Workshop Manual 6-19 / 6-29

Modifying the Fork Cartridges by Flange May 2005

Most who have ridden a bike with truly good suspension will agree that the SV's forks don't cope particularly well on anything other than fairly smooth surfaces. Here in New Zealand our roads tend to be a bit rough and whilst I quite enjoy pushing a bike on bumpy surfaces I've felt for a while now that I need something a bit better than the standard SV forks to safely indulge in this sort of behaviour.

I've already done springs and messed around with different fork fluid weights but always found that there is simply too much compression damping (both high and low speed) and not enough rebound damping. Plus I was getting some weird results when damping adjusters were set close to their full damping positions.

Unable to find anything on offer from the likes of Racetech and having a real cunning engineer available I decided to pull the forks apart and have a play.

The Service Manual states that the cartridges are not serviceable but they appeared to be closed off by a screw cap as indicated by the arrow in the photo.

Suzuki must use Araldite or something similar to lock this screw cap because even with judicious heat and as much torque as we dare use it wouldn't budge so we put it in the lathe and machined the end off the cap. As soon as the end was off the body of the cap unscrewed easily, a conventional thread.

The compression stack is mounted on the end cap and includes a fixed orifice feeding into the banjo bolt which holds the cartridge into the fork lower. From the banjo bolt the fluid passes through a gallery via the low speed compression adjustment screw, then back into the lower fork leg. As the fork compresses fluid is forced through this circuit - this is the low speed damping. On a sharp bump when the fluid flow rate becomes too great for the compression adjustment screw to handle, the hydraulic pressure through a valve body causes the shim stack to deflect and allows excess pressure to bleed out of the cartridge and into the lower fork leg - the high speed damping.

Here's some better shots of the cartridge caps which also act as the holders for the compression stacks. The large ends have been machined away to enable the units to be unscrewed from the cartridges. This
became the major part of the project because Jimmy had to make up new ones - a very fiddly operation taking seven or eight hours of calculation and machining
 

At the other end of the cartridge is the rebound stack, mounted on the rod which passes up through the fork and screws into the underside of the top fork cap. Part of the rebound stack acts like a piston and it is this which generates the hydraulic pressure (above and below itself) for the damping circuits to act on. The rebound stack is of very similar construction to the compression stack.

The cartridge internals appear to be of pretty reasonable quality, almost exclusively aluminium and so very light. These SV forks are in reality pretty good items. One thing we did note though is that despite quite a few fork fluid changes over the 16,000km the bike has covered, there was a lot of furry muck inside the cartridges. I'm absolutely sure that I didn't introduce that during fork fluid changes, so I'm picking that this was stuff not cleaned out properly during manufacture.

We saw two options to get the desired result. One was to play with the shims, maybe move some off the compression stack and put them onto the rebound stack, the other was to increase the size of the orifices in the compression stack's valve body, then use thicker fork fluid to boost the rebound damping if necessary. We chose the second option as we figured it was easier to predict the effect of a given increase in orifice size than it was to predict the effect of changing shims. It was a relatively simple job to bore out the valve body orifices, but not so simple to reproduce the end caps which we had needed to machine away, but as stated my mate Jimmy is a pretty cunning engineer and about five hours later we had two sparkly new screw caps, complete with an extra bleed orifice to further reduce the low speed compression damping.

The end result is very good. Using the same fluid (about 7wt) as previously the forks are much more compliant on compression which means that the bike does not tend to jump off the tops of bumps when pushing it fairly hard over rippled surfaces around corners. In other words, less understeer in bumpy corners. Turning in to a bend, the feel is also more positive and stable too. And as a welcome bonus, a lot less jackhammer effect through the handle bars - a much more comfortable ride.

Probably because there is now less pressure build up between the compression and rebound stacks, the rebound damping is now more effective than before without having to go to heavier fork fluid.

It's a shame that it was necessary to destroy the screw caps to get at the internals of the cartridges, otherwise the cost of this very effective mod would be as near as dammit zero. The cost of making up the new cartridge screw caps and boring out the valve bodies would be about five hundred Kiwi bucks at Jimmy's normal chargeout rate, and that's not too bad at all.

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Update: Apparently Ohlins now has a revalve kit - will post more info when I get it.
 

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