I broke this today...

[Skipton]

I built this bike to be bulletproof.





One of the main things I made sure this bike had was a tough as nails Profile BMX three piece crank set.

On my ride to work this morning the bike felt weird when I was hammering after a red light. Or when braking. When I looked down I saw this.



Cracked right at the weld. Good news I was able to turn around and ride home carefully.

 

[Rat Rod]

Is that how you make a front freewheel?

 

[LukeTheJoker]

Wow! That is an unexpected place for a break!

Luke.

 

[Skipton]

Is that how you make a front freewheel?

That is..

 

[Skipton]

Wow! That is an unexpected place for a break!

Luke.

Seems like a weird spot. Maybe more torque there they we think.

 

[heartlandcruisers]

it is possible that that particular crank was not tough enough to run fixed gear.
I do not know much about the fixies but I would imagine there is a lot of pressure on the crank arm when you try to slow/stop.
sorry to see it broke but maybe you can reweld it? or if it is under warranty possibly get a replacement.
either way hope to see you fix it soon.
Sean

 

[Bicycle808]

This is common on FG bikes that happen to be running traditional BMX 3-piece cranks, and it's also the reason why many companies (Profile included) are moving towards spline-drive cranksets. Coaster-brake set-ups are not as stressful to a drivepin as FG is, but it is a lot more torque than that drivepin would have twithstand on a freewheelin' bike with rim brake. It's not surprising at all, in my view.

 

[jamesdart]

I've neve been on a fixed gear, but I can see a ton of force there. With the spline drive, doesn't the sprocket still have a pin/ bolt?

 

[Skipton]

Whoa!
I'm not coordinated enough to ride a Fixie.
It's a coaster brake hub.

A year ago when I frist built the bike I had trouble with the chainring bolt coming loose. And the clearance is very tight to the bottom bracket. A couple days ago I checked the bolt... It was a tiny bit loose... Less the a quarter turn.

When I took it apart today you can see were it was rubing on the BB. This is what ultimately put to much stress and or heat on the post.

 

[Bicycle808]

I've neve been on a fixed gear, but I can see a ton of force there. With the spline drive, doesn't the sprocket still have a pin/ bolt?

Some do, many don't. It doesn't need one. Some new cranks don't even have them.
http://www.profileracing.com/estore/product.php?productId=474

 

[Bicycle808]

Dang, Skip!

 

[jamesdart]

Ok, haven't seen a splined sprocket. Cool.

 

[Bicycle808]

The force on a fixed gear is actually no greater or any lesser than any other configuration.

Yeah, i guess the more accurate description would be frequent opposing forces---both forward and backward-- that will be placed on a drivepin or bolt with a FG drivetrain. In my experience, riding fixed with a OPC will lead to gradual loosening of the BB's driveside cone, and the shearing off of drive bolts on 3piece cranks working with fixed gear drivetrains was one of the factors in developing the splinedrive set-up, which ended up having other advantages....

So, while the forces would be the same with a FG versus a freewheeling set-up (if all other factors were equal), load-bearing components will experience that force in both directions--mostly forward, but significant amounts of backward forces as the rider slows or stops the bike by resisting forward crank motion. Other drivetrains won't be effected by the backwards forces much-- a very small amount in c/b applications, and virtually none at all with a freehweeling bike equipped only with handbrakes.

 

[Bicycle808]

Ok, haven't seen a splined sprocket. Cool.

 

[ifitsfreeitsforme]

for you, and your "take no prisoners" commuting style, NOTHING is bullet proof.

 

[Bicycle808]

FGF=FF+BF/BS
(FixedGear Force equals Forward Force plus Backwards Force over BullSnot)

Conversely, FWF=FF/M
(FreeWheel Force equals Forward Force over Malarkey)

In all seriousness, anyone who has lived with a fixed drivetrain for any amount of time knows that it's different in terms of how it wears on many components. While the force at any given time may not be greater, the total force is greater b/c it's under load any time it's moving, and the kinds of loads vary greatly, sometimes with opposing forces occupying the same instant. Imagine you're rolling 18mph and you gotta slow down right quick-- the energy being exerted on a FG drivetrain in a situation like that is possibly greater and certainly more complicated under those circumstances than it would be on a c/b bike, or a freewheeling handbrake bike....

 

[B607]

It ain't the bike, it's your legs. They're too powerful from all that riding. Skipton, the incredible hulk. Gary

 

[Skipton]

It ain't the bike, it's your legs. They're too powerful from all that riding. Skipton, the incredible hulk. Gary

 

[user 135]

Maybe you should try a Warranty, if you were the original purchaser of those cranks. http://www.profileracing.com/tech/warranty-information/

 

[GuitarlCarl]

I'm not sure there'd be any more force applied on the crank arms, but when you stop a coaster, the brake actually slows the rim from the hub out. Now a fixed gear seems like it's hub out too, but I'm willing to bet aggressive braking on a fixie puts more torque on the rear gear because that's whats doing the initial stopping instead of the hub. Notice how looking at 70 yr old drivetrains you see hooked gears in only one direction... BUT, the crank pin would be subject to similar torque with both systems... MAYBE more often aggressively on the fixie IF you're a crazy bike messenger or something, but not to a normal rider. (whatever that is ) but that's hard to prove having seen no 70 yr old fixie drivetrains...

Carl.