Slow Monster rebuild

[350TSS]

This afternoon whilst admiring my new (to me) S4 purchase, I realized that the front fork legs from an S4 are a larger diameter than ’93 M900 at the bottom yoke. I should really have known that, I definitely do now.
No problem I thought, just use the S4 bottom yoke. But the steering stem is a different diameter which means I will have to do a mark 3 regulator rectifier bracket or think of some other means of accommodating it. Also, the clip-ons bought early on in the project will not fit. I got a bit discouraged when I realized also that the steering head tube in the frame is a larger diameter so the tapered roller head bearings may have a larger diameter. Will I need compromise bearings to fit an S4 steering stem to an M900 frame? When I take the yokes out of my S4 I will have to measure up and work out what to do – I am sure it is not insurmountable just annoying buggeration.
I really do not want to use an S4 top yoke which is, in my opinion, perhaps one of the worst pieces of design to come out of a Ducati factory. The S4 handle bars are bolted to the top yoke in a casting that partially shrouds the fork leg top, which means that there is no possibility of dropping the legs through the yokes, the top of the fork legs are retained by a single bolt clamp, there is no depth to the clamping face onto the leg and on top of that they have that stupid aluminium central retaining nut that requires a 6 pronged tool to undo it. Certainly not their best day in the Bologna design department. It is a styled component to pander to Japanese competition, all the best design follows the maxim that “form follows function”
Rant over.

[utopia]

My internet connection keeps going down .. so here is the short version of the post that I keep losing.

Some yokes are 50mm top, 54mm bottom.
Others are 53mm top and bottom.
Boring top yoke to 53mm and shimming the bottom to the same size is a common mod when installing upgrade/superbike forks.

[Darren69]

Will the stems swap over? eg original monster stem into S4 bottom yoke? I agree the S4 top yoke is a hideous carbuncle, they just had to go, the handlebars are just wrong the problem I had was getting something to replace them with that would suit clip-ons and still fit my steering damper. Got there in the end but it wasn't straightforward.

[Albie]

Been meaning to pop in this week to see updates but been busy tearing around no time. Will pop in next week Richard.

[Dukedesmo]

Originally Posted by utopia

Some yokes are 50mm top, 54mm bottom.
Others are 53mm top and bottom.
Boring top yoke to 53mm and shimming the bottom to the same size is a common mod when installing upgrade/superbike forks.

I did exactly this; M900 has 50/54 and I'm using 916 forks which are 53/53. I had the top bored to 53mm (plenty of material to do this BTW) and had top-hat shims machined to fit the bottom.

As an aside I have Ohlins forks (from 1198) in my 916 and these are 53mm top/56mm bottom and I would say that Monster yokes could be bored to this with no issues...

[350TSS]

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.

[350TSS]

After servicing my daughter’s car in preparation for an MOT (it passed) I did manage to get into the garage today. I made the other two ribs for the skeleton and the started on loading the skeleton with foam. It very soon became obvious that I did not have enough foam to complete the job. The original foam came from Easy Composites (EC) and was very good consistent quality. I spent a lot of time filling the gaps between the wooden skeleton with bits of offcuts of the EC foam but it was obvious that I could not progress further without more foam being available.
I went to the local florist and bought 5 house brick sized blocks of “Oasis” foam for artificial flowers for £7.50. it was less dense, therefore easier to fit in the spaces between the ribs in the skeleton, but, when I came to rub down to get the approximate shape it rubbed down so easily, that, when it had been placed adjacent to the original foam, it wore away too quickly. This will obviously make getting an even contour a challenge.

[Dukedesmo]

What diameter are the S4 forks?

[utopia]

The shim in the bottom yoke would only be 0.5mm thick, not 1.5mm, would it not ?
The shim itself would not be under any particularly serious stress at all.
The only problem, in theory, might be the reduced wall thickness of the top yoke after boring, which would be somewhat weakened.
This is a valid concern but the amount of material removed is very small and it is a long established procedure which has stood up over time .. at least I haven't heard of any failures.
But yes, the first time I heard about it I was sceptical about the reduced strength of the top yoke.
The shim at the bottom yoke however, will not be under any critical stress at all.
If you think about it, the shim is contained/restrained over virtually all of its surface area .. only the 0.5mm wide rings at each end are exposed. There is simply nowhere available for deformation to take place.

...is my personal view.
And I was an engineer (until Thatcher/globalisation happened).

[Dukedesmo]

Originally Posted by utopia

The only problem, in theory, might be the reduced wall thickness of the top yoke after boring, which would be somewhat weakened.
This is a valid concern but the amount of material removed is very small and it is a long established procedure which has stood up over time .. at least I haven't heard of any failures.

There appears (to me) to be plenty of material left after opening up the top yoke to 53mm.

As an aside I fitted the 56mm Ohlins to my 916 and at the time also purchased an Ohlins bottom yoke (916 Showa forks are 53/53 and Ohlins 53/56) as it was a good price and is a nice billet piece that is clearly better than the original.

However I later discovered that the standard yoke for both 53mm and 56mm forks (where supplied by Ducati) were the same part simply machined out larger for the Ohlins fitment. So, in this instance at least, Ducati feel it is OK to remove material - now these are different bikes/yokes but I would imagine there is enough over-engineering in the Monster yokes to take some oversized forks.

I certainly don't have any concerns over the strength or integrity of my (either bored top or shimmed bottom) yokes under braking or any other conditions.

For the record, my lower shims are 0.5mm;

Shims;



Bored top/shimmed bottom yokes;



Shim in place;



I did put a cut in the shims before fitting BTW.

[350TSS]

I bow to superior knowledge

[Darren69]

My aftermarket yoke set runs a shim in the bottom yoke with the standard S4 forks. That gives the option of running larger forks (Ohlins) if required. No problems with doing so, as it is much stronger and better metal that the standard one anyway. I would be more concerned regarding removing material from the original castings it may weaken them and as they are brittle it could crack, although plenty have done that mod so it should be ok.

[utopia]

I would strongly suspect that the original yokes are forgings rather than castings, and if that is the case then they won't have the brittleness of a casting.
In fact, a forging would be stronger than the same item machined from billet (all else being equal).

And I have also had confirmation this afternoon that Ducati machine the same basic part (same cast/forged number) to different bores, and also that some aftermarket upgrade yokes come with shims as std.

Incidentally, I happen to know that B&Q used to sell sheets of 0.5mm thick ally for gas fire backplates.
I see no reason at all why you couldn't cut some shims from flat sheet.
They wouldn't have the flange of course but, in my opinion, this is really only a fitting aid and indeed the aforementioned supplied shims have no flanges on them.
Obviously, ally shims are needed rather than steel, to avoid electrolytic corrosion amongst other reasons.

[350TSS]

For the last 3 days. I have been preparing the fly screen mold. I am quite pleased with the look, the bottom half looks a bit like a bevel 750SS half fairing, and the top is slightly more angular as I think a continuation of the rounded look above the clip-ons would definitely have looked over powering/odd. I may need to trim the sides above the clip-ons so that the screen part is more of a blade, subject to hiding the back of the instruments from the side view.

[350TSS]

Since these pictures were taken I have filed away on either side where the clip-ons go to fit the LED indicator strips vertically and underneath the headlamp location have filed a flat for an LED running light. It is now about a day away from being ready for primer resin.
I have learned quite a bit about mold making in these last few months
1. Whenever you manufacture a mold just as you think it is finished you realise that there is some (essential) modification that would make it better or you find a parameter that you hadn’t considered so it has to change, so pondering before manufacturing is not wasted time.
2. Make your skeleton about 5mm smaller in very dimension than the expected finished product.
3. Make the skeleton accurately, the human eye can spot amazingly small deviations in the radius of a curve or flat spots on a double curvature.
4. Ply wood is about the worst material for the skeleton because when you break through it is difficult to shape edge on. I used ply because I had loads of offcuts cluttering up my garage. MDF would be better and others have suggested balsa wood but I have no idea where to obtain any locally.
5. Male molds are a lot simpler to make than female molds and a lot easier to rub down but require a lot of “reverse” thinking. Obviously there will be a “smooth” side to the finished article and the choice is sometimes made for you depending on what is on display when on the bike.
6. When designing the mold do so bearing in mind the ability to form in one piece so you do not have to go to the complication of split molds, not always possible for example with the tank. Female molds must have tapering non – parallel sides to enclosures otherwise the finished article will not release from the mold.
7. Consider carefully the mounting arrangements of the finished article in the design of the mold – something I have absolutely failed to do and will probably suffer from later.
8. Use only one grade of foam as a filler between the skeleton, differential rubbing down capability leads to more filling downstream.
9. Buy filler in big tubs and wet and dry in bulk off the internet– it is lot cheaper that way.
10. The grades of wet and dry you buy in bulk will not accord with what you actually need to do the job. 180 and 240 grade gets far more use than 600 800 and 1000 grade.
11. Resign yourself to accepting that whenever you mix filler you will always have precisely either 95% or 130% of the filler you need to complete the task.
12. Also accept that you will discover the most difficult place to fill when the mixed-up filler has less than 20 seconds workable life.
13. It always rains on days when you have rub down so you have to stay in the garage and all the dust goes everywhere.
14. When rubbing down, never attempt to rub out pin holes, you will either break through or spend a lot of time taking the depth of the pin hole out over quite a large area. Filling and rubbing down the pin holes takes virtually no time, filling and rubbing down a breakthrough can take hours
15. The moment you feel the filler starting to coagulate throw the mix away.
16. Accept that the most difficult place to fill and to rub down will be the most “in your face” part of the finished product.
17. Every job, making the skeleton, filling the skeleton with foam, skimming with filler takes approximately 2 x the time you think it will. Rubbing down and completing the mold take 3 x the time you hope it will.