Dr_C

Normally around 0,7-0,8 mm. Whats the deal with that large play? To cope with little and big end being out of parallel, and the crank not square with the bore?

bushmechanic

Hi Dr_C

no nothing that complicated ... making the axial play larger just provides more room for the crank to spread

since a large component of the load on the bearings is from axial crank spread this reduces the load on the bearings.

from my um er -- ejumicated measurements of forces required to re true an 80mm crank after it has spread under "normal use" this is in the region of 5-6 Ton at the center of the shaft.

if you were to have zero axial play for example then almost all of that axial force would be transmitted to the bearings.

if you increase the axial play the bearings have less work to do ... some of the axial force is instead able to be taken up by the crank and pin bending.

at some point this would lead to other issues possibly with the pin especailly if its tiny tack welded but Im again ejumi-guessing from watching my crank in the press that this happens at 3-4mm + of axial play ...

the DCR collar being mentioned i think will have no function at all with low axial play values since the crank will spread and reduce axial play to zero at quite low rpm pushing the crank axially into the both the main bearings with great force..

nice gearbox spanner .. I noticed the diameter of the gears is very large also reducing loads on them , ours are only at 60mm shaft centers which is very very tight compared to other similar HP applications and as Dr_C mentioned usually with multi cylinder engines. even with a knobby tyre you can see the dirt spray pulsing in time with the engine note on my 700

edit -- may need to increase big end axial play with larger crank axial play .. i found the factory spec of 0.5mm to be too tight for big end axial but this is with 0.8mm crank axial play instead of factory 0.5mm

Dr_C

Hi Bushie! I have never tried to translate the spread to real movement, but if the crank is spreading 1,0-1,5 mm at the bearing location - thats huge! But if imagining that crank in my lathe at 9000rpm, it would propably take the lathes 1000kg and jump around my workshop with it til the power cable was ripped off, so its comprehendable.

Ok. I'm off to my machinist to get these new cases milled out to be able to achieve that large play. Big end axial play is 0,65mm.

Dr_C

These cases I'm now getting race ready, offered no axial play at all with a crank 106,5 mm wide including bearings!

Taffy

we run 0.45mm big end axial float. you're going to have to fiond some space somewhere!

Taffy

Dr_C

Yes. I had the bearing seats bored deeper on my other cases. I'll just have to do that again on this pair!

tourist

Out of curiosity, what cases are you using?

bushmechanic

sounds good, ive been using 0.7 to 0.8mm axial play on the 35mm big end just as the RFS builders do. IIRC lineaweaver suggested 0.8 long ago for you on the 32mm pin

regarding the flex yes I agree it is quite impossible to believe such things occur down there between our toes !

but if the crank requires such a large axial force to become spread open to the degree they obviously do (we can measure the spread on engine disassembly) then we must justify it somehow other than jedi mind tricks

id love to see a slo mo film of the crank doing its little dance through a plastic window in the cases, then perhaps i could do more than guess what is going on

have you seen this picture of mine ? roller main bearing outer race PULLED 1.5mm inwards by the crank

if i had not made the retainer and seen it nearly broken off like that nobody could ever have convinced me the outer race of a roller bearing moves inwards at all let alone nearly twice as far as the axial play.

Taffy

Mats

you need to seperate 'flex' from 'expansion'. if you can let the crank grow to the right at no cost then the actual flex can be absorbed perhaps by the balls in theiir trenches?

you need a ball that will take flex and not have to worry about the expansion?

whenever I grab a ball bearing, there is no flex in them!

maybe you can get that by going for some special bearing?

Taffy

Dr_C

While waiting for machining of the cases, I amused myself with making a 3D-model of the existing crank. Then it´s easier to make the mods and wishes for a new design.

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