LCGB Forums

Fully expect this to be shot down , but interested to know the thoughts of those that know.
My only experience of layshaft failure was by proxy. I was with Adam when he had his moment.
I guess the manner of his failure was the same as most of not all of the others. The threaded end snapped off.
That being the case, would there be any improvement by machining off the threaded section drilling into the shaft , tapping and securing the hub by means of a bolt. Similar to the method used to fit the sliding dog to the crank.
The thinking being the bolt can be changed periodically eliminating the worry of fatigue.

Thoughts please.

Ahh got it

Engine braking.

Got what?

Forget the second post.
The effects of engine braking shouldn't effect the bolt or stud any differently.

The trouble with drilling and tapping and using a bolt to secure the hub is that you wont be able to get the same amount of torque as with the large nut and washer.The problem that arises when a nut is torqued and then undone quite a few times over the years is that it weekens the threaded end of the layshaft and it eventually fails.At least this is how I see it.

Yep, I'm watching this thread with interest as Scooterlam has pointed out that it was my incident that he witnessed, with me being very lucky that he was there for, helping me to get the bike across a busy slip-road junction and then lifting it over a crash barrier!

I've no doubt that the heavy bag that I had on the back would have played a role in the failure, however, these bikes are designed to carry 2 and I was only 1 up. I'm also not surprised that my failure happened at the bottom of a long right hand and fairly tight curving slip road, which meant that the bike was leaning over to the right and putting a considerable force through the rear wheel/hub/layshaft (all perfect condition Italian originals). However, just like the 2-up comment, these bikes are also designed to go around corners, rather than simply be ridden bolt upright on straight roads.

The worrying thing is that when this happens to you and you start to talk about it, the more and more instances you hear of! I've since heard of layshafts failing on others, after being correctly fitted and torqued up (not over-torqued), with the layshafts ranging from original Spanish or Innocenti to brand new SIL layshafts from a known source and/or on complete SIL machines (when available from new), and through to modern/new expensive aftermarket layshaft from a respected tuning house.

It seems as though age. brand or cost may not be the way around this issue so I'm interested to see what ideas this thread generates, as well as any feedback on what may be not so good!

Here's hoping that it doesn't happen again to me or any of the rest of you. With age being a potential factor, let's hope this does not become more frequent.

Adam

Adam_Winstone wrote:
I've since heard of layshafts failing on others, after being correctly fitted and torqued up (not over-torqued), with the layshafts ranging from original Spanish or Innocenti to brand new SIL layshafts from a known source and/or on complete SIL machines (when available from new), and through to modern/new expensive aftermarket layshaft from a respected tuning house.

Adam

That's a bit worrying really, I was looking at a strengthened layshaft available from a certain Northern based tuner and thinking 'ah good to see there is a cure to the snapping layshaft problem' seems even that isn't without failure or are referring to another supplier/manufacturer Adam? Paul

I think HxPaul is right about the size of the bolt being a factor of concern. I doubt whether you'll get much larger than an M8 which would not fill me with confidence.

Something Adam said about cornering being a factor in the failure reminded me of the 80's. When lambretta were first launched we could not corner as we do now because the tyres were square! We are now cornering harder and faster than we used to and that has to be a factor in the failure of the lay shafts especially the older ones.

I've had a layshaft go and pure luck meant I didn't even fall off; It was an original italian snapped in the normal spot. I don't think there is ever going to be a perfect solution. My personal solution is to hope it doesn't happen again but I carry a spare shaft with me.

The reason for the failure of any layshaft that conforms to OEM (Original equipment manufacturer) standards will be due to a number of quantifiable issues.

Most people might well attribute age alone, but that is, in my opinion, rather a ‘Red Herring’

It would be good to think that ‘we’ could sing from the same hymn sheet to try to resolve this issue & I don’t personally believe that a new replacement is the cure.

I’d hope that there is a common consensus of opinion that we could sort out the reason for failures, between us, with what should be a sense of goodwill.

So, having thrown down the gauntlet & with apologies for repeating myself in any way, I’ll state my belief.

The following presumes all components are first class OEM quality, but there is one component that sneaks under the radar:

I think that the integrity of the assembly of the rear hub, cone & layshaft is fit for purpose as long as the hub nut is tightened to the correct torque & maintained @ that torque as long as all components are OEM.

However, as a nod to what others have stated, then, of course, if the layshaft has ever been subjected to the nut being over or under tightened, then it will have been subjected to stresses that may manifest in failure @ any point.

However, it is my experience that one of the key components in maintaining the correct torque of the hub nut (which stretches or ‘pre-loads’ the layshaft to the desired amount) is the hub cone which may not be to the correct standard of bore hardness & allows the layshaft splines to indent the surface, thus reducing the ‘tightness’ of the hub nut.

Does anybody agree or disagree or am I alone in witnessing badly burred bores of those new replacement cones when I remove rear hubs?

Wasn't there a spate of layshaft shearing a little while ago with people who were running R1 shocks on their machines? (could be wrong here) but I thought that the extra stiffness of the shockers were passing the bangs and crashes of the road impact onto the layshaft causing a guillotine effect? Paul

Wasn't there a spate of layshaft shearing a little while ago with people who were running R1 shocks on their machines?

Paul

Thank you for the response.

Personally, that’s news to me, but others may verify your suggestion.

TBH, I immediately thought of sprinters that use no rear shock absorbers @ all. The tracks they use are not renowned for being smooth, either!

So, it would be great to have feedback from sprinters that have, or have not for that matter, experienced layshaft failure.

Warkton Tornado No.1 wrote:

Wasn't there a spate of layshaft shearing a little while ago with people who were running R1 shocks on their machines?

Paul

Thank you for the response.

Personally, that’s news to me, but others may verify your suggestion.

TBH, I immediately thought of sprinters that use no rear shock absorbers @ all. The tracks they use are not renowned for being smooth, either!

So, it would be great to have feedback from sprinters that have, or have not for that matter, experienced layshaft failure.

Not a problem, I kind of go along with the theory of if a component (layshaft, hub etc) is worn or damaged then this could have a knock on effect. still would be nice to get to the bottom of this. Paul

Does everyone use the same torque setting? It's hard to achieve if you're 11 stone wet through (but I manage )& if locking ring holes not lined up - squeeze a bit more out of torque wrench or back it off. (I know what the book says) Be interesting to know if state of engine where failures occur, in terms of one extreme being standard and other extreme monster tune, are a factor or an irrellevance.

I always torque to just under the book value and then work up in smaller increments towards the correct value, meaning that I never find myself aiming for the exact figure and finding that I've just passed a hole and would have to torque considerably more to reach the next hole.

Funny thing is that there is video footage from the factory assembly line that shows the hub being done up without a torque wrench and then knocked round to the next hole with a spanner and mallet, hardly scientific or accurate!

Adam

Spent ages on a post then lost it

Summary is, if the cone is fitted correctly any force is transmitted to the lay shaft gearbox side of the threaded section.
Cornering forces shouldnt be a factor as centrifugal forces will keep the load perpendicular to the lay shaft.
If as warkton suggests the cone is made of the wrong grade of steel and loses the interference fit its possible load will transfered to the nut via the lock washer.
As Warkton says examination of high milage set ups could confirm this.
This would explain why the origin of the failing layshafts isnt consistant.

Adam_Winstone wrote:I always torque to just under the book value and then work up in smaller increments towards the correct value, meaning that I never find myself aiming for the exact figure and finding that I've just passed a hole and would have to torque considerably more to reach the next hole.

Funny thing is that there is video footage from the factory assembly line that shows the hub being done up without a torque wrench and then knocked round to the next hole with a spanner and mallet, hardly scientific or accurate!

Adam

In a perfect world the same number of turns would result in the same tourque.

I have wondered if there might be any evidence of wear in the fit between the cone and the hub. Repeated removal and tightening might cause some reduction in the efficiency of that tapered joint, especially if the two materials are of inequal hardness?

If this was the case then it might lead to pressure elsewhere in the set up.

Of those that have seen a failure of the layshaft do they usually fail at the bottom of the thread?

Adam_Winstone wrote:Funny thing is that there is video footage from the factory assembly line that shows the hub being done up without a torque wrench and then knocked round to the next hole with a spanner and mallet, hardly scientific or accurate!

Adam

I've visited the workshop of a very well known European scooter restorer who uses the spanner and mallet method rather than a torque wrench

As my "shit happens" post wasn't all that positive I'll put forward a more productive solution.

When I was involved in sorting out construction defects the first thing we needed was accurate data from a qualified independent expert, At the moment we are bouncing ideas around about the possible causes of layshaft failure with lots of variables.

If we want to take this issue any further we need proper research.We first need a volunteer from within the LCGB to record any reported failures. Then we need funding to instruct an engineer to investigate the failure. We need one expert to look at them all as if we get more than one (expert) the results may not be consistent.

In my view that would need the broken parts to be forwarded to the expert for inspection / testing.

At the moment I suspect the expert would need the following:
[list=]Broken layshaft / rear hub and cone (I doubt most people will be able to supply the nut and thread).
Origin of layshaft hub and cone
Engine specification
Torque settings used (this needs to be as reliable as possible)
Road conditions.

I have no doubt others and any expert will have more suggestions.

The next problem will be who pays for all of this investigation; experience tells me it won't be cheap. Someone will need to put a case to the committee or does the rider pay?

If members want to take this seriously then this is (in my view anyway) the only way forward

If we want to take this issue any further we need proper research.We first need a volunteer from within the LCGB to record any reported failures. Then we need funding to instruct an engineer to investigate the failure. We need one expert to look at them all as if we get more than one (expert) the results may not be consistent.

Jim.

I agree with the sentiment of your response, but, I believe there may yet be a number of ways in which we can ‘conspire’ to root out the main suspects.

Personally, I have gathered up the cones that I have to hand in the conservatory. I have not ventured down to the Garage. The Butler has yet to finish ironing my overalls….

Of the six cones, four were the older, steeper taper & two the later, shallower taper.

Of the two shallow taper as utilised in Spanish hubs, the oldest was the best with no burrs & the newest, the worst with significant burrs.

Of the four steeper taper cones, all were burred up to an extent, but, considering their likely age & history, not exceptionally & I would be prepared to use them again after dressing the burrs & possibly a hone.

I think five of the six cones would be thirty years old or more & the worst, the newest, less than five years old & supplied by a reputable dealer.

Using my ‘seat of the pants’ file hardness tester, I would say that the newest is the least hard in the bore.

None of the cones appear to have any indication of manufacture though one or two may have some lettering. As it is, I don’t know where or what to look for as all I’ve ever done is check the taper of cones in the hub & grind them in with paste as necessary.

So my own test is by no means conclusive, but it would be great to have feedback from other members.

As far as hardness testing I will have a word with Warkton Tornado No. 2 to see if he can sneak anything into work for Rockwell/Brunel testing. He appears to be happily married & has three children but understandably doesn’t like anybody knowing that he’s a voluntary test pilot for the manufacture of Butt Plugs which have to remain hard during use….