Commando 850 Forks very stiff damping.

Well I stripped down both forks today, completely and this was my first time doing this. There was oil in both legs but only about 100cc and I could find nothing wrong. There were no rusty springs, oil seals all good and I took the damper tubes apart and all seemed as it should be...BUT i did not measure the length of the springs or check to see if anything was bent as it all looked ok. Put it all back together and I did notice that one fork did sound a little "scrapy" when I was pushing it up and down to get the new oil to go down. I did not use proper fork oil but 140cc of 20/50 engine oil in each leg. I know this is not the correct oil but it should be ok just to see if the forks were going to work better. When I lowered the bike off the jack the legs did not compress and when I applied front brake and pushed hard they dropped a little and stayed there, they did not rebound. I have not ridden it yet. Should I bite the bullet and replace the springs, damper tubes, damper rod and valves etc?? To see if this helps. Help!! please:) This bike has not been looked after properly, this I am finding out.
 
Hi Pete IMHO no don’t ride it…possible top bush and slider “stiction”. Mine stuck on a test ride after rebuild and not fun getting it home. L.A.B. and others with greater knowledge may confirm stanchion should “drop“ thru slider so strip down and check.
 
By the way you describe things I suspect the issue is more likely stichtion than dampening.

One way to check is to stand the bike so you can remove the front wheel and guard. Undo and remove the top nuts. Move each slider up and down. They should be smooth without much friction. Refit the axle only and see if the movement is still smooth. Refit the guard and again check for smooth movement. I expect you will find you get friction somewhere.

It's very common for triple clamps or the fork stantions or both to be bent. They need to be checked for straightness using a granite table. Correcting it is really a job for an expert. To do it yourself you need a dead flat surface like a granite plate and a pair of large calipers.Roll the fork tubes and see if they are dead straight. Assemble them in the triple clamps and again check they are straight fkat and parallel. You will need to take the triple clamps off the bike.

If the forks are straight but not flat or parallel on the plate then you will need to straighten the triple clamps. Use an old fork tube to twist and straighten them. Fixing this is not easy and takes a lot of trial and error.
 
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Did you check the stanchions were absolutely straight
And that each fork plunged perfectly without the spring fitted
Upon assembly you loosely assemble the whole front end
Tighten the top nuts plunge the forks a few times
Then tighten the bottom yoke pinch bolts and plunge again
Make sure the fork slider pinch bolt side is free to move sideways,lube if necessary
Then plunge the whole front end again a few times as you tighten the pinch bolt (don't crack it)
Then check if the front end is working correctly before you fit the mudguard
 
Peter Waller said:
There were no rusty springs, oil seals all good and I took the damper tubes apart and all seemed as it should be...BUT i did not measure the length of the springs or check to see if anything was bent as it all looked ok.
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Some important checks appear to have been missed then, such as checking the straightness of the stanchions and if each moves freely in its slider without the damper assembly installed and then with the damper assembly and the cap bolt fitted but without the spring.

Peter Waller said:
Put it all back together and I did notice that one fork did sound a little "scrapy" when I was pushing it up and down to get the new oil to go down.
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Some spring noise isn't unusual.


Peter Waller said:
I did not use proper fork oil but 140cc of 20/50 engine oil in each leg. I know this is not the correct oil but it should be ok just to see if the forks were going to work better.
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The original fork oil specification included engine oil (Factory manual, Section K, Routine Maintenance, Lubrication Chart, ).

Peter Waller said:
When I lowered the bike off the jack the legs did not compress and when I applied front brake and pushed hard they dropped a little and stayed there, they did not rebound.
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That could also be due to misalignment of the yokes or twisted mudguard bridge.

Did you check the lower yoke was free to rotate on the stem (as it isn't unknown for the lower yoke to be rusted to the stem) before refitting the fork legs and bouncing the forks and that both yokes were marked 'ANG' as this is important?

Peter Waller said:
Should I bite the bullet and replace the springs, damper tubes, damper rod and valves etc??
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No, I suggest you spend a little more time trying to identify what the actual problem is.
 
If the forks are as bad as you describe you definitely should NOT ride it like that. It’s honestly not safe or even legal to ride like that.

You really need to follow the advice already stated by Baz and LAB and others.

Mechanic work is not only about slinging things together and going for a ride. You really do need to assemble and check, assemble and check, every step of the way. Usually with some fettling / finessing required and often in a two steps forward, one step backwards kinda dance !
 
Thx for all the replies and will strip again and go through everything more carefully and do the checks as suggested. Thx again and will inform of the results!
 
Peter Waller said:
Thx for all the replies and will strip again and go through everything more carefully and do the checks as suggested. Thx again and will inform of the results!
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Even if you had a set of forks in good fettle and you didn't plunge the forks during the tightening sequence there's a chance they would bind as you describe
Also if you tightened the axle pinch bolt first it could give the same results
 
baz said:
Even if you had a set of forks in good fettle and you didn't plunge the forks during the tightening sequence there's a chance they would bind as you describe
Also if you tightened the axle pinch bolt first it could give the same results
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Further to this and I don't know if others do it but whilst the whole front end is still loose including the steering stem nut I stand astride the front wheel and push the handlebars left and right a few times before letting the wheel settle in the centre
Also If available check your stanchions for straightness with a pair of vee blocks and a DTI guage
If nothing available at least try the stanchions against each other
Good luck with it ,slow and methodical here will save time later
Cheers
 
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Baz seems to have "hit the forks on the head" I loosened up the lower yoke clamping bolts, the fork axle clamping bolt and the axle nut.....plunged the forks a few times with the front brake on and now they work:) Albeit a little stiff still but the damping is now working so thanks everyone for there input. Perhaps a lighter oil may help as they now have 20/50 engine oil in them BUT they are now working. Thx again everyone and now onto the next issues!!
 
Peter Waller said:
Baz seems to have "hit the forks on the head" I loosened up the lower yoke clamping bolts, the fork axle clamping bolt and the axle nut.....plunged the forks a few times with the front brake on and now they work:) Albeit a little stiff still but the damping is now working so thanks everyone for there input. Perhaps a lighter oil may help as they now have 20/50 engine oil in them BUT they are now working. Thx again everyone and now onto the next issues!!
Click to expand...

I would suggest you buy a workshop manual.
 
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