Can I shorten carbon cranks?

[Mountain Bru]

3 minutes ago, JA-Q001 said:

Those crank boots get dust between the boot and the crank. I have seen more failed cranks because of that dust than any other reason.

It grinds the carbon away around the insert and creates a weak spot. That black dust all over the tip of his crank, thats carbon dust.

What about on alu cranks?

 

[Jewbacca]

2 minutes ago, JA-Q001 said:

Those crank boots get dust between the boot and the crank. I have seen more failed cranks because of that dust than any other reason.

It grinds the carbon away around the insert and creates a weak spot. That black dust all over the tip of his crank, thats carbon dust.

I knew thre was a reason I rode Alu cranks without boots!

A serious question, why does the boot move? 

[Zebra]

4 hours ago, JA-Q001 said:

And PS, stop using crank boots.

 

Keen to learn WHY not to use crank boots? Thanks, Chris

[JA-Q001]

@Mountain Bru Alu cranks are fine, no bonded insert there.

@Jewbacca Not all of these boots fit really snug, some are also from a harder plastic. It can vibrate on rough terrain probably. If you silicone them onto the crank it should eliminate the problem probably.

Not sure 100% myself why, but I have seen it on some of my own cranks and have seen it mentioned a lot.

(for the life of me I cannot find a nice pic of it anywhere)

[Jewbacca]

2 minutes ago, JA-Q001 said:

@Mountain Bru Alu cranks are fine, no bonded insert there.

@Jewbacca Not all of these boots fit really snug, some are also from a harder plastic. It can vibrate on rough terrain probably. If you silicone them onto the crank it should eliminate the problem probably.

Not sure 100% myself why, but I have seen it on some of my own cranks and have seen it mentioned a lot.

(for the life of me I cannot find a nice pic of it anywhere)

I'm too poor to buy/use carbon cranks so it's a moot point for me. I also have never had shoes on my cranks, so it's something I have no idea about.

Harder plastic makes sense as surely the point is to protect the crank end from rock strikes, which a soft material wont do. 

With my newly cut and tapped 145mm cranks I doubt I will have too many rock strikes going forward ????

[Steven Knoetze (sk27)]

5 hours ago, JA-Q001 said:

Do you want to shorten just to save the crank?

Shortening is a very complicated and laboursome job. You cannot just drill a hole and tap it.

It is quite easy and safe to just fix the loose insert.

the hardest is cleaning the cavity, then drilling a hole to inject resin into the cavity formed by the loose insert and then keeping it aligned while setting, so that your pedal does not have a wobble.

Graeme can fix it back to perfect for you, check his instagram, he has done it loads before.

And PS, stop using crank boots.

 

Out of interest, why stop using crank boots?

 

[Mountain Bru]

43 minutes ago, droo said:

I'm 1.82 and ride 165mm cranks on my MTB. Much preferring them to the 175mm I was on before, although they take a bit of getting used to on steeper climbs.

Would really love to see some actual data to go along with this dudes subjective feelings about things and possibly misguided conclusions...

He says stuff like "my muscles don't fatigue as much on short cranks" but never says anything about if he's doing the same power. Maybe he's doing half the power and now he feels like his muscles are super strong. And speaks about mashing the pedals on long cranks.... But is he even doing the same cadence? Speaks about how long cranks tax your legs more so he wants short cranks for long rides, but is he even doing the same times on the laps he does? If he's going way slower, its not the cranks that are taxing his legs more, it's the fact that he's putting out more power. He adjust the height of his saddle twice in the video (but maybe he did every time) but never adjusted the fore/aft, so his position was slightly off for every crank but the one his bike is set up for, and that make all his comments about easier/smoother spinning etc completely useless. 

I'm not saying that short cranks aren't good, just that this video is seriously lacking in objectivity and usable information. It would have been way more useful if he just did 250w at 85rpm for 5 minutes on an IDT (with the correct saddle height and fore/aft) and then drew conclusions. 

[droo]

3 hours ago, Mountain Bru said:

Would really love to see some actual data to go along with this dudes subjective feelings about things and possibly misguided conclusions...

He says stuff like "my muscles don't fatigue as much on short cranks" but never says anything about if he's doing the same power. Maybe he's doing half the power and now he feels like his muscles are super strong. And speaks about mashing the pedals on long cranks.... But is he even doing the same cadence? Speaks about how long cranks tax your legs more so he wants short cranks for long rides, but is he even doing the same times on the laps he does? If he's going way slower, its not the cranks that are taxing his legs more, it's the fact that he's putting out more power. He adjust the height of his saddle twice in the video (but maybe he did every time) but never adjusted the fore/aft, so his position was slightly off for every crank but the one his bike is set up for, and that make all his comments about easier/smoother spinning etc completely useless. 

I'm not saying that short cranks aren't good, just that this video is seriously lacking in objectivity and usable information. It would have been way more useful if he just did 250w at 85rpm for 5 minutes on an IDT (with the correct saddle height and fore/aft) and then drew conclusions. 

As with most things I take this with a pinch of salt, but some interesting observations that coincide with my experiences as well, bearing in mind that while I can strip and reassemble a bike to a degree most can't, I kinda suck at riding them.

 

He does state at the beginning that his is a subjective approach, and your average recreational MTBer (internationally, the average in SA is a marathon rider and therefore a completely different animal) cares not much for power outputs and efficiency and would rather just have a jol on the bike.

 

In my case I was willing to take the knock on leverage (which is noticeable, but not insurmountable) for the benefit of kicking way fewer rocks. And so far I haven't regretted my decision. Which goes (I think) a fair way towards explaining why someone might want shorter cranks, and would consider modding a stuffed pair since they're not always easy to find, cos efficiency etc.

 

OP - Lyne have just released 165 AMP cranks, which are a pretty affordable way of testing whether or not you'll agree with them.

[mountainfun]

20 hours ago, Veebee said:

Yeah Graeme Kidson aka Carbon Ninja will be able to sort this out for you.

He posted new contact details yesterday, here we go.

 

Thank you, will definitely get in touch.

 

[mountainfun]

20 hours ago, Mountain Bru said:

Possibly a more relevant question than "Can I cut these shorter?" is do you want shorter cranks? If they're currently 170mm, you'll probably have to go down to like 160mm, which is maybe not a good idea in itself and so it's irrelevant if they can be cut shorter. 

I'm also interested to hear why crank boots are a bad idea though. I've never heard anything bad about them. 

I'm happy with the 170's, but after doing some reading I'm sure 165's would suit me better.

I've also submitted warranty claim, and if they honour that, i'll go for 165's.

For now i'm back on my old 175's and it's incredible the number of pedal strikes i'm getting.  so 165 would be perfect i reckon

 

[TNT1]

34 minutes ago, mountainfun said:

it's incredible the number of pedal strikes i'm getting.  

Hope you put your crank boots back on...