though---

But today, temperature dropped below 6 centigrade, and
I stutterered all the way home (SAE 30) on my (newer) MG88ZN.
This got me puzzled as to "I drive all seasons, What is it that I
really want?" Dunno yet.

Rob.

 

How about a motorcycle that originally came equipped with a
fully-adjustable cartridge fork?

Or, how about swapping a fully-adjustable cartridge fork into *your*
triple clamps? A swap like that is very common in California, the
SV-650 racers will stick a GSXR front fork on their motorbike.

Or, how about checking www.racetech.com to see if they make an emulator
that will fit your present damper rod fork?

When you install the emulator and racetech springs, you will throw away
the preload spacer and use the longer springs. And you can fiddle
around with different weight oils.

The major hassle with the emulators is that you have to remove the fork
caps to adjust the compression blow off point.

The externally-adjustable compression relief valves on my GSXR come
from an era when the endurance racing guys wanted a motorbike that
wouldn't nose dive under heavy braking on the same corner in a long
race. The street riders still wanted suspension compliance, so the
compression blow off valve was the solution.

Canyon riders don't do a lot of heavy braking, or they would complain
about nose dive too.

Canyon riders want a fork that can handle frost heaves in the spring,
so the cartridge fork with the adjustable shim stack works just fine
for them.

 

30 weight and heaters for the shocks?

Ian Singer


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T Shadow schreef:

Right, I didn't think of 'single weight'.

I googled for thumpertalk and front suspension.
Probably the best solution will be to use Dextron ATF
and experiment with the oil level in the forks.

Rob.

 

Ian Singer schreef:

Or 90 weight and making the shocks part of the cooling system. ;-)

Rob.

 

Is that OK on the seals or will it destroy them?

Ian Singer

--


=========================================================================
See my homepage at http://www.iansinger.com
hosted on http://www.1and1.com/?k_id=10623894
All genealogy is stored in TMG from http://www.whollygenes.com
Charts and searching using TNG from http://www.tngsitebuilding.com
I am near Toronto Canada, can I tell where you are from your reply?
=========================================================================

 

I wonder what the effect would be if you used a multi-weight oil like
10w-30 or 10w -40 ????????????????????
George C[/QUOTE]

A dude on the VFRlist did this. It's not a comprehensive evaluation, but
there is it:

http://www.deleveld.dds.nl/forkoil.htm

--
Later,
John



'indiana' is a 'nolnn' and 'hoosier' is a 'solkk'. Indiana doesn't solkk.

 

Actually, the viscosity never goes up. It just doesn't thin as much at
higher temps like a single weight oil would.
In other words; "The multi-weight oil is never thicker at higher temps
than it is at low temps,."



Wudsracer/Jim Cook
Smackover Racing
'06 Gas Gas DE300
'82 Husqvarna XC250
Team LAGNAF

 

No, that wouldn't be the "best" solution, it would be a *cheap*
solution because of the ready availability of ATF.

And, it begs the question about what is supposedly "better" about ATF,
compared to the unknown quality original fork oil or a locally
distributed aftermarket fork oil that comes out of a 55 gallon drum,
has some pretty colored dye added, is decanted into a plastic bottle
with a snazzy logo, and is marked up accordingly.

I had a local trickster try to sell me a 55-gallon drum of soybean oil
for $5000 USD several years ago, but that's a whole other story, beyond
the fact that he was trying to
recruit people into his pyramid scheme.

Back in the 1970's, one of the motorcycle magazines did a series of
fork stroking tests on a suspension dynamometer and they published nice
load cell readout graphs showing how the damping forces compared from
fork to fork.

The readership really ate that shit up...

Of course the simple damper rod forks mainly damped in one direction
(rebound) and the other direction was either a flat line or showed
annoying spikes as the fork was unable to
compress rapidly enough while it was in orifice limitation.

And, for consistency, they used a certain hydraulic oil, referred to by
a mysterious number. :-/

Interested racers called in, wanting to know where they could get that
magickal oil, and the magazine confessed that it was a special high
quality laboratory oil that was used only because it was consistent
from batch to batch.

Ordinary people couldn't easily get that oil, so they couldn't
duplicate the magazine's results. But, the article was typical of a lot
of the things that the amateur engineers at motorcycle magazines do
when they are pretending to do research. :-(

It was about that time I first started hearing about racers who used
ATF. The claim for ATF was that it didn't foam up, and that is
important because a motorcycle fork is essentially an air over oil
strut, similar to what an aircraft uses, except that motorcycles use a
dependable steel spring, instead of compressed air that leaks away.

So, high speed stroking would cause air bubbles to entrain in fork oil
and the fork wouldn't damp consistently as the bubbly mixture passed
through the fixed orifices.

Look at the design of REAR shock absorbers of the early 1970's. Rear
shock absorbers were being pressurized with nitrogen gas, either
directly over the oil, or the gas was kept out of solution with the oil
by putting it into a plastic bag, or a remote reservoir, behind a
piston.

We don't see remote reservoirs on front forks very often. Why? Maybe
it's because front forks don't stroke very fast under normal highway
use. And they don't stroke very far, either.

In fact, the most comfortable-riding fork is one where the laden ride
height is only about 1/3rd to 1/2 of full compression.

And, when a rider is going down the highway, using very little
suspension travel at all, the
air-over-hydraulic fork or the gas-over-hydraulic fork just isn't doing
much damping at all.

How can a hydraulic damping system work, if it depends upon motion to
produce a damping force?

Also, if street forks aren't moving much, and they have a large enough
quantity of oil in them, how much air can get entrained in whatever
fluid is in the fork, ATF or Super Slick Stroker oil?

The ATF solution was originally for *dirtbikes*, where the front end
strokes up and down rapidly and uses all the travel half a dozen times
in 100 meters.

Finally, the ATF solution also becomes problematic in that the
viscosity of ATF can vary wildly from one brand to another. Nobody
seems to be able to tell me exactly what the viscosity of the red stuff
is.

About all I can say for sure about ATF is that any experiment you do
with ATF that allows you to decide on sticking with that particular set
up would indicate that you always use the exact same brand of ATF.

But that would also be true of any set up arrived at through the use of
Super Slick Stroker oil, or whatever the dustributor calls his
decantment.