clutch slip ?

[Adam_Winstone]

^... cool, I'll keep scratching my head. Interestingly, the most recent time that I suffered from this was after fitting one of the BGM front sprockets that turned out to be soft (issue rectified and new sprocket, gasket and gearbox oil sent out by Scooter Center, without questioning it... good service) and I too bought an AF clutch and new front sprocket spring in an attempt to rectify the problem. I ended up still having to fit a shim under the front sprocket assembly to stop the sensation of clutch slip, which is still in there to this day

I guess that you should be checking out that gearbox and sliding dog arrangement and continuing to rule one thing out at a time.

Best of luck with it.

Adam

[isthmus]

i had similar prob,front spro seemed to slip when decelerating, went through 3 chaincase gaskets,front spro asem, decided to look at low tension and starter coil replaced them no more problem a little knowledge can be an expensive thing,hope this helps,plain sailing,stay safe. kna i still have same probs i had before,i,ll,have to try again and look at my pre-load of primary drive assembly where i think the problem lies,see ya Si

[steve chapman]

Thanks everyone for your input , all sorted i had only gone and put the second gear in the wrong way round , ive built many engines over the years and never cocked up like that before , oh well cost me a few quid , but we all live and learn what a tool

[Warkton Tornado No.1]

Re: clutch slip ?
Postby dickie » Thu Jul 14, 2016 9:41 pm

Second loose gear wrong way round and not catching sliding dog properly?

Dickie sussed it!

Steve. Thanks for your honesty. A lot of people wouldn't have been man (or woman) enough to admit that, but I think it shows that we are all only human.

[Adam_Winstone]

Good to hear that you've got it sorted.

Adam

[JETEX]

I think it's best to post your outcome ( good or bad ) as it may help somebody else in the future. As my mate says - the man who hasn't made a mistake hasn't been born yet. My biggest mistake ?.....putting a new rear diff unit in a car. The recovery people brought it back the next day - diff seized. Yep, I forgot to put oil in it !

[isthmus]

You can look for wear on the front sprocket, which might help to identify the issue. As for curing it, you can change the sprocket (some machined too low) and fit a new sprocket assembly spring, however a good check to see if this is the case is to leave everything as it is but to fit a suitable washer under the sprocket assembly to jack it up and to add more preload to the spring. This may or may not put your sprockets out of alignment, which in turn can lead to chain wear, however, it allows you to test it to see if this is the reason for the feeling of clutch slip.

CamLam sell, or did a while back, shims of various thicknesses as they had found incorrectly machined sprockets to be an all too common problem, however, they were good enough to tip me off to the shim that fits between rear sprocket and endplate bearing being the correct size to do the same job. The shim needs to be fitted between the oil thrower washer and the splined column of the front sprocket assembly.

Adam

surley (to preload the spring) that would just raise the sprocket assembly if put under the packing washer before the sprocket sleeve,not take up the height of the sprocket to tighten the spring,to do that the packer should go on the sleeve,bombom I think that is right if you think about it,

[Warkton Tornado No.1]

Post by isthmus » Mon Aug 15, 2016 12:00 pm

Adam_Winstone wrote:
You can look for wear on the front sprocket, which might help to identify the issue. As for curing it, you can change the sprocket (some machined too low) and fit a new sprocket assembly spring, however a good check to see if this is the case is to leave everything as it is but to fit a suitable washer under the sprocket assembly to jack it up and to add more preload to the spring. This may or may not put your sprockets out of alignment, which in turn can lead to chain wear, however, it allows you to test it to see if this is the reason for the feeling of clutch slip.

CamLam sell, or did a while back, shims of various thicknesses as they had found incorrectly machined sprockets to be an all too common problem, however, they were good enough to tip me off to the shim that fits between rear sprocket and endplate bearing being the correct size to do the same job. The shim needs to be fitted between the oil thrower washer and the splined column of the front sprocket assembly.

Adam

surley (to preload the spring) that would just raise the sprocket assembly if put under the packing washer before the sprocket sleeve,not take up the height of the sprocket to tighten the spring,to do that the packer should go on the sleeve,bombom I think that is right if you think about it,

TBH I did see the well-intentioned advice & come to the same conclusion.

It all can get very confusing:

• To add preload/increase cam engagement of the front sprocket requires shimming on to the flat(ish) end face of the sprocket. Alternatively, shimming on to the ends of the spring would work but should be avoided as any shim fitted there will be subjected to far greater dynamic loads.

• Any shimming fitted between the front sprocket & the splined carrier will result in misalignment which can be corrected as pointed out in earlier responses.

I hope this helps.

[Adam_Winstone]

The shim sits on top of the oil thrower washer and under the splined column that it all revolves on.

Shimming does put the sprockets out of alignment if they are correct to start with and if it is wear to the ramped faces that is being corrected. However, it can actually be improving alignment if the front sprocket was machined badly to start with.

Adam

[isthmus]

so we are saying,to align the chain and the rear sprocket, you can safely,with the proper packing washer(shim)if you like, raise the sprocket sleeve,in turn raising the front drive sprocket,if needed for alignment.also using the correct packing washer(shim) to raise the sprocket preload (washer fitted on sleeve before primary drive cog)and also affecting the sprocket alignment with the rear sprocket,also as an add-on a shim could be fitted below the spring proper, can add extra preload also,not affecting chain allinement then i think we have lift-off,are we agreed Adam and TBH .yours Isthmus

[Warkton Tornado No.1]

I hate to appear pedantic, but placing any shim between “the oil thrower washer and under the splined column that it all revolves on” will only have an effect on chain alignment.

To increase ‘cam engagement,’ ‘preload,’ call it what you will, the shimming as stated above will not achieve the desired effect.

The critical dimension of the spring & sprocket assembly is controlled by the splined column & the splined ‘top hat’ washer.

A decrease in that critical dimension may be achieved in a number of ways, but not by shimming between the oil thrower washer and the splined column.

I just wanted to make that crystal clear

[Adam_Winstone]

I see what you're saying but know that this IS a working fix so need to better understand why it does work as it does.

The perfect example for me is the import motor that a pal stripped from a running bike (no sensation of clutch slip when brought in) when doing a strip and rebuild. He is a regular Lambretta engine builder with 30 years of Lambretta engine building behind him but had never experience this problem and/or fix before so he phoned me to discuss it. Knowing that there should not be a shim on top of the oil thrower, he left it out when rebuilding, but instantly ran into the clutch slip sensation caused by sprocket assembly spin. Stripped it down, found nothing wrong, then remembered the additional shim that had been in there, replaced it and found the problem cured... then phoned me.

Having used this as a sprocket spin fix a number of times (reality, rather than theory) I know that it works. All I need to do now is determine 'why' it works and the engineering behind it.

My first thought is that it may be down to the top bolt distance not being fixed by the top plate but rather being set by the distance of increased diameter bolt between threaded bolt bottom and underside of fixed bolt shoulder (loose washer on some later/aftermarket assemblies). This would set the distance between thread/splined face of crank and fixed distance of bolt shoulder, rather than the idea that the top tightens against the end of the splined column. That would explain why this does work in practice, as it does, as it would be the bolt 's unthreaded length that determines the assembly's final length.

Keep the top plate in the same fixed position and compress the assembly into a shorter distance, set by the shim face now acting as the lower extent, and the spring is force to compress more, resulting in increased spring load.

OK, so it is a working theory at best but we're understanding the assembly and mechanics incorrectly if we think that this is not a working fix, which I can assure you that it is. This is not me trying to say that I am right or that anyone else is wrong, rather, this is it dawning on me that the space available cannot be determined by the bolt's fixed shoulder sitting tight against the top of the splined column... which is why I'm starting to think that it is unthreaded shaft length of the bolt, which is independent of a slightly shorter splined column length.

For the above to be the determining factor then the crank's threaded length would need to be deeper than the thread length of the bolt (allowing bolt to determine final assembly space), which is something that I'll need to check.

Hmmmm, interesting.... we may all learn something from looking deeper into this issue!

Adam

[Adam_Winstone]

"The critical dimension of the spring & sprocket assembly is controlled by the splined column & the splined ‘top hat’ washer.", WT1... this is something that we have both taken for granted and I totally agree with you. However, having used this method of fixing sprocket spin a number of times (first hand experience that it does work) I simply need to figure out why we were wrong to think that to be the case. My response above is intended to be the first stab at considering what the determining factors might otherwise be.

This is in no way a challenge of who is right or wrong, simply a case of me wanting to figure out exactly why this does work in reality. This is something that I'm happy for us all to figure out, for the benefit of all.

Adam

[Adam_Winstone]

Right, well having just been out to the garage to see if I could better understand the critical dimensions, I'm currently leaning towards backing WT1 in this discussion

I've just compared a number of Innocenti SIII front sprocket assemblies to ensure that differences are not down to component variation and all of the splined columns are the same height, and the 2 original bolts that I compared are the same spec (plain shaft and thread length). Happy that they are close enough in tolerance to be considered the same, for this purpose anyway, I then offered them up to the casing on the bench that has a standard Innocenti crank in and went about trying to determine what the critical elements were for controlling the stop of the bolt's fixed shoulder in relation to lower lip of column (the limit of assembly and spring influence).

Without any doubt, whether the spring is fitted or not, the bolt does tighten against the top of the splined column as the limit to controlled space/distance available. In this case, WY1 is right to say that no amount of shimming between thrower washer and underside of the column lip will reduce the space available and/or increase spring preload.

TBH this has me more baffled then ever?!

Why do I know it to be the case that fitting a shim here has eliminated sprocket spin in 2 motors that I've worked on, for no other change? Why was a shim in this location also the factor that eliminated the sprocket spin in the import motor that my pal stripped and rebuilt to replace it?

If, as WT1 correctly stated, such a shim can only influence alignment or misalignment of the whole assembly in relation to rear sprocket, can there be any other factor(s) here that result in the assembly spinning or clutch slipping?

I'm completely stumped with this one now and I have no explanation for why this might work. Funnily enough, the guy that I used as the example (stripping and discovering a shim in this location and running into clutch slip/sprocket spin when he removed it) is a welder/fabricator/machinist and he'll be even more baffled than I am His OCD will really cause this to play on his mind, to try to understand the mechanics behind it.

WT1, hats off to you for being correct that the bolt shoulder/fixed washer tightening fully against the top of the column, thereby fixing the second end of the space available (lower lip of splined column being the other). I'd always assumed this to be the case but hadn't actually confirmed it on the bench.

More confused than ever.

Adam

[dickie]

I'm thinking out loud here, so forgive any bs please.....

The misalignment of the chain/sprockets will place a side load on the sprockets which MAY force the two halves of the front sprocket together depending on direction of misalignment. But I don't know how much due to several factors: shim thickness, available side-play in chain. It's probably unreasonable to calculate this due to the effects of side-play and chain speed. Like calculating friction; you can get in the right ball park but you can easily be out by a factor of 3 or 4 until you've actually measured it.

Personally I would have thought this had diddly-squat to do with it but maybe food for thought.

[Adam_Winstone]

"Personally I would have thought this had diddly-squat to do with it but maybe food for thought.", I've thought about this too and came to the same conclusion that, surely, it must not be enough to result in slip.

Knowing that the top cup and spring is different on SI models, I wonder if there are differences in other components too (e.g. different splined column or bolt length to allow for the assembly not having an oil thrower washer)? Could it be that the wrong combination of parts taken from a mixture of SI and SIII parts create a situation that encourages the assembly to spin, which could be resolved by shimming under the bottom?

Thinking back to when I've resolved the sprocket spin in this manner, I cannot be 100% sure that have not inadvertently swapped out another component that I'd assumed to be the same (a mixed tin of components). Obviously, I cannot categorically state that my pal didn't also swap out components, especially as he has other spares to choose from.

Still baffled

Adam

[isthmus]

what would you say,to bring up preload,milling or grinding down where the top hat sits(top end of sleeve)to negate shiming of rear sprocket upwards to maybe cause problem with clutch nipple and kickstart mechanism, to rectify preload of under sized sprocket or cush,olso what degree of pre can be achieved if problem persists. I can see this as speculative of what is safe,also I need to read about the front sprocket use of cush and sprocket configuration ,because it is there in order to slip in the first place.I think I,ve covered the use of overshiming the sprockets and why shortening the sleeve would help this,regards

[dickie]

Personally I'd be happy to put a shim directly below the spring. Just don't use really soft steel.

[Adam_Winstone]

Providing the components are to the correct specification and not worn then the front sprocket should not spin anyway. It is only when parts are worn, the spring has gone soft over time/through decades of use, or parts have been incorrectly machined/manufactured that the sprocket can start to spin. As such, I would only recommend that parts be replaced to achieve, rather than modified anyway... with this discussion being more a case of remedial of fix rather than best practice.

Rather than modified, replacement of worn or substandard parts is really the way to address it.

Adam

[Wack]

Having a few sprocket carriers in my parts bin, I often try several to get the sprocket alignment correct as there can be differences in the shoulder thickness. Perhaps this then causes the ramps to jump when using an original spring that may have weakened and lost its height over the years, certainly if you compare an old springs height to a new SIL one they're different.