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350TSS · 13-10-2017, 01:06 AM

Thanks guys – all good information/advice.
Darren69: I do not think the stems will swap over, I haven’t measured them and I do not have them in front of me but the M900 one is c 25mm diameter x 17mm bore (definitely) so c 4mm wall thickness whilst the S4 one is c 28mm diameter and c2.5mm wall thickness so 23mm bore. In either case, I am also not sure how the steel stem is fitted to the aluminium bottom yoke. It could be press/interference fit or cast in with some form of retaining serrations/barbs (I am pretty sure it is not threaded in). The old stem will almost certainly fall through the hole left if I ever managed to get the stem out of the S4 bottom yoke.
I think my ideal solution, if and only if I can get compromise bearings is to use the S4 bottom yoke and bore the M900 top yoke.
The second best is to bore both the M900 top and bottom yokes, compromise bearings are not an issue in this case.
The third option is to bore the M900 bottom yoke and buy a new top yoke with the right bore. When I had the exhaust made, Mark Lumb of MADASAL had various yokes available which I was quite taken with (milled from solid with nice machined windows and at quoted prices around c£100 - £120 which I considered very reasonable). Also, he said I could have what I wanted in relation to other fixings, threaded holes for attachment of instruments, indicators etc., etc.
You will note the S4 top yoke does not feature at all in any of the above scenarios.
Whilst others, I am sure, may well have had successful shim /top hat arrangements I think I am fortunate that none of my scenarios would ever need them. I do not think they are a sound solution.
If you think of braking hard from 100mph + speed with an all up mass of shall we say 250 kg, assume that the front tyre would accept (without locking) about 1g deceleration and all the force is transmitted to the top and bottom yokes and at the bottom yoke there is a 1.5mm wall thickness 53mm diameter shim made of soft aluminium. Bearing in mind also that the shim is subject to the force being magnified by the effect of the “lever” acting between the tyre contact patch and the bottom yoke - c 600mm. The force acting on the shims may be ameliorated by the fact of the suspension moving. I am not an engineer but this solution does not look good to me.

13-10-2017, 01:06 AM	#351
350TSS Too much time on my hands member Join Date: Jan 2013 Location: Shipbourne Bike: M900 Posts: 1,420	Thanks guys – all good information/advice. Darren69: I do not think the stems will swap over, I haven’t measured them and I do not have them in front of me but the M900 one is c 25mm diameter x 17mm bore (definitely) so c 4mm wall thickness whilst the S4 one is c 28mm diameter and c2.5mm wall thickness so 23mm bore. In either case, I am also not sure how the steel stem is fitted to the aluminium bottom yoke. It could be press/interference fit or cast in with some form of retaining serrations/barbs (I am pretty sure it is not threaded in). The old stem will almost certainly fall through the hole left if I ever managed to get the stem out of the S4 bottom yoke. I think my ideal solution, if and only if I can get compromise bearings is to use the S4 bottom yoke and bore the M900 top yoke. The second best is to bore both the M900 top and bottom yokes, compromise bearings are not an issue in this case. The third option is to bore the M900 bottom yoke and buy a new top yoke with the right bore. When I had the exhaust made, Mark Lumb of MADASAL had various yokes available which I was quite taken with (milled from solid with nice machined windows and at quoted prices around c£100 - £120 which I considered very reasonable). Also, he said I could have what I wanted in relation to other fixings, threaded holes for attachment of instruments, indicators etc., etc. You will note the S4 top yoke does not feature at all in any of the above scenarios. Whilst others, I am sure, may well have had successful shim /top hat arrangements I think I am fortunate that none of my scenarios would ever need them. I do not think they are a sound solution. If you think of braking hard from 100mph + speed with an all up mass of shall we say 250 kg, assume that the front tyre would accept (without locking) about 1g deceleration and all the force is transmitted to the top and bottom yokes and at the bottom yoke there is a 1.5mm wall thickness 53mm diameter shim made of soft aluminium. Bearing in mind also that the shim is subject to the force being magnified by the effect of the “lever” acting between the tyre contact patch and the bottom yoke - c 600mm. The force acting on the shims may be ameliorated by the fact of the suspension moving. I am not an engineer but this solution does not look good to me.