Reeses

I'm currently running SG75s with SG75 chainring and an RPM BB7420 bottom bracket (I'm aware that that's the incorrect spindle length for SG75 cranks). Anyways I was riding and did a few skids and now my drive train is making a really bad noise. It's like some sort of crunching sound every pedal stroke, like the sound of a really bad chain line. I've ridden with this setup for a couple weeks and had no problems until I did a few skid stops yesterday.

So I took off my chain and spun the cranks. Judging by the chainring, the drive side crank is wobbling side to side (away and towards the frame). It's not the chainring, because I removed it and placed it flat on the ground and it didn't seem bent. I'll be pulling the crank off tomorrow to see if I'm f*ed, but right now I wanna know how this could have happened? I've skidded on $50 Vuelta Pista cranks with no problem for god's sake...

IvyCap

skidding has nothing to do with this. you probably set up your drivetrain wrong. chainline, spindle length, bolt torque, etc etc. The problem here isn't the cranks it's how you installed it. inspect the drivetrain everytime the crunching sound appears and check what seems to be out of line.

BigglyPuff

Sounds like a BB problem.

Not a lot of possibilities if it's a fixed gear bike and a crunching noise.

Reeses

Must I get the correct size bottom bracket? I checked my chainline and it looks near perfect. Or is there something more to it...

IvyCap

I think what leuky mentioned is correct. you are using a different type of spindle rather than the length of the spindle.
this article should clarify: https://sheldonbrown.com/bbtaper.html

BigglyPuff

What did you check your chainline with? Are we talking about a tool, or just eye-balling it?

I could say I tightened my stem bolts so they look perfectly even, so it's not gonna have issues. But a torque wrench would be a much more accurate option.

And like Leukybear said, you gotta use a compatible BB with the cranks.

Reeses

The thing is I rode it with no problems (and barely any sound) before, so maybe it was undertorqued? And Sheldon Brown says "I have a lot of experience mixing ISO/J.I.S. in both directions on my own personal bikes, and it as never given me a lick of trouble." and "In practice, you can very often get away with mixing these sizes, as long as you select a spindle length that gives the desired chainline."

I took a tape measure to my chainring/seat tube and my chainline is off by 3 mm. Measured from the center of the seat tube to a tooth and got 45 mm

hairnet

you said it

wont know for sure until you use a torque wrench

seau grateau

Derp.

calv

Did Sheldon ever bust Mad skids like you did?

Soo__Fuego

Spoonrobot 

Sounds like you didn't have the correct torque. Not a JIS/ISO issue.

Correct torque specs are all over the map for 75s. Despite what the quote says, I'd recommend something like 28-33 ft/lbs since you've already biffed them once.

Reeses

I removed the cranks and BB and reinstalled everything with sufficient torque but the chainring/crankarm still wobbles side to side.

Scrodzilla

Sounds like either the square taper interface is damaged or the BB is crap.

Crandrew

still using the incorrect bb?

Reeses

Scrodzilla

Reeses

Currently I'm trying to work a stripped crankarm off of a Sugino 75 BB so I can use the correct size bottom bracket.

rudypyatt

Timely topic.

I run a freewheel on my TimeLine, so no skid stops. I do ride over the local bridges and some hills in the area. I've been doing more and more hill rides lately. I don't stand and mash when I climb, but maybe the spate of climbing over the last three weeks has something to do with today's problem. I rode over the Brooklyn Bridge to the Greenway and up to 44th Street: No problems until, on the way back, this really annoying squeaking started coming from my bottom bracket. It sounded almost like shoes on a basketball court.

The crank arms and pedals look okay, so I'm guessing the BB needs to be lubed and torqued. Since I don't have a torque wrench, I'll take it up to the LBS this week.

Scrodzilla

Cool story bro.

For what it's worth, "mashing" is not standing up and pedaling like a rabid gorilla. To "mash" simply means to push a big gear ratio.

rudypyatt

Yup. Thanks for the reminder. To clarify, I do run the stock gearing, which is pretty big (46/15). What I meant to say was that while I do mash in the sense of the big gear, I climb seated. Sorry about the misuse of terms .

This actually belongs in the Ask Scrod thread (so, sorry again for the mis-post; please move at your discretion, Mr. Scrodmerator), but does this sound like a matter of lube the BB and re-torque it, or is it likely that my stock BB is toast? I've read elsewhere on the board that a few people have had issues with the BB on The Hour, which I believe is the same on the TimeLine.

"To stand up and pedal like a rabid gorilla, or to sit and spin like a gerbil on crack? That is the question..."

Bat56

Great hijack. Lube the BB and re-torque it... Do you know what you are talking about?

BigglyPuff

He must mean grease the threads and install it to the right torc speques

rudypyatt

Yup. Thanks. Sorry for the thread hijack.

Reeses

Today I tried to separate the stripped SG75 crank from the SG75 bottom bracket - it did not go well. But in curiosity I attached the drive side SG75 I skid on a couple days ago onto the non drive side of the BB and put the chainring on it. I gave it a spin and the wobble was gone. I guess it was that cheap RPM BB that I trashed and not the crank arm, thank goodness.