Repair options seat tube crack
08-07-16, 11:54 AM
#1
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Repair options seat tube crack
Hi all,
I got given a Cannondale Bad Boy frame with a crack in the seat tube, just below the clamp.
And the likely culprit, a 30.9 seat post in a 31.6 frame, still in place.
But I'm still tempted to build it up.
The size is right, and it has all mounting points I like to see on a commuter frame.
And I think I've got most of the parts in my stash already.
So what to do about the crack?
1) Basically nothing. Fit proper seat post(or shim), ride and be happy.
2) buy a double clamp, fit proper seat post, ride.
3) drill the crack to stop it propagating, buy double clamp, fit proper seat post, ride.
4) buy a more substantial shim, like a 31.6-to-27.2. And a double clamp. And a 27.2 seat post. Drill the crack, bond the shim in place using either epoxy or a bushing mounting compound as a reinforcement. Ride.
How ambitious do you think I should be?
Double/split clamps are a little expensive, but otherwise none of the fixes above are particularly costly or time consuming.
And the worst that can happen if the crack were to continue is that the saddle would lose alignment.
Still, I'd prefer not to waste effort on unneccesary and/or doomed projects.
I got given a Cannondale Bad Boy frame with a crack in the seat tube, just below the clamp.
And the likely culprit, a 30.9 seat post in a 31.6 frame, still in place.
But I'm still tempted to build it up.
The size is right, and it has all mounting points I like to see on a commuter frame.
And I think I've got most of the parts in my stash already.
So what to do about the crack?
1) Basically nothing. Fit proper seat post(or shim), ride and be happy.
2) buy a double clamp, fit proper seat post, ride.
3) drill the crack to stop it propagating, buy double clamp, fit proper seat post, ride.
4) buy a more substantial shim, like a 31.6-to-27.2. And a double clamp. And a 27.2 seat post. Drill the crack, bond the shim in place using either epoxy or a bushing mounting compound as a reinforcement. Ride.
How ambitious do you think I should be?
Double/split clamps are a little expensive, but otherwise none of the fixes above are particularly costly or time consuming.
And the worst that can happen if the crack were to continue is that the saddle would lose alignment.
Still, I'd prefer not to waste effort on unneccesary and/or doomed projects.
08-07-16, 12:21 PM
#2
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In addition to 2) above, I would put a shaft collar below the crack, and even or above where the bottom of the seat post. The collar is to prevent the seat tube from cracking due to the force from the seat post.
08-07-16, 12:56 PM
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First a comment.
You're not the first, and it's all too common. WHY do people asking for help force readers to look up specs instead of just providing the relevant info? (you've been around and should know better)
I was able to infer that you're dealing with an aluminum frame, but it would have been all to easy to say so.
Meanwhile, you don't provide key info about your crack, which way does it go? how long, where in relation to the seat stays, etc.? A photo will really help, along with knowing your weight, riding style, length of post below the crack, and whether you only ride locally, or are planning a multi-day tour.
There are general approaches to most repairs, but when dealing with each specific case the devil will be in the details, and you provide none.
You're not the first, and it's all too common. WHY do people asking for help force readers to look up specs instead of just providing the relevant info? (you've been around and should know better)
I was able to infer that you're dealing with an aluminum frame, but it would have been all to easy to say so.
Meanwhile, you don't provide key info about your crack, which way does it go? how long, where in relation to the seat stays, etc.? A photo will really help, along with knowing your weight, riding style, length of post below the crack, and whether you only ride locally, or are planning a multi-day tour.
There are general approaches to most repairs, but when dealing with each specific case the devil will be in the details, and you provide none.
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FB
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An ounce of diagnosis is worth a pound of cure.
Just because I'm tired of arguing, doesn't mean you're right.
“One accurate measurement is worth a thousand expert opinions” - Adm Grace Murray Hopper - USN
WARNING, I'm from New York. Thin skinned people should maintain safe distance.
Last edited by FBinNY; 08-07-16 at 01:00 PM.
08-07-16, 03:21 PM
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I need new glasses... can't see the pictures.
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08-07-16, 03:44 PM
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Hi all,
Am on vacation. Didn't bring bike or pictures of the damage. Thought I'd get a head start at what might be the project for this autumn by researching options.
Crack(s) are perpendicular to the main axis of the tube, between the collar and the top tube/seat tube/seat stays junction. Maybe 1/2" long, starting from the compression slit.
It's unlikely to be enough seat tube protrusion to simply cut the damaged part out.
Plan was to build the bike up as a disc brake equipped commuter, since I don't seem to ever get around to fitting disc brake mounts to Old Faithful.
As such, the bike would see 30-32 miles daily.
Am on vacation. Didn't bring bike or pictures of the damage. Thought I'd get a head start at what might be the project for this autumn by researching options.
Crack(s) are perpendicular to the main axis of the tube, between the collar and the top tube/seat tube/seat stays junction. Maybe 1/2" long, starting from the compression slit.
It's unlikely to be enough seat tube protrusion to simply cut the damaged part out.
Plan was to build the bike up as a disc brake equipped commuter, since I don't seem to ever get around to fitting disc brake mounts to Old Faithful.
As such, the bike would see 30-32 miles daily.
Last edited by dabac; 08-07-16 at 03:50 PM.
08-07-16, 04:01 PM
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Hi all,
Am on vacation. Didn't bring bike or pictures of the damage. Thought I'd get a head start at what might be the project for this autumn by researching options.
Crack(s) are perpendicular to the main axis of the tube, between the collar and the top tube/seat tube/seat stays junction. Maybe 1/2" long, starting from the compression slit.
Am on vacation. Didn't bring bike or pictures of the damage. Thought I'd get a head start at what might be the project for this autumn by researching options.
Crack(s) are perpendicular to the main axis of the tube, between the collar and the top tube/seat tube/seat stays junction. Maybe 1/2" long, starting from the compression slit.
Cracking perpendicular to the axis, usually starting at the bottom of the slot, will not be reinforced by any collars, nor can they be glues, welded or bonded.
They're caused by metal fatigue of the cantilevered section owing to excess flex, or lack of support from the seatpost within. This is becoming a more common failure as frame design moves away from seat lugs, and cross member support to the top of the tube.
The unbuttressed tube flexes under load dependent only on it's own strength, and the support from the post within. That internal support from the post is an iffy thing, because it depends on the relative stiffness of the tube vs the post.
In your shoes, I'd put minimum dough into the project, insert the stiffest, correct size post I can find, (no top notch lightweights) so I'm depending on the post to carry the stress below the top tube, and counting on the cantilevered set tube only for clamping and twist prevention.
At this point, nothing will stop this crack from growing, but a shaft collar directly over it, might slow it down.
With luck, and depending on riding conditions, it might last years (or outlast you), or it could continue to total failure in days or weeks (I've seen both).
If the design allows, after it fails, you might file the new end clean, saw a new slot, and fit a seat post clamp to the stub and start fresh. Of course, the devil will be in the details.
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An ounce of diagnosis is worth a pound of cure.
Just because I'm tired of arguing, doesn't mean you're right.
“One accurate measurement is worth a thousand expert opinions” - Adm Grace Murray Hopper - USN
WARNING, I'm from New York. Thin skinned people should maintain safe distance.
FB
Chain-L site
An ounce of diagnosis is worth a pound of cure.
Just because I'm tired of arguing, doesn't mean you're right.
“One accurate measurement is worth a thousand expert opinions” - Adm Grace Murray Hopper - USN
WARNING, I'm from New York. Thin skinned people should maintain safe distance.
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