Sherpa 198A Swing Arm Resistance

[JonnyWobbles]

Hi, the rebuild of my Sherpa 198A is progressing 'well' but I have a question on what is the expected / acceptable swing arm free-play and 'stiffness'. I have replaced the nylon/alloy swing arm bearings and spindle with new items, but find that with the newly powder coated frame and new replacement rear engine mount (all from In Motion) its all a very tight fit - requiring the rubber mallet persuader to fit! The result is that although nice and smooth, the swing arm does not freely move under its own weight (a rough measure I've used on road bikes with needle bearings) but requires some force (approximately a 1kg weight placed on swing arm) to move even without the swing arm spindle nut tightened. It has to be said, the resistance is very similar to that when I stripped the bike down but I'd put this down to worn components etc.

I've no experience of the nylon/alloy bearings and whether these will ease with use, but these themselves did tighten after being driven into the (clean) swing arm which will account for some of the resistance.  But it has to be said longitudinally with the powder coating there is no free play between swing arm bearings and frame even before the nut is tightened etc. incidentally i couldn't find a torque setting for the nut as the manual just says 'tighten', but obviously the tighter it goes the stiffer the swing arm gets.

Before I strip it all back down and take grinder to the lovely powder coating and possibly wet and dry to the bearings, could any one advise on what is acceptable resistance from their experience as I can't find any info in the manuals? Unfortunately I don't have a wheel built up to fit to gauge the effect of the increased weight etc. but it does not seem quite right to me!

Thanks as always!

 

 

[woody]

These can be an absolute bitch sometimes.  I've never used the alloy/nylon bushes, only the bronze/steel but the same principle applies and there is more than one cause.

Before the bushes are fitted the inner bush should slide and rotate freely in the outer bush. When the outer bush is pressed into the arm the inner bush should still slide and rotate with little or no resistance, but the ID of the outer bush can shrink once pressed in and this can cause the inner bush to bind up so they might need skimming to rotate freely.

Once they rotate/slide freely the next problem can be the clearance of inner to outer bush when the spindle is tightened. The inner bushes contact against the engine bracket so that when the spindle is tightened the inner bushes tighten against the bracket and leave clearance between the collars of inner/outer bush. No clearance means the inner bush collar tightens against the outer bush collar and locks up the swingarm. There are two solutions, one is to put a shim between the inner bush and the engine bracket if there is enough room, the other is to remove material from the inside edge of the collar of the inner bush - both are trial and error as to shim thickness or how much material to remove. When working properly the spindle tightens the inner bushes up against the bracket leaving the swingarm free to rotate around the inner bushes. You can see the outer bushes rotating with the swingarm and the inner bushes remaining static when it is working correctly and the swingarm moved up and down

Powder coating can be thick enough to stop the swingarm being fitted into the frame and sometimes it has to be removed from around the area where the swingarm sits. It shouldn't really be a cause of the binding up, it can just prevent the swingarm actually being fitted

[JonnyWobbles]

Thanks Woody, fundamentally all of the issues you mentioned plus one of the swing arm tubes itself was not absolutely parallel. Not sure if it was built that way or had suffered a knock at some time. The powder coating was quite thick making fitting the swing arm difficult - but it was tight when I removed it. So to get some clearance I sanded off the powder around the holes, filled and squared off the swing arm ends and used emery cloth on the alloy bushes to get a nice smooth fit as the nylon was pinching when pushed into the arms. Net result easier to fit and just the right amount of friction when all tightened up! So result! Thanks for the advice!

Wheel building next!

?

 

 

[carl ekblom]

Actually, I don´t think the plastic bushings are made of PA(nylon). I guess they are made of POM which often by mistake is called Nylon. POM is a totally different material and should be superior in that application. Other "good" plastics are available, for instance (if protected from sunlight) PE MOS2 (Robalon) and PET PTFE (if not subjected to lot of water).

PEEK is also good but it has a very high price tag.

High-density PE (HDPE) is used for petrol tanks and withstand almost everything but is not as far as I know used for bearings.

In addition to what Woody says and he is correct

For proper fitting I think the frame should be about 0.5mm wider then outside the swing arm bushings. It will then be tight when you tighten at swing arm axle. 

After tightening the axle and rear engine mounting in place there should be some side play in the swing arm bearings.

With the axle tightened and everything is OK the swing arm should move freely up and down by hand without any tight spots.

My never ending 199B project. Someone long time ago painted the frame blue? Will be red a la Yrjö Vesterinen

[JonnyWobbles]

Thanks Carl, good info. I agree a clearance of 0.5-1 mm seems about right, makes fitting swing arm possible without resorting to rubber mallet and allows the spindle nut be tightened without affecting swing arm movement!

My 198A frame was in a pretty sorry state with chips and rust and had at sometime been painted very badly in grey paint of some description, obviously with the bike fully assembled as there were drips all over the shocks and wheels etc! Its obviously been well used! 

[carl ekblom]

I think the white framed 199B/198B looks nice?.

As my 199B will have a number of modifications (longer rear shocks and modified mounting points, swing arm axle moved forwards and downwards, better brakes etc) inspired by this Yrjö-bike I think it should have red frame and red tank. I think the red Yrjö-bike on picture is a modified 198B.

[JonnyWobbles]

Hi Carl, interesting project you’ve got! I’m new to trials bikes so I thought I’d stay fairly stock, although due to the nature of the sport all the bikes have been modified and alternative components fitted, which is part of their DNA! I guess when I start riding it I’ll find out it’s shortcomings or maybe not being a novice! The build is coming along, just been fiddling around with throttles and cables this week. Next big job is to tackle the wheels, I had a guy lined up to rebuild these but with our current lockdown can’t get over to him. So I thought I’d have a go and can always get him to fine tune them! 

[pschrauber]

I have been using needle bearings very much but discovered silentbloc to be much better in all occasions, as theses are water proof, gives extra cushing to the drivetrain and do have even a
tiny bit of extra damping to the rear suspension.
And to older Bultaco bikes these silentbloc came with them too, like the model 187 or 221 which I had.

I "re-" learnd all about it with my SWM and too the Panda which have also silentbloc's mounted, so to newer builds I use them now with good effect. All I do is to put some never seize between the silentbloc bushing and the swing arm shaft.

If in need I will even to my 199b replace it with  silent bloc "bearing" so convenient.

[carl ekblom]

4 hours ago, pschrauber said:

I have been using needle bearings very much but discovered silentbloc to be much better in all occasions, as theses are water proof, gives extra cushing to the drivetrain and do have even a
tiny bit of extra damping to the rear suspension.
And to older Bultaco bikes these silentbloc came with them too, like the model 187 or 221 which I had.

I "re-" learnd all about it with my SWM and too the Panda which have also silentbloc's mounted, so to newer builds I use them now with good effect. All I do is to put some never seize between the silentbloc bushing and the swing arm shaft.

If in need I will even to my 199b replace it with  silent bloc "bearing" so convenient.

You have a good source for silent blocks? Buying 100´s of them to get 2 is not very attractive.

[carl ekblom]

5 hours ago, JonnyWobbles said:

Hi Carl, interesting project you’ve got! I’m new to trials bikes so I thought I’d stay fairly stock, although due to the nature of the sport all the bikes have been modified and alternative components fitted, which is part of their DNA! I guess when I start riding it I’ll find out it’s shortcomings or maybe not being a novice! The build is coming along, just been fiddling around with throttles and cables this week. Next big job is to tackle the wheels, I had a guy lined up to rebuild these but with our current lockdown can’t get over to him. So I thought I’d have a go and can always get him to fine tune them! 

Yes, they are nice bikes. Good project?.

Little curious about the throttle. To me it looks like the one for Beta 4T and it should be a very quick throttle for a normal round throttle valve trials carb. I think they from the beginning were made for 38mm(?) motocross carbs

[JonnyWobbles]

It’s an old Doherty throttle, but I’ve had to junk it as it just too worn and the sleeve very distorted and shortened. The throttle just wouldn’t snap back. So I’ve fitted a new Amal T80/200 slow action trials throttle. Nice bit of kit! (I initially bought a cheap Domino but couldn’t bring myself to fit it, very plasticky!)

[pschrauber]

5 hours ago, carl ekblom said:

You have a good source for silent blocks? Buying 100´s of them to get 2 is not very attractive.

I only buy as many as I need buying 100 why that?

Here is an aniline shop that does sell too to private people in low numbers, you can get just one,
As mentioned at the web page not everything is on their onlinelist you have to ask for rare dimensions.

https://gummi-metall-puffer-shop.de/silentbuchsen

A better resource but only for companies, and if you ask them they also provide just one! 

https://silentblock.de/produkte.php?sprache=Deutsch

 

I have also made my rubber parts and silent block using this set set sold by Corrosionschutz Depot, with good effect,
My set up, which I have at hand for making new rubber parts, you burn out the old rubber and reuse the tubes:

Their web page: 

480 gr,; https://www.korrosionsschutz-depot.de/restaurierung/reparatur-kits/gummiteile-anfertigen/ksd-flexi-mittel-480-g

960 gr.: https://www.korrosionsschutz-depot.de/restaurierung/reparatur-kits/gummiteile-anfertigen/ksd-flexi-mittel-960-g

 

[feetupfun]

I also have used silent blocks for swingarm pivots and in Australia, individual silent blocks can be bought from Blackwoods

[feetupfun]

8 hours ago, JonnyWobbles said:

It’s an old Doherty throttle, but I’ve had to junk it as it just too worn and the sleeve very distorted and shortened. The throttle just wouldn’t snap back. So I’ve fitted a new Amal T80/200 slow action trials throttle. Nice bit of kit! (I initially bought a cheap Domino but couldn’t bring myself to fit it, very plasticky!)

I've got a mix of AMAL 80/200, Domino slow and Suzuki DS80 twistgrip assemblies on my twinshocks and they all work very well but have found that the AMAL and DS80 types are more easily broken in crashes