Damage from Crank Grabs

My crank is getting all scratched up from crank grabbing. Is this bad, or anything? I also occasionally have to tighten it, I think from the same reason.
I use a KH20 trials uni, if that makes any difference.

Next step: try to learn to peadal grab. pedal grabs enable you to get onto poles and other uneven objects.


Yeah, I was considering learning to pedal grab. I wouldn’t use them for jumping on to poles, because I’m probably not good enough yet. So they’d be about as useful as crank grabs are now.

Re: Damage from Crank Grabs

Well, yeah it’s bad.

The strength of a piece of metal (or any other homogenious material for that matter - glass, ice, etc.) depends in part on the condition of its surface. If the surface is polished and smooth then the stress flows evenly throughout the material. If there is a gash (aka a notch in mechanical engineering terminology) then the stress that was flowing evenly through the material has to bunch up to get under the gash. This concentrates the stress at the bottom of the notch, which either causes the notch to stretch out (if the material is ductile and the stress is slowly applied), or fracture (if the material is brittle or the stress is rapidly applied). The damage can be sudden (a crack or tear) or cumulative (fatigue damage).

This is why spokes usually fail in the threads (lots of deep notches), why glass cutters work, why mechanics drill a hole at the end of a crack to stop it from running (round holes concentrate stresses less than sharp angles), and why high-end components are usually highly polished. (Connecting rods in high-end race cars usually look like jewlery, for example.)

So what can you do about it? Well, if you like crank grabbing, I’d budget for new cranks. You could try polishing the scratches out, however the next time you gash the crank you’d be gashing a thinner piece of metal, which would be even worse. I think your best bet is to treat your cranks like any other part with a limitied lifetime (e.g. tires and tubes) and have a set of spares on hand.

I see… Well, I suppose I’ll live with these cranks until they get really bad and then go for some stronger ones. And try to learn pedal grabs, in the mean time.

Re: Re: Damage from Crank Grabs

Ummm… I thought spokes usually fail at the bend where they run through the hub. :thinking:

That’s my experience with spokes anyway.

Mojoe out…

Are the scraches really deep? or is it that you’ve just scrached the nice black paint off of your kris holm cranks. I really dont see scraches as long as their not deep to be that big of a problem. My Profile cranks are plenty scrached up and I’ve seen some really bad ones from riders who grind alot, and Not alot of failures in the cranks.

Its a factor and you can put some rubber on it if your just doing grabs. But I dont seem to let it bother me really. If its not scrached your not riding.


Don’t worry. it’s not a big deal. it will compromise the strength, but only very minimally. Just egt out and Ride, and don’t care about your cranks being all scratched, and less pretty :astonished:


Re: Re: Re: Damage from Crank Grabs

Spokes fail at the flange if the wheel isn’t built right. There is a step where the wheelbuilder bends the spokes at the flanges to make them point to the right spot on the rim with no tension. And during stress relief the aluminum in the flange is deformed to make a nice cradle for the spoke elbow.

Of course, this assumes that the wheel is hand built. Machine built wheels don’t get these steps so the spoke is constantly working a bit at the hub end. This might be why your experience is what it is.

To be honest, I don’t have much experience with spoke failure. I’ve never had one. My wheels die from dents and dings. I replaced the rims on one rear wheel four times and the spokes are still ok.

dont worry about it.
you’ll probably break the hub flanges first.

my cranks are scuff up pretty badly and it dosent realy bother me.

Don’t grinds mess them up even more? Kevin Tugboat person has a summit, and those have the same cranks as the KH20 Trials, and he grinds a lot. But it doesn’t seem to bother him.

if the paint is coming off it’s no big deal, get over it. however, if they’re loose that’s kind of odd. i’ve never had to tighten mine and i got it when they came out. like i said in the grinding thread, if you’re grinding directly on the crank, don’t. when you grind on the pedal you’ll be partially grinding on the crank, but you won’t be putting all your weight on it which will not get you as far, and won’t damage it as much. the same goes for crank climbing, but i can’t pedal climb yet so i can only say it obviously will be relieving the pressure on your crank.

Don’t worry about it. I do crank grabs all the time so my cranks are very scratched up. Unless your scratchs are very deep, I don’t think they will weaken the crank. My paint is almost all gone, but its okay.