Crank arm shortening - any recommendations in the UK ?

[M-Power]

I can bung you a set of MT-60 that are a bit tired, posted to me as spares so can pass them on for postage only

Cheers LGF, I may just do that if mine are a no go. I will try to let you know asap - sent him an eml enquiry. Are they 175mm ? You can only lop a max of 20mm off so I would need 175 to leave 155, which should be spot on for her.

 

[FluffyChicken]

That site says MT60 can be done, but M730 can't, aren't they the same shape etc?

I've not looked why as I don't have a pair but from memory there is not a lot of flat area around the pedal, in the diagram it shows they need the area they drill to be flat I guess.
No doubt you could do it. But you'll be on the slope at a narrow point to the sides and maybe narrow to the rear? You would need to create the flatface and you're into getting thinner again to the rear.


They are cold forged cranks though so technically should be stronger.

 

[FluffyChicken]

Re:

P.S. I'm only throwing things in the ring as talk and ideas are good, you know the metals and what you can do far better.

I dare say a lot is based on aldult riders.


You could drill, face and helicoil the thread in place?

 

[M-Power]

Re:

I emailed Chris at

http://www.highpath.net/highpath/cycles/shorten.html

But he's on a charity cycle ride and cant do them until Dec...by the looks of it the cranks may need flatting off before the hole is recut and tapped using a special jig to ensure accuracy. It's prolly a bit of a pita to do without precision set up etc any variation from 90 deg in the angle of the recut hole will result in unrideable cranks.

 

[FluffyChicken]

Re:

My thought is screw the crank to something flat via the pedal hole, face down, you then use a pillar drill to drill the hole. It should then be corect
That's if you have one. Or just find a 150/155 crank say on sjs or other readily available crank
.. Forget about the granny ring...

But that is no fun.

 

[M-Power]

Re:

Cranks now sent off to Highpath Engineering. Will update this thread when I get them back sawn off and retapped to 155mm

 

[02gf74]

155 mm? You know there is no reason why they need to be multiple of 5 mm, or even a whole number.

Did you do the crank length calc according to leg dimensions?

 

[Rampage]

Re:

That's the only length that really works with M730.

 

[M-Power]

155 mm? You know there is no reason why they need to be multiple of 5 mm, or even a whole number.

Did you do the crank length calc according to leg dimensions?

I did consult a plethora of 'specialist' threads on this subject and decided to go for a crank length of 155mm as used by premium kids bike suppliers for her age group and approximate height. She's very tall for her age.

I also just had this feedback from Chris at Highpath today. This guy knows what he is doing.


re: 'Other specialists'. I've had to correct a couple of crooked holes
drilled and tapped by other people. I use a custom jig on my CNC
milling machine, which means that my holes are always parallel to the
bottom brackt axle - it even works for bent cranks! I don't see how
anyone can offer this service unless they have an equivalent setup.

And it's the same for repairing stripped threads with a 'Helicoil' type
thread insert - the same jig is needed to for that too.

Anyway, as for your cranks, I'll try not to keep you waiting too long.
Thanks for your PayPal payment.

Best wishes,
Chris.

 

[Rampage]

Re:

It should be fairly easy to line up with jigs for the square taper and the previous pedal thread.

I don't doubt that he's seen loads that have had people hand drilling them then using the wrong size tap at an angle though!

Is he going to put the chamfer back on the end to make them look original? That would be really good!
Post pictures when done please.