Broke a Crank Arm !

[1FJEF]

The cruisers right pedal felt slightly askew, it was late and only a mile from home. I gave it a cursory inspection, I'd check it at home on the stand.
I gingerly rode on a few revolutions and BAM!

1) Right foot slams to the ground and pushes forward off the toes
2) Gonads pound into the nose of the seat
3) Chest hurtles towards the the bars
4) Body rotates slightly to the right as the right foot is dragging
5) Left foot tries to slide outwards off the studded pedals
6) Every ounce of strength in my being tries to lock my shoulders, pecs and biceps to stop my chest plant into the bars

It must have looked comical. Especially the ride home where I rocked the left pedal back & forth at the bottom of it's arc with the sole of my right shoe impaled on the stub of the right crank arm, hacking back & forth like a monkey humpin' a football.

Pulled the muscles so badly at the front of my armpits I took 3 days to get to where I could lift my arms in front of me.

Sun 7 speed cruiser (LBS, not a Wally world type bike). One piece (Ashtabula) crank. 3500 actual miles, + whatever the previous owners put on it.

[nfmisso]

Glad you survived

At least the bike is an easy fix

[GeorgeET]

Bummer , that's a nasty way to get it, no warning, like getting hit from behind. Glad you did not high side over the bars. Tha arm must have had a crack in it from bad casting.........

[1FJEF]

Thanks guys! I can't believe how sore I was for awhile.

SO- I was gonna change it to a square taper BB with the adapter kit


BUT
1) I need a one ring crank set with 44T. Or 38T min 46T max.
2) It's gotta fit the OEM chain guard
3) The only sets I've found are extremely expensive OR the pedals aren't (or don't seem) far enough apart. Width, as in Q, not arm length

My cruiser seems to have extraordinarily comfortable width between the pedal thread faces.
The one ring crank sets I find seem to look to be purpose built for speed and have little offset like this "trackset" under $50




I could steal a 3 ring crank set from a MTB & just use the middle ring, but then that bike would need a new one & I'd have to grind up the outer ring to use as a donor for crank bolt spacers.

[nfmisso]

I might have something that would work - I will try to take a look this weekend - or next - the boss has my booked......

[1FJEF]

Thanks.

[nfmisso]

Let's get the math started

Is this a single speed (or IGH) or 7 speed RD or ?

What is the desired chainline (in mm please)?
* OLD ?
* center of center cog to face of drive side lock nut? Call this "C"
* chainline = OLD/2 - C

Distance from center of chain ring to inner face of crank arm? This is the clearance for your chain guard.

With the above; I can see if what I find will work, and recommend a BB axle length for you to purchase.

[1FJEF]

(08-13-2014, 02:38 PM)nfmisso Wrote:  Let's get the math started

Is this a single speed (or IGH) or 7 speed RD or ?

What is the desired chainline (in mm please)?
* OLD ?
* center of center cog to face of drive side lock nut? Call this "C"
* chainline = OLD/2 - C

Distance from center of chain ring to inner face of crank arm? This is the clearance for your chain guard.

With the above; I can see if what I find will work, and recommend a BB axle length for you to purchase.

I'll work on it. When it's under 100 degrees!
Chain line to follow. Whatever it is works well.
135 OLD
3/32" Chain
7 Speed RD
No FD

The distance from the cog C-line to pedal thread face is very large & comfy. I would rather not use pedal spacers, especially at my weight.

[nfmisso]

measurements required.

[1FJEF]

Thanks, there's been some confusion. I am going to keep the existing chainline with whatever new crank I install, I have a few sq. tpr. BBs laying around I can use for reference, matching is no problem.
My problem is that 3 piece 3/32" single ring crank sets are extremely expensive OR the pedals aren't (or don't seem) far enough apart. Width, as in Q, not arm length.

[nfmisso]

(08-15-2014, 05:50 AM)1FJEF Wrote:  Thanks, there's been some confusion. I am going to keep the existing chainline with whatever new crank I install, I have a few sq. tpr. BBs laying around I can use for reference, matching is no problem.
My problem is that 3 piece 3/32" single ring crank sets are extremely expensive OR the pedals aren't (or don't seem) far enough apart. Width, as in Q, not arm length.

Jeff; I need the current chainline measurement to be able to tell you what BB axle length is required so that the chainring on the crank matches that.

I need to the other measurements so that I can find you a crank that will put the pedals where you want them and maintain your current chainline. By playing with BB axle length and position of the chainring on the crank (inner vs outer), I hope to be able to find a crank set in my collection that will work for you.

Your goals as I understand them - and my understanding has not changed.
* keep current chain line.
* keep current chain guard
* keep current pedal position side to side (width).
* 36-46 chainring; 44 target

[1FJEF]

(08-15-2014, 11:33 AM)nfmisso Wrote:  Your goals as I understand them - and my understanding has not changed.
* keep current chain line.
* keep current chain guard
* keep current pedal position side to side (width).
* 36-46 chainring; 44 target

Exactly what I need.
I have to figure a way to get an exact measurement of "center of center cog to face of drive side lock nut? Call this "C""
C line of bike to C line of front cog is no prob but eliminating error from the other will take some thought. It's a cruiser with a one piece crank, the tolerances are large and some parts aren't true, like the front sprocket, lol.

[nfmisso]

(08-15-2014, 11:28 PM)1FJEF Wrote:  .....I have to figure a way to get an exact measurement of "center of center cog to face of drive side lock nut? Call this "C""....

ruler ? within 1.5mm (1/16 of an inch) is close enough.

[1FJEF]

My existing front cog c-line is 41.25mm from the c-line of the bike. The front cog center line is 25.75mm from the outer rear lock nut face (inside of the rear drop out).
The big illustration of my issue is that the existing one piece crank, on the drive side is 47mm from cog c-line to the face of pedal thread opening. My narrowest looking MTB crank set is 18mm less to the middle ring. I haven't measured my other bikes that look a little bigger there, as it's moot as they are riveted 3 ring.]
In that instance my "Q" or "tread" would decrease by 36mm or just under 1 1/2", which is a lot when what you had seemed to fit just right.

Ignore the inner & outer cogs, it's the only pic I had.


By the way a couple exercise bikes I checked are wide also.

[nfmisso]

I will check my inventory Sunday at the latest. I may have something will work

[1FJEF]

The BB adapter to convert 1 piece crank to 3 piece came in.

[nfmisso]

Hi Jef;

This is what I am looking for this morning:
[attachment=5316]

[nfmisso]

Hi Jef:

This is what I got:
[attachment=5317]
[attachment=5318]
[attachment=5319]

The chain ring center line to the pedal mounting surface is 45mm. It was made up from the 118mm BCD crank set originally on my Miyata 310. It has a 42T ring, extra long bolts with two sets of nuts, a set of thick spacers and a set of thin spacers.

Sorry, I have no idea what the BB axle length needs to be - I am running out of time. And will not be able to work on bikes again until mid October.

If you normally need a 68mm BB shell width, you can use a 73mm and add a lock ring from an old pre-cartridge BB.

If you would like this franken assembly, PM me your address; I'll include a lock ring or 2 also. You can also salvage the parts, and use the same strategy with any crank set with bolt on rings. It will be in the mail on Friday the 29th. Sorry, very busy this week, getting ready for being away along time - business trip.

[1FJEF]

That actually looks like someone was making it for me! Excellent design. Very close to what I thought it would need to be.
I had read somewhere that I would need to do it like this picture: Sorry for the big pic-I wanted the detail. Your way makes more sense & gives great spacing.





1) Remove the inner 3rd small ring if it's a triple.
2) Remove the outer chain ring. Cut it up so each chain ring bolt has a shaped & sanded chunk in place, so it's like the outer ring keeps the old chain ring bolts in play.
For some reason I thought you had to do it this way. I would have rounded them off better though.