172,5 and 175mm arms on same bike

[rainerhq]

I have 172,5mm 4iiii left arm PM, which I use on roadbike and CX bike. I want to know the TSS, I get from MTB race, but MTB has 175mm arms. Is it a very bad idea to mount the 172,5mm crankarm to MTB for one race? Use thicker insoles in right shoe, to somehow compensate the difference?
The MTB race is ca 2h long, 50km.

[Marin]

Try it. You probably won't even notice.

[Stitchking]

My coffee bike runs this combo. It's all good, until you try to ride No handed. Then you really know. Zero balance. Its amazing how badly 2.5mm can mess you up.

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[DamonRinard]

Overhauled my bike late one summer. Couldn't find the left crank arm to reassemble. There was this spare crank arm on the workbench, obviously not mine, since it was a different length. (This is not surprising, I was working at Kestrel and lots of spare parts were always lying around...)

It finally dawned on me I had been riding two different crank arm lengths all season and never knew...

So go for it, like me, probably you'll never notice.

[rainerhq]

Thank you all.
I can´t fit the road left arm to MTB crankset, sad. Wasn´t aware of that.
Shimano 105 and XT.

[User Name]

I once did this (sort of: a 5mm difference, though) accidentally, before rushing off to a group smash-fest. I was changing the cranks from 172.5 to 177.5 for some off-the-saddle stomping over short hills, but got distracted after changing only one arm. Yes, I noticed the 5mm difference.

[NickJHP]

Years back I mangled a left hand XTR M950 crank in a crash, and gave a friend the RH crank so he could use smaller chainrings on his touring bike that had come equipped with an Ultegra 130BCD crankset. The only problem was that the Ultegra cranks were 170mm and the XTR 175mm, but he replaced the RH Ultegra crank with the XTR and rode it for the next couple of years with a 5mm L/R length difference. I never heard him complain about any physical or handling problems as a result of this.