Project SV650 Part One, Fork Mod

in howto, test

The Suzuki SV650 is probably one of ths most fun machines you can buy. They are relatively cheap and the word entry level comes to mind. But they are much better than a normal "entry level" street machine that comes to mind when when using that term. They have an incredibley strong frame for their displacement, the engine is bullet proof and makes excellent power for a 650 vtwin.

As we all know, you can always improve upon a good thing. The SV650 has a few glaring weaknesses when compared to modern sport bikes. Well, what do you expect, it is an "entry level" machine. It lacks cartridge forks, which by itself, isn't that big of a deal. The SV for some unknown reason feels like it has the forks of a moped. They are probably the worst forks I have ever felt on a street bike from the factory and is in my opinion one of the most glaring problems with the bike.

Cartridge forks are all the rage these days and have been for some time now. They allow forks that experience a large quick hit to respond rapidly to that hit without sacrificing slow speed damping. It allows a form of progressive damping based on flexible washers that control the flow of hydraulic fluid through the damping ports. Most people won't notice the difference on a street bike. I elected to try a simpler and cheaper fix for the forks first. I basically wasn't going to lose anything but time and some fork oil if it failed.

After changing the fork oil from the stock 7 weight to 15 weight and some testing I determined that there were two problems with the forks on the SV650. They were incredibly undersprung for anything more than a trip to the Quickie Mart for a slushy. Just mild braking would have them diving through ALL of the available travel and they would dive through that available travel with enough speed to bust the front wheel loose in a turn when they reached bottom. It was only fun in that sick twisted way when you realize you are still alive and how close you came to actually tossing a bike down the road. Next, when in a turn and encountering stutter bumps, or just about any type of bump the forks would compress and rebound so quickly the front end would lose traction and start hopping towards the outside of the turn. Even novice riders noticed that tendency.

The plan of attack was to increase the spring rate and increase the damping rate. Spring rate is what controls how far the suspension compresses with a given force. Damping is what controls how quick the suspension goes through that amount of travel. The stock springs are 315mm long and are rated for .70kg per mm of compression. After some figuring, that I won't go into here but will write another article about suspension to cover it, I determined that I thought I'd like to have about .9kg per mm as my rate. I figured that cutting the front springs to about 250mm length would get me in the ball park. Cutting the springs will give you and increased spring rate, but it will shorten the life of your springs since they are going to be flexing more to produce that spring rate. Cutting that much out of the springs for the SV will be pushing it pretty close to the limits of what the spring can handle without deforming. In my opinion the stock springs are worthless to start with, so I decided to go for it. Suspension damping is a bit tougher to figure out. I knew that the damping with 15 weight fork oil was still too soft, so I opted to try some 20 weight. I decided to run the fork oil at 140mm from the top of the tubes to start with and then see how that worked.

The first thing to do, obviously, is to remove the forks from the SV. Back in the good old days when they actually put drain plugs in fork legs, this was not necessary. Thank you engineers. Loosen the handlebar clamp for SVs with clip ons.

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Loosen the top clamp and then loosen the top cap before loosening the bottom clamp. It makes it much easier to take the cap off if you do this instead of trying to clamp the fork legs in a vise to loosen the cap.

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Remove the fender, front brakes, front wheel and then slide the fork legs out of the clamps.

The springs prior to butchering them.

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I needed to remove this much...

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I used a dremel with a cut off wheel to cut the springs.

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Use a torch to heat the end of the springs and then bend them down flat once the metal is glowing red. DON'T TOUCH THE END. Use vice grips or something like that to hold the springs. And yes, that is a bottle of vodka on my work bench. It makes a great solvent.. honestly..

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I made some spacers out of some pvc pipe. Make them the length of the stock spacer plus the amount you cut off if you want the static sag to be roughly the same or a bit less.

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Once you are done doctoring the springs and spacers then it's time to assemble the mess. Compress the fork legs and pour in your fluid. Use a ruler or a pre-measured stick to determine your fluid level, in my case I was going to use 140mm from the top. Once that is done, extend the fork legs and slip them into the clamps. Go ahead and set the height of the fork leg in the clamp and tighten down the bottom clamp. Slip the springs and spacers into the fork leg and put the top cap on. Make sure everything is really clean before you put it into the fork legs. You don't want bits of plastic shavings or metal shavings floating around the fluid eating away at the bushings.

Tighten down the top clamps, clip ons and install the fender, front wheel and brakes. I'd have more pictures to show if the batteries for the camera hadn't died. I also apologize for not having the torque specs handy. I used my best guess method based on bolt size, thread and material and won't give up those secrets since you may sue me if you use my specs.

Once the forks were assembled a test ride was in order. The difference was absolutely amazing. The wallowing pogo stick of a front end had been transformed into what felt like real sport bike front fork. It just blew me away how much of a difference it made in the handling of the SV. No more bouncing to the outside of a turn. No more tendency to stand up or wash when braking mid turn. Landings from wheelies were plush. Big bumps, little bumps and everything in between were a joy to encounter. The SV now steers with confidence. It's now an awesomely fun machine to ride. Would I consider it as a daily mount? Now I would. It just needs some more power for my tastes. If the liter bikes terrify you and you don't have the dough for a supersport, this bike is it.

Since the front end was sorted out it has become easier to evaluate the rear suspension. My thoughts on the rear are that the spring rate is pretty good. It's a bit odd that Suzuki completely blew the front end of this bike, but got the rear so close. The rear rebound damping is a bit weak. I'm not sure I'd change the compression damping on the rear any. I'm not even sure the rear shock is rebuildable. In any event, it's not bad enough to worry about at this point. Next on the project list is to search for a bit more power and sound. The carbs on the SV could us a bit of work. The CV carbs are a bit sluggish and it's pretty lean on the bottom end. The mid range and top end will have to wait until I open up the intake and exhaust a bit.

Stay tuned for more on project SV.

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