Bicycle Mechanics - Crank gives when pedaling hard
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08-19-11, 08:18 AM
I have a 1994 GT Zaskar that I adore! The components are from 1992. They are Shimano XT. I recently had shifting problems so I replaced the chain and cassette. The bike was tuned up. This seemed to solve the problem until I took it out for a real ride. If I stand-up to pedal really hard it will give. This was the same problem but it was much worse before the tune-up and chain/cassette replacement. This seems to happen in the middle and big ring. I am simply an avid rider and far from a bike mechanic. However I have and am not afraid to fix things. The work done the recent replacements and tune-up were done by a mechanic. The mechanic suggested replacing the cables and replacing the hub. I was wondering if a BB change was needed since it is original. Maybe even the chain rings. However I feel like I'm throwing darts in a dark room. Everything is original accept the chain and cassette.:(
Usually you can replace several chains before needing to replace the cassette, and several cassettes before you need to replace the chainwheels. but if conditions are just "right" it is possible to run even the chainrings into the ground.
Apart from that your reported symptoms and the suggested fixes really aren't making sense. Shifting problems are usually due to excessive friction in cabling, then eventually slop in derailer, or the fine mechanics in the shifter sticking. Worn chain/cassette can certainly cause the chain to slip/skip, but usually don't affect shifting much.
Replacing the hub is even more picked out of the blue. Sure, the freehub pawls can become gummed up, but compatible freehub bodies are still readily available, and a far easier fix than a hub replacement. Unless one wish to try to flush it or service it first.
BBs do wear out eventually. But I can't see how a BB could have a part in your current issue unless it was worn beyond belief, where failure would be self-evident.
08-19-11, 10:34 AM
Thanks. I appreciate the feedback. Shifting is not the problem. It was part of the original problem along with slipping. This remaining problem is not happening when shifting. I am pushing the pedals from a standing position and it gives (slips). It only happens with extreme force like standing and pedaling. Are you suggesting a hub replacement. Thanks
EDIT -- Never mind, I see you replaced the cogs. Are the chainring teeth excessively worn down? But it would be unusual to have the big ring get that worn out.
It doesn't sound like a hub problem to me. If the hub had slipping problems under load, it probably wouldn't work at all, just spin freely all the time.
Are the cogs worn out? If they are really worn, and you use a new chain, the chain could jump a tooth when pedaling hard. They'd have to be very worn out, though.
From Sheldon Brown's Chain Maintenance page (http://sheldonbrown.com/chains.html):
The illustration above shows two formerly identical sprockets, viewed from the right side. The one closest to us is badly worn. On a new sprocket tooth, the surface that the roller presses against is perpendicular to the pull of the chain. The worn teeth have become ramps, causing the chain to ride up under load.
In addition, as the roller follows its tooth around the sprocket it rolls up the "ramp," while under load. This promotes wear to the insides of the rollers and to the "bushings" they roll on. With a properly meshed chain, the roller only turns a tiny bit as it rolls onto and off of the chain.
Due to the pitch mismatch, the chain will not reliably mesh with the sprocket under load, and will tend to jump forward, skipping over the teeth.
08-19-11, 10:42 AM
The cassette was changed. I have not switched out the chain rings though.
08-19-11, 10:50 AM
It's most likely the chainrings. I suggest replacing them as your next step. As it is, the slippage poses a safety hazard as well as a nuisance, so be careful in the meanwhile.
For replacements with a long wear life, I'd consider some Salsa rings. They're not too expensive and they have thick full-height teeth, and are 7075 aluminum, which tends to wear well and resist gouging. For Shimano Deore XT from 1992, I believe you'll need 74mm for the inner ring, 110mm for the outer, so here you go: http://aebike.com/product-list/salsa-b359/chainrings-74-110-bcd-t203-m14628-qc30/
08-19-11, 10:52 AM
The problem could be a simple adjustment is needed to get the chain mating properly with the rear cogs, or a bent derailler hanger. If you are serious about "extreme pressure" your pedaling technique could be exacerbating the issue, especially if you never encounter it otherwise.
I'd suspect that perhaps your chain was only cleaned and not changed out with the cassette if all adjustments/alignment look good. An old chain on a new cassette will frequently result in skipping.
08-19-11, 10:54 AM
Hey thanks. That's exactly what I was going to buy. The Salsa were recommended to me because of the age of the components. I just didn't want to spend $100 if the chain rings were not the issue.
08-19-11, 10:55 AM
I bought the chain and the cassette. It's new. The bike was tuned up too. Thanks
08-19-11, 11:15 AM
I don't see how the chainrings could be the problem. I'm not sure what this "give" is ... but maybe the cassette cogs are slipping on a cassette hub that has trashed splines. Does it happen in specific gears in back ?
08-19-11, 11:29 AM
Hard to imagine chain rings are worn enough to slip with 20+ teeth engaged. To be that worn it would be conspicuously obvious that the teeth are practically gone.
I once bought a Team Marin with XT (1990). The chain had stretched so I replaced it. Then the chain slipped on the cassette. I replaced the cassette and then, like you, the chain slipped on the middle chainring. The small chainring was fine (it was steel) and the big ring was okay (it hadn't been used much). The middle, however, being alloy it had worn down enough to cause this.
The problem was all of the parts with regular contact had worn to each other. The consequences of buying a used bike with a stretched chain. And it didn't take long - I bought the bike in 1991.
As mentioned above, be careful in the meantime. I'd stay in the saddle to minimize groin surprises.
08-19-11, 11:45 AM
I've personally worn down chainrings enough to get full-circumference chain slippage. Including the big ring! Sometimes you can see this by pressing down on the crank while watching how the chain reacts... if the chain begins "climbing" the teeth under load, trouble's brewing. When it does happen to me, it's usually at a very inopportune time, such as trying to pull away from a stoplight very fast with traffic stacked up behind me. Dangerous and embarrassing to have the bike suddenly "go into neutral" while trying to accelerate. Adding a new chain to the mix will concentrate the problem because only some of the teeth are getting proper loading.
Anyway, after 20 years, those rings have definitely delivered a good service life for your money :)
08-19-11, 12:27 PM
I ordered the chain rings. I am hoping this solves the slip. mechBgon and P_M this is exactly what is happening. It happens when I need to accelerate quickly and now I'm skiddish. I will be afraid to test it but will keep you updated. Glad I joined this group. If any one else has a thought please post. Thanks
08-19-11, 12:36 PM
Does it happen in every gear or only in the higher gears? Maybe your rear derailleur isn't adjusted to wrap enough of the cassette cogs?
cycle_art, I hope this helps. And the way I looked at it at the time - I had essentially a new drivetrain.
All the best!
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