MK Indy Propshaft snapped - fundamental design flaw?
Hi All
So I took the MK Indy to Le Mans last week, except I got to about 20K outside Alencon and my propshaft snapped!!! Thankfully I still have legs, but
sadly my car is still in France and will be for a few weeks waiting for repatriation. I didn't get a great deal of chance to look at it at the
side of the Autoroute but it's basically snapped along a weld on the front section of the prop I think. My conjecture is that the prop adaptor
bolt worked its way loose (but not off) for reasons below, which induced significant vibration and the prop decided it wanted out.
So, to the issue. My car has a Fireblade engine, and as we all know they are installed at an angle. This however means that the prop installation is
far from perfect: -
Basically I know you want a small angle at the diff to stop the rear UJ brinneling, and you also want the gearbox output and diff input to be in the
same plane. However, because of the engine installation the input and output are out by quite a few degrees (can't measure at the moment,
car's in France etc etc...)
What it should be like for ideal prop installation I believe is this: -
So basically the engine and diff are parallel to each other (horizontally and vertically), the UJ's counter each other and everything runs
smoothly.
It's always been a concern of mine that it's not ideal, and now I've seen the full consequences of what happens I'd like to
look to how it can be improved. The misalignment is noticeable on part-throttle where you get the rattle ("they all do that sir".
Has anyone else engineered this problem out? Either by re-mounting the engine (will it fit? I guess there's a reason it's at an angle.
Obviously cradle, manifold etc etc would need work), or even by re-mounting the diff to be in the same plane as the engine???
I'm not an engineer so all the above is just what I've read and learned, but would like to hear any suggestions - I can't be the
only one that's not happy with how this is designed???
I built my bec as in your second picture not an indy though. I've never been a fan of the angled engine.
It baffles me how more secondary locking devices are not used In the assembly of these parts. Maybe it's because in my line of work most
critical parts are safetied. Along with the loctite and the lock washer I wirelocked every single bolt and nut from the engine output to the diff.
These engines are high revving and usually rigid mounted. All this activity and vibration is a recipe for stuff coming loose.
Glad the legs are in one piece these failures can be incredibly nasty.
In reality you have to be a long way out before it causes any noticable problems, especially if you have a TRT prop (which is highly recommended).
Also remember that it is quite feasible to have non-parallel drive faces and still have the propshaft in phase as long as the sum of the angles is 0
degrees.
More likely cause of the prop adaptor coming loose is the sprocket adaptor being too thick. The nut shouldn't be torqued onto the adaptor, but
onto the shoulder on the shaft, stopping any torque transfer into the nut.
quote:Originally posted by sdh2903
I built my bec as in your second picture not an indy though. I've never been a fan of the angled engine.
It baffles me how more secondary locking devices are not used In the assembly of these parts. Maybe it's because in my line of work most
critical parts are safetied. Along with the loctite and the lock washer I wirelocked every single bolt and nut from the engine output to the diff.
These engines are high revving and usually rigid mounted. All this activity and vibration is a recipe for stuff coming loose.
Glad the legs are in one piece these failures can be incredibly nasty.
They definitely should have a secondary locking device, but experience shows that these only work if the above rule is adhered to. Otherwise it ends
up twisting the locking washer and the nut.
quote:Originally posted by gingerprince
So basically the engine and diff are parallel to each other (horizontally and vertically), the UJ's counter each other and everything runs
smoothly.
You don't have to have the angles equal and opposite, i.e. so engine and diff are parallel. The UJ velocity cancellation still works if the
angles are equal and not opposite e.g. if you needed a total angle of 10 degrees, and you had two UJs working at 5 degrees each they would cancel.
This might be easier to arrange in your case.
quote:especially if you have a TRT prop (which is highly recommended)
Interestingly, I have a TRT and was considering going back. If only because as it's heavier I seem to be able to feel more vibration than my
old prop - figured the extra rotational momentum had an effect. Maybe if alignment can be solved as has been said though that issue may go away.
quote:More likely cause of the prop adaptor coming loose is the sprocket adaptor being too thick. The nut shouldn't be torqued onto the
adaptor, but onto the shoulder on the shaft, stopping any torque transfer into the nut.
Ah, wasn't aware of that. I have a fat machined washer on there to make sure it clamps up tight to the adaptor. Are you saying that when the
bolt is tight that there should be some float on the prop adaptor? I guess the torque transfer thing makes sense, but won't it rattle around
like that or are we talking fractions of a mm?
quote:Also remember that it is quite feasible to have non-parallel drive faces and still have the propshaft in phase as long as the sum of the angles
is 0 degrees
and
You don't have to have the angles equal and opposite, i.e. so engine and diff are parallel. The UJ velocity cancellation still works if the
angles are equal and not opposite e.g. if you needed a total angle of 10 degrees, and you had two UJs working at 5 degrees each they would cancel.
This might be easier to arrange in your case.
Are these 2 saying the same thing? I've read a little about "broken-back" configurations, which generally talk about vertical
alignment but is this essentially the same thing. So in theory if I move my centre bearing to the right and try to make the angle between centre and
rear be the same as between centre and front then this will cancel out?
quote:I'll just leave this here...
Thanks. Explains the reason for vibration perfectly.
Looking like hopefully there'll be a way around this without major surgery to engine mounts, which is comforting.
quote:Originally posted by gingerprince
Are these 2 saying the same thing? I've read a little about "broken-back" configurations, which generally talk about vertical
alignment but is this essentially the same thing. So in theory if I move my centre bearing to the right and try to make the angle between centre and
rear be the same as between centre and front then this will cancel out?
Yes. The angles need to be in the same plane, (e.g. you couldn't have one pointing left ten degrees and the other pointing up ten degrees), but
provided they are in the same plane, if you share the total angle equally between two joints then the velocity change will cancel.
e.g. in the following pic, as long as the angles 'a ' are the same, then the velocity change will cancel
Photo Archive
Building: Hatred of Loughborough's Speed Humps
posted on 18/6/15 at 10:45 AM
The full consequencies of what can happen, quite amply demonstrated with my reverse gearbox failure at Combe, with the output shaft shearing off. The
car was destroyed, the scars on my legs are still there after 8 years. The combination of a Hayabusa, 10" slicks and all that goes with it, not
a happy one.
All bike engines are at an angle, the Hayabusa moreso than others, it's a size thing. With the smaller engines and their stacked gearbox units,
the sprocket output is now becoming less of an offset problem. The R1 went in the Westfield, with the engine gearbox output pretty much in a straight
line.
Many hours racing were 'safely' carried out with a Bailey TRT, Centre Bearing, no reversing sprocketry (etc.), despite the angles on the
engines not being ideal. This is what I'd recommend.
So I believe my car is based on an early Indy chassis. My centre bearing is floor-mounted, whilst I see photos of newer Indy's with the centre
bearing mounted sideways on to the passenger tunnel. Was this a design change to improve alignment?
. 
