Some good Forkin Questions

 

Question: I am currently preparing my girlfriends 1999 SV650 for her first track day.
I am installing a much more readily available 120/70R17 front tire as
apposed to the stock 120/60R17. What adjustments should I make to compensate
for the change in diameter? Also I have noticed when riding her bike that
when starting to push harder in sweeping corners (on ramps) that it starts
to feel a little unstable, any suggestions how to improve this with the VERY
LIMITED available adjustments on the stock SV suspension? I think the only
thing that can be adjusted is the height of the forks in the triple clamps.
Obviously changing the front tire should have a fairly major effect on the
handling of the bike, would you expect switching from the current tire to
the new tire to improve this situation, or worsen it? FYI I think they are
currently Bridgestone Battlax BT56's or BT57's on it, and I will be
installing Dunlop D207's. Or would you recommend sticking with what's on it?
I was under the impression that the 207's were a stickier/better tire.

 

When doing any tire swap, front or rear, I will take a narrow tape measure (or a piece of string will work) and measure the circumference of the tire.  To do this, tape the leading edge to the tire while it is still in the rim and on the bike and slowly turn the tire while keeping the tape perfectly in the centre of the tire.  Do this a few times to confirm your result.  You will get a number something like 1960 mm. Divide this number by Pi to get your tire diameter (623.9mm for our example), half this number will be your radius (311.9mm).  Now mount up your new tire, install the rim on the bike and do the exercise again (use the same tape measure or string, don’t switch methods now).  Lets say your new tire’s radius is 315.5mm.  You now know that you would have to slide the forks up in the triple clamps by 3.6mm to lower the bike back to its original position.  Be cautious though, you have corrected for the change in ride height, but you must still be aware that all tires have a different profile and carcass that will change the way the motorcycle steers and feels.  Dunlops historically have a more triangulated profile with a stiffer carcass than most.  This results in a bike that will snap onto its side quick, maintain a large contact patch at full lean and the stiffer carcass will give the rider plenty of feedback at full lean. 

 

The instability you mentioned may have a couple of causes.  The SV is a great street bike, but definitely needs some front-end attention when being prepared for the racetrack.  The stock fork springs in the 99’ measure out to be about .70 kg/mm, which is definitely too light for high speed riding.  What will happen when the bike is pushed hard is that the fork will fall through its stroke and engage the hydraulic cushion right near the bottom limit of travel.  What you are feeling is either the chassis’ general nervousness when the suspension is collapsed (thereby reducing the rake and trail to its minimum), or the loss of compliant compression experienced while being engaged in the hydraulic cushion for more than just a fleeting moment.  If the instability is caused by the former reason, the new larger diameter front tire should probably help the nervous feeling (unless of course you corrected for it as discussed above). 

 

You are correct; the 99’ SV has a very value conscious fork.  You can add stiffer springs to raise the overall ride height at all speeds, raise the fork oil level 10-15mm to increase the resistance to bottoming and go with a heavier fork oil to slow down the forks compression and rebound to manageable levels.  Steen Hansen stocks Ohlins springs for your bike that are the same length as stock (no spacer cutting or fabricating is required) but are about 10% stiffer, they can have them to your dealer the next day.  With these quick mods, you will notice a dramatic improvement.  If, albeit at a much higher speed the symptoms return, the forks should go to a competent suspension service centre to perform more in depth modifications.

 

Question #2:  I have a 1999 Suzuki DR350S and recently decided to change the fork oil,
removed the forks and drained them by turning upside down (had USD forks for
awhile).  I reassembled everything OK but found that the damping adjusters
on the top of the fork no longer 'bottom', I can screw them in way past the
point where they used to bottom.  Any ideas?

 

Ideas yes, and concerns.  First off, when changing fork oil, it is important that the forks come right apart.  You would be surprised at the metal filings and general break in crap that collects in the bottom of the fork after even one year of use.  In most forks the compression piston sits right in the bottom of the fork and seems to collect all the big chunks right in the valving stack.  So, for a set of forks that are going on six years old, get them right apart and have them cleaned and serviced properly.  Fork seals on a dirt bike that haven’t failed after 5 years deserve a medal, so replace them too.  The other observation is that without complete disassembly, you will only be changing about 60% of the fork oil.   The other 40% is residing safe and sound inside the valving cartridge that no amount of tipping up and down will drain out.  Now on to your problem…  Inside most conventional or USD forks, the top of the damping rod threads into the fork cap and is secured by a jam nut.  Inside the damping rod is another rod that converts your turns of the top screw into downward movement of the rebound needle.  If your top screw is going deeper into your cap then it was before, either you didn’t set the damping rod into the top cap properly before tightening the jam nut, or worse, the jam nut is loose and the damping rod is unthreading from your top cap and your fork is about to fall apart from the inside out!  So, either way, get your forks to a reputable suspension service facility RFN for some luvin!