Hitchcocks Forum

Hello all

I wonder if anyone could shed light on what is hopefully a simple problem with front forks on my 2006 Bullet. This has the later disc braked forks and as I was finding the damping action rather harsh, particularly on broken road surfaces, I decided to change the fork oil. After checking through the blogs here and elsewhere, it seemed like a simple job, and indeed it was - bottom nuts loosened off with no problems and about 150ml of blackish oil drained out of each. Almost certainly the same oil that was in them from new. I refilled with 200ml of fresh SAE20 and refitted nuts with new copper washers. Job done I thought. A quick test ride confirmed all seemed well - I thought maybe the damping was a little smoother and no clonks or noises. As I wheeled the bike back in the garage and stopped it using the front brake, the forks compressed slightly and then stayed there. Put bike on stand - forks extend to full length, pull off stand and with weight on bike on them, they compress about one and a half inches. There is still some suspension travel, but they didn't used to do this. The front end is now sitting low enough to bend the front brake hose out at a sharp radius, something that probably only happened before when I went over a pothole that would have compressed the forks a couple of inches momentarily.

I rode it carefully a couple of times to see if something needed to settle or if air was somehow in system - excuse my lack of knowledge here - but nothing changed. Just as a check, I drained and refilled both forks carefully again, but no improvement. The bike still settles lower than it originally did. Interestingly, although just about 200ml of my new oil drained out of one fork, only 150ml came out of the other. There are no signs of leaks anywhere externally and no bits of rubber, dirt etc in the oil that came out. Mystery to me, but I'm sure more experienced hands will have come across this before. I'm assuming a fork rebuild is the likely outcome, not something I will attempt on my own, but just wanted to check before incurring expense.

Best regards,

Phil

mtype - When you say it was a simple job, did you have to remove the fork legs from the casque at the top? The one which houses the speedo etc?
You mentioned removing the fork nuts at the bottom of the forks but nothing about where you actually poured in the oil mate?
Did you have to take out the springs or any of the fork internals to do this?

I suspect you now have the forks working properly.
I've always found SAE20 a bit stiff and settled on ATF (which I understand is about SAE7)

Hi, what type of oil did you fill the forks with? If too thick it can cause the problem you describe. When draining, its always best to pump the forks up and down to remove the oil. Try again and see what the action is like without oil. Vince

Or possibly if you removed the front wheel you have refitted the spindle with the fork legs being held too close together or too wide apart which will cause the forks to jam when compressed.

When you drain the oil you need to pump the forks a few times to push out any stuck oil. If you didn't then it's likely you have now got too much oil in.p perhaps it accounts for the missing 50ml. The recommended oil is 10w/30 fork oil. If there is any doubt as to the grade you put in Halfwords do that motorcycle fork oil in their bike section.

It does sound as if the forks are binding because they are not parallel.
Perhaps the brake hose has simply been reattached to the front end slightly differently than previously.

Thanks for all the quick replies - much appreciated. To answer a few of the points brought up: I didn't have to disturb the forks in the casquette, or remove wheel. Oil drained via nuts at bottom of forks and topped up via screwed chrome plugs at the top. Nothing else dismantled i.e. brake hose is exactly where it was before, except now has a bend in it! I did give them a half hearted pump or two after draining oil, but didn't appear to be any more to come out, so refitted bottom nuts and refilled. Oil was SAE 20 motorcycle shock absorber oil - I was a bit dubious about going as thin as ATF even after reading the various pros and cons on this site and others.

Pete F's comment that now I have forks that are working correctly is interesting. They were pretty solid feeling before I drained the original oil and gave a really hard ride, tending to crash over bumps and holes in road. So maybe not right initially? No idea what they were filled with at the factory, but it didn't look too good when it drained out and was definitely thicker viscosity that the 20 grade I put back in. My thoughts with the 20 grade was - its what Hitchcocks specify and I hoped it would give a slightly softer, more forgiving front end. Not sure about the comments on binding, as the fork action is quite smooth through its travel, its just the initial settling down as the weight comes on the forks that is unsettling me!

However, its MOT time tomorrow, so will see if tester picks up on anything. I will also try draining, pumping and refilling again at weekend. If all remains the same, I think I will try and adjust the fixing point for the brake hose to avoid the kink and then get on with riding it.

Best regards,

Phil

Old dodge - loosen the two lower yoke pinch bolts and flex the wheel from side to side, rather like straightening the steering on a push bike, the top yoke being clamped the fork legs should now find a comfortable parallel position; re-tighten. Forks can twist out of parallel and this won't have been done for a long time.



The forks do need plunging to force the old oil out, and if the old oil has been in there a long time then you may well need to flush with fresh oil a few times. Indian oil is basically rubbish and blackish is not good.



The bike's weight is supposed to compress the forks. The new oil should let them move more freely but the improved damping should stop them bottoming out or over-extending.



Do we know the correct ride height?

I thought it could be useful to know a little about the oil and specifications for this application. The SAE (Society of Automotive Engineers) spec was originally made for engine oils and the ranges are quite wide. For example an SAE 20 oil could have a viscosity of between about 35 CSt (CentiStokes - Stoke was a British Physicist) and 70 cSt at 40 C. Kinematic viscosity measured in cSt is a more accurate measurement of the oils ability to flow. and is generally used in an ISO scale for industrial grade lubricants. The temperature is important because oils change viscosity as they change temeperature and this is measured by an index called the VI (viscosity Index). So, depending on the quality, an SAE 20 oil could be twice as 'thick' at the top of the scale compared to the bottom and it would still be SAE 20. Automatic Transmission Fluids (ATFs) generally have a viscosity of around 37 - 38 cSt at 40C (bottom end of the SAE 20 scale) so fit in a much narrower band of viscosity. They also have to meet some reasonably severe specifications established by manufactures (Dexron, Mercon, Allison etc) An extra benefit of an ATF is that the viscosity index (VI) is very high and usually >150. This means that the viscosity does not vary as much as the temperature changes. Engine oils (mineral based ones) generally have a VI < 150. To sum this up it means that it is possible to have quite significant difference in damping between SAE 20 oils of varying manufacture or quality. ATFs, on the other hand, should provide more consistent damping across manufactures and temperatures.