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Broken Crank Arm

Old 05-14-20, 08:43 PM
  #1  
seaverd
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Broken Crank Arm

I just purchased a used 2017 Trek Dual Sport 4 a couple of weeks ago. On my bike ride this afternoon my left crank arm came flying off the bike. Fortunately I was 1/4 mile from home, so I walked it back. Upon inspection the splined teeth on the aluminum left crank arm show damage and the threads of the crank arm fastener that is pressed in the arm...looks like the threads are damaged/stripped. The splined teeth that this arm meshes to seem fine. The specs for the 2017 Trek DS4 indicate that this is an S1000 crankset. I tried to find it on SRAM's site and can't find it. Can you purchase just the left arm? Wondering what the fastest way is to get back up and riding. Ebay has a SRAM left arm listed but it says its a SRAM Left GX Eagle GXP...was wondering if this would fit? I also see on modern bike dot com (new user so cant post url) a SRAM Left X5 GXP Crank Arm. Logo looks different but description makes me think its a possibility.

Would love any advice.

Thanks,
Dan
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Old 05-14-20, 08:56 PM
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Take it back to the dealer. (hopefully, you purchased the bike from a Trek dealer). That sounds like an improperly torqued crank arm, to begin with, with which on a bike that you've only owned for a couple of weeks should be covered by whom you purchased the bike from.
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Old 05-14-20, 09:07 PM
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Yes, as @Le Mechanic said, if you ride a loose crank, it will damage it. But, it is the shop's responsibility to make sure it is set up properly when they deliver the bike to you.



If it was a recently purchased 2017 bike, then it would be either new-old-stock (may have a warranty), or more likely a used bike. If purchased through a shop, one may have some recourse, but if purchased through a 3rd party, usually stuff is sold as-is.

As far as replacing an arm, usually the specs of the crank is printed either on the back side of the crank arm, or on the back side near the pedal threads. Verify what you have.

You should see a Model Number, and a length (165, 170, 172.5, 175, 177.5, 180, etc). Matching the length is very important if you wish to purchase a single arm.

A brand will usually have similar attachments for several models, but some brands such as Shimano have multiple types of cranks, even sold concurrently.
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Old 05-14-20, 09:10 PM
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Originally Posted by Le Mechanic View Post
Take it back to the dealer. (hopefully, you purchased the bike from a Trek dealer). That sounds like an improperly torqued crank arm, to begin with, with which on a bike that you've only owned for a couple of weeks should be covered by whom you purchased the bike from.
This purchase was from a private seller, so no dealer is involved....as is purchase.
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Old 05-14-20, 09:23 PM
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The specs indicate that the crankset on this bike is the S1000 crankset. However the arm is stamped with X56LAAA 1 on the back side at the top and it also has a 175 on the backside along the bottom edge...which I assume is the 175mm length.

Does the first two digits mean it's an X5 crank arm? It looks similar to an X5....the screen printing is different than mine. The s1000 listing from trek is what is confusing me.
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Old 05-15-20, 05:51 AM
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Did you try re-installing the crank arm? Chances are pretty good that it will be fine, if the splines and threads are not too badly damaged. You probably lost the star shaped spacer/dust cover, when the arm fell off, so that will need to be replaced or even a new arm will never tighten correctly. There is no pressed in part on the non-drive arm, that is a threaded in self extractor and it should have a large hex shaped hole in it for removal if necessary(it usually never needs to be removed).
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Old 05-15-20, 06:27 AM
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It was probably damaged when you bought it and the seller knew it and screwed you. I wouldn't trust a used left crank arm either. The answer is go to a reputable mechanic and pay for a new crank with installation.
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Old 05-15-20, 06:37 AM
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Originally Posted by seaverd View Post
I just purchased a used 2017 Trek Dual Sport 4 a couple of weeks ago. On my bike ride this afternoon my left crank arm came flying off the bike. Fortunately I was 1/4 mile from home, so I walked it back. Upon inspection the splined teeth on the aluminum left crank arm show damage and the threads of the crank arm fastener that is pressed in the arm...looks like the threads are damaged/stripped. The splined teeth that this arm meshes to seem fine. The specs for the 2017 Trek DS4 indicate that this is an S1000 crankset. I tried to find it on SRAM's site and can't find it. Can you purchase just the left arm? Wondering what the fastest way is to get back up and riding. Ebay has a SRAM left arm listed but it says its a SRAM Left GX Eagle GXP...was wondering if this would fit? I also see on modern bike dot com (new user so cant post url) a SRAM Left X5 GXP Crank Arm. Logo looks different but description makes me think its a possibility.

Would love any advice.

Thanks,
Dan
assuming that the splines and threads are trashed and you need another crank, amazon appear to have one for ~$40. Search using “SRAM left crank arm”. However, I don’t know if this comes with a bolt, which you say is stripped
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Old 05-15-20, 07:18 AM
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Worst case is the whole crankset will be replaced. If this bike had been priced for $100 more would you have bought it? Sometimes the true cost of an item is only realized after you've taken it home.

The components, like cranks, can be different when purchased in the quantity that a bike brand buys, compared to those that the after market does. But there's a good chance your crank is very similar to one that is available in the after market. Given the lack of photos or our being able to have the crank in hand to confirm the replacement will either have to be determined by you or a LBS. I suggest the LBS because they will be better able to insure that all is compatible and they will be there is something goes wrong when you take it home, unlike that used seller. Additionally this bike will wear and have other needs as the miles add up. better to establish a relationship with those who know what they do and can actually touch the bike. Andy
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Old 05-15-20, 09:12 AM
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Originally Posted by seaverd View Post
...It looks similar to an X5....the screen printing is different than mine. The s1000 listing from trek is what is confusing me.
The previous owner might have changed the cranks.

Some pretty good deals out there right now for Ultra Torque cranks. 'Just sayin'...
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Old 05-15-20, 09:26 AM
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So I have tried to post some pics and URLs but as a new user I haven't met the criteria yet.

I have removed the crank. Splines on drive side look good. The flat washer that is grooved for the splines fits through fine. Crank bolt from left arm definitely stripped. Splines inside left arm are mangled to point that I cannot force left arm on drive side even with crank bolt out of the way. I scouted entire area of the incident and did not find the wavy washer...just the flat one that goes right over the bearing.

In terms of the bike, was purchased from a 70 year old. It is his son's bike and he moved far away over a year ago and the both decided it was time to sell it as it was taking up space in the 70 year olds garage. I paid $200 for the bike, so I think I bought it right and am happy with the price even when I factor in this needed repair.
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Old 05-15-20, 09:57 AM
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Old 05-15-20, 09:58 AM
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...I am a big fan of used stuff in my bicycling. I have, in the past, made some decisions to reuse stuff that was less than optimal. (For example, I used patched tubes from the co-op for a number of years, carrying a spare tube in case something went wrong.) I finally got tired of fixing stuff on the road (even though there's a real sense of satisfaction in pulling off another roadside repair. So in my case, I would not reinstall that crank. There are a lot of deals on cranks right now, as stated by an earlier poster. And so long as you get the threading and the shell width right, you ought to be fine ordering one online. You can go with that 175 length, or switch to 170 or 172.5 (some other common arm lengths). If it's a road bike, the shell is most likely 68mm wide. If you want to calculate (and change) your gearing, you can order one with different tooth counts for the chain wheels. But if you do that you might need to change the chain length a little bit.

It's pretty hard to go wrong
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Old 05-15-20, 10:15 AM
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I'm of the used camp too. I have never had a crank arm spontaneously explode on me. Although, be careful you don't get someone else's used trash (thus, a local sale lets you inspect).

Looking at the photo above.

The greatest wear on the chainrings is at 90° from the crankset. And I see significant wear on the large ring at the 90° spot, near the shift pins. Note the groove between teeth appears elongated and angled back towards the tooth.

The small ring also shows similar wear at the 90° point, but to a lesser extent.

This means you probably have a bunch of stuff to replace.
.
  1. I'd replace the whole crankset. It will be cheaper to get the crankset + 2 new rings than replace everything one at a time.
  2. CHAIN. Always replace if you replace your rings.
  3. Rear Cassette is likely also toast.
  4. Is the bottom bracket (bearings) smooth?
.
Of course, the other option is to simply cobble it all together, and get another year or two out of it before replacing the entire drive train.
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Old 05-15-20, 11:17 AM
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Let me first thank everyone for their responses, I really appreciate it.

My issue is trying to figure what 2x10 crankset to get. My current chainrings are 42T and 28T. The rear cassette is 11-36T. For most of my riding I am on the 42T chainring. When I look at Mtb cranks they usually max out at 38T and I am concerned that they are too small.

I see Shimano has a new gravel bike crankset....the grx fc-rx600. This has a 46T and 30T....so that would be great.

Current bottom bracket is smooth....but this would also have to be replaced if I switch to Shimano.

​​​​​​I have found a new crank arm for $25 with free shipping.

Wondering if I should try that to at least get up and running while I figure out a complete drive train replacement.
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Old 05-15-20, 11:40 AM
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Originally Posted by seaverd View Post
​​​​​​I have found a new crank arm for $25 with free shipping.

Wondering if I should try that to at least get up and running while I figure out a complete drive train replacement.
As long is the new arm is also 175mm long, then it would give you time to start exploring options. Make sure you have a good bolt.

I think a number of 24mm bottom brackets are compatible, perhaps requiring spacers. But you may not need to replace immediately.
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Old 05-15-20, 11:49 AM
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Oh, if you add about 4 teeth to your chainring.



1/2" per tooth, 2" longer circumference.

Circumference = πD
Circumference = 2πR

R=2/(2π)

So your derailleur will have to be able to be moved up about 0.32", or just under 3/8"
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