TOPIC:

I recently changed the oil in my front forks and used 10 weight oil. On taking the bike for a ride I noticed a "clanking" noise coming from the forks on a relatively rough road at slow speed. On reading the trouble shooting guide it mentions that a low level of oil or too thin a oil can cause this. My oil levels were perfect. Investigating further, most 1300 riders recommend a 15 weight oil. I have changed to a 15 weight and still have the noise, albeit slightly less. Should I go to a 20 weight oil? Can any of you gurus out there explain to me what causes this noise when using a lighter weight oil? A technical explanation would be great. Thanks.

There's a series of orifices in the piston in the fork with control valves that will allow a few or all of the orifices to pass oil from the lower chamber to the upper chamber controlling the rate of compression. Then there's another series of orifices that control the rate of extension which is controlling the rate at which the fork is allowed to extend. The flow rates are dependent upon the viscosity range of the oil used. If the viscosity of the oil is too high, the fork will be stiff and if the viscosity is too low the shock will be too soft.

There are fork oils available in the market with viscosity modifiers that hold the viscosity under a wider range of operating temperatures to help maintain a more constant ride experience. They are more expensive than standard engine or transmission oils, but are well worth the price.

Your clunking could be caused from too much or too little air in the shocks and the clunk could be when the shock fully extends or fully retracts. So that being said, knowing that the front fork suspension has a travel of 200mm, you should set your air pressure in each shock when you're sitting on the bike and set the initial ride height at 100mm of compression. This is a 2 person job. You need to measure the distance between the oil seal and the lower triple clamp when the front wheel is off the ground then subtract 100mm. That's your ride height. Now put the bike on the ground and you sit on the bike and start adding air to the shocks equally until you achieve the correct ride height, then go for a ride and see if you've got rid of your clunk. If you find it's still clunking, try to determine if it's when the forks are being compressed or on extension then adjust your air pressure to accommodate. If it's still clunking after air adjustment it could be caused by the control valves on the orifices being held open with debris allowing the oil to free flow uncontrolled from the upper chamber to the lower chamber and vice vresa.

Thank you Kawboy. Will check the ride height and adjust air pressure accordingly if necessary. Should I still have the clunking noise after those adjustments and prior to striping them to check for debris etc. what are the thoughts on going up to a 20 Weight fork oil. I currently have 15 weight in them. A little concerned that if I go too heavier a weight that I may blow my fork seals. Any thoughts on that?

The pressure in your forks is only dependent on the air pre-charge and the compression of the fork tube. Oil viscosity doesn't have much to so with that. You wont "blow the seals" with heavier fork oil. Not on forks like these anyway.

After reviewing comments from past postings on the forum I went with 15w fork oil, but 20w is probably not excessive for bike as heavy as the 1300, it will depend on your riding style and preference. 10w is probably too light in most cases though.

Craig wrote: Thank you Kawboy. Will check the ride height and adjust air pressure accordingly if necessary. Should I still have the clunking noise after those adjustments and prior to striping them to check for debris etc. what are the thoughts on going up to a 20 Weight fork oil. I currently have 15 weight in them. A little concerned that if I go too heavier a weight that I may blow my fork seals. Any thoughts on that?

20 weight oil would be fine. the manual calls for 10W20. I would start with setting the air pressure and see where that leads first.

A shock expert told me a while back that fork oil viscosity varies between brands something shocking .
So it might be wise to use the same brand somebody else is having success with that is about the same weight as you.
I did find a comparison chart a long time ago but this an example .
s158.photobucket.com/user/WadeHarley/med...ybrandname2.jpg.html