Damaged frame replacement help

[econrad]

Hello, I'm having trouble sourcing a replacement frame for my ebike.

After 6000kms my seat tube developed a crack and a bike mechanic I trust recommended replacing the frame. The bike is an "Urban Cruzer" from greenlightcycle, though I have added a BBSHD and a second battery under the rear rack.

I like the frame a lot -- ideally I would like to find where greenlightcycle sources their frames (I tried contacting them and asking and they were not able to tell me where they get their frames). I've found similar looking frames in pre-made bikes on alibaba, though I cannot listings with only the frame.

The next best option to finding the exact frame would be a different frame that is compatible with my existing wheels (26 x 1.75 inch), BBSHD, disc brakes, and a locking mechanism for seat tube battery (I think its called a silverfish blade connector?).

Any help would be greatly appreciated.

Thanks,
-Eric

 

[gromike]

It would be easy to find a used donor bike, if you ditch the seat post battery mount and the step-through. Otherwise you need to look for any ebike with that battery mount and the frame style you desire.
Why did your seat post crack on the Urban Cruzer?

 

[e-beach]

If the crack isn't too extensive find a welder to weld it for you. Frame metal dependent of course. To get better advise post a picture of the frame so we can see what you have going on.

 

[Chalo]

After 6000kms my seat tube developed a crack and a bike mechanic I trust recommended replacing the frame. The bike is an "Urban Cruzer" from greenlightcycle,

If you had a car that broke its back at 6000 kms of ordinary use, would you want another one exactly like it?

Diamond frames aren't just normal and boring because of tradition. They're normal, boring, and traditional because they've been designed, vetted, and refined for 150 years. The more you diverge from their most normal boring configuration, the more likely you're trying out something that's been tried over and over again for generations and found to be lacking.

I recommend getting a diamond or mixte framed bike, fitting a good stiff strong luggage rack, and carrying over all your electric components to the new frame. Don't repeat the mistake you already made.

 

[econrad]

If the crack isn't too extensive find a welder to weld it for you. Frame metal dependent of course. To get better advise post a picture of the frame so we can see what you have going on.

See the attached photo.

I discussed with the bike mechanic about potentially cutting below the crack with an angle grinder, though the issue is the seat tube becomes a different diameter (35mm) not too far below the crack. though I think this is also a standard seat tube size, I would just need a new seat post and collar.

I may proceed with this plan for now.

 

[econrad]

If you had a car that broke its back at 6000 kms of ordinary use, would you want another one exactly like it?

Diamond frames aren't just normal and boring because of tradition. They're normal, boring, and traditional because they've been designed, vetted, and refined for 150 years. The more you diverge from their most normal boring configuration, the more likely you're trying out something that's been tried over and over again for generations and found to be lacking.

I recommend getting a diamond or mixte framed bike, fitting a good stiff strong luggage rack, and carrying over all your electric components to the new frame. Don't repeat the mistake you already made.

Hi Chalo, though the frame itself failed, I would honestly put the blame on the original seat post (and myself). The seat post was far too short, and given that I am a taller rider, I did not have enough of the seat post inside the seat tube.

At first it was just warped, but over the past ~500kms it developed into the crack (I attached the photo in another reply).

I don't think I'd be willing to give up the step-through frame if I end up replacing the frame, though if you know of a more robust step through frame, I'd be open to suggestions.

 

[econrad]

It would be easy to find a used donor bike, if you ditch the seat post battery mount and the step-through. Otherwise you need to look for any ebike with that battery mount and the frame style you desire.
Why did your seat post crack on the Urban Cruzer?

The seat tube cracked since I was riding with the stock seat post too high. I replaced the stock seat post with a much longer one, though the warping caused by misuse of the original seat post turned into a crack (or maybe a tear is more accurate?). I attached a photo of the damage to another reply.

I may end up instead modifying the existing frame by cutting below the crack, there is wider portion of the frame that measures 35 millimetres, which I believe is a standard seat tube size. I'd just need a new collar and seat post provided the surgery goes well.

 

[Chalo]

The seat tube cracked since I was riding with the stock seat post too high. I replaced the stock seat post with a much longer one, though the warping caused by misuse of the original seat post turned into a crack (or maybe a tear is more accurate?).

I recently broke off a long, high quality seatpost that I had been using at its maximum height marking. So I ordered a much longer, very thick walled piece of tubing in the correct diameter (26.0mm), and ran it much deeper into the frame.





Seat post flex eventually caused cracks to form at the weld joining the bike's top tube to the seat tube, and those grew longer. Eventually the top broke off the frame. It was an uneventful failure because of various reinforcements I had introduced by then in the attempt to save the frame.

This is only to note that if you had the problem before, you'll have it again if you use the same frame the same way.

 

[2old]

As above, plus the frame tube will be thinner if its diameter is wider. However, if you go this route, ensure the "slit" (for the collar to tighten the seatpost) in the "new" seat tube doesn't have any stress risers and examine it carefully every few rides.

 

[lnanek]

There's double seat tube clamps you could probably use to avoid making a new slit entirely. They have separate clamps for the seat tube and seat post, basically.

 

[econrad]

There's double seat tube clamps you could probably use to avoid making a new slit entirely. They have separate clamps for the seat tube and seat post, basically.

Interesting -- I've never heard of these. I've already made the new slit, though this may be a good route for redundancy.

With a cursory search I'm not sure one of these double seat tube clamps would work for my exact setup. The outer diameter of the seat tube is 35mms, and the inner is 30mm; I plan to use this spacer to use my existing 27.2mm seat post in the new, larger 30mm inner diameter seat tube.

I am concerned if a collar can adequately compress all of that material, so a double collar would be a good addition if they make one in a 35 / 27.2mm configuration, but that seems quite niche.

 

[derekderek]

I would like to see a pic, including more of the seat tube on the frame. I think a better bet than that spacer would be to get a thicker seat post so then you don’t have a spacer in there and the thin seat post and then the thicker seed post is in contact with the inside of the seat tube for a much longer distance. And since at this point, the frame is kind of butchered anyway, you could also consider drilling a small hole through it and cross bolting it down lower.

 

[derekderek]

Also, you will need to cut a notch for clamping the seat tube clamp onto the larger diameter down lower spot.

 

[econrad]

I would like to see a pic, including more of the seat tube on the frame. I think a better bet than that spacer would be to get a thicker seat post so then you don’t have a spacer in there and the thin seat post and then the thicker seed post is in contact with the inside of the seat tube for a much longer distance. And since at this point, the frame is kind of butchered anyway, you could also consider drilling a small hole through it and cross bolting it down lower.

Sure thing, here is a pic of her post surgery:

Not sure if its clear in the photo, but I stopped the notch above the two lateral welds.

The inner diameter of the seat tube is 30mm, which seems like a rarer seat tube size, hence me opting for a spacer. I agree that a 30mm seat post would be ideal -- I can look into sourcing one.

The outer diameter of the "new" seat tube is 35mm exactly, so I have ordered a new clamp of that diameter. I don't intend to reuse the existing clamp.

 

[derekderek]

Assuming, of course, that there is enough seat tube above the frame rails, both going down to the dropout and forward to the forks. It’s my reasoning for wanting to see a picture of the center of the frame.

 

[Chalo]

It's customary to put a 1/4" or larger round hole at the bottom of the clamp slot to reduce stress risers and prevent cracking at the end of the slot.

 

[econrad]

It's customary to put a 1/4" or larger round hole at the bottom of the clamp slot to reduce stress risers and prevent cracking at the end of the slot.

1/4" seems large, are you certain?

Sounds like a good idea, though I'm worried that the distance between the top of the seat tube and the hole will be to small if I drill from the bottom of the slit. Should I enlarge the slit and then drill?

 

[Chalo]

1/4" seems large, are you certain?

Here's a typical Chinese made consumer frame:


And here's a USA made Independent Fabrications frame, featuring not only a generous keyhole but also a brazed-on reinforcement:


The main precaution is don't have notches in the perimeter of the hole.

 

[glennb]

I reckon I've got exactly the same frame. Yes, 34.9mm I.D. seat tube.

I've used all sorts of posts and homemade shims with it. Much experience with this annoying beast.

It's an unusually thick-walled seat tube. Shortening it puts the clamping pressure too close to the welds, which will resist the force.

It's a bad idea, you'll need so much clamping pressure that you'll strip threads or snap the clamp.

You'd only just get away with a very burly clamp using M7 or M8 bolt to secure it. That's with an actual 35.9mm post. With a shim, which effectively increases the post wall thickness and requires even higher clamping force ... forget about it.

If you don't want to use a 34.9 post, and must shim a narrower post, use your 34.9 post as the shim. Insert it well into the seat tube and rivet it in place. Or hammer a nail all the way through the seat tube if you're that way inclined. Leave at least 50mm of the shim/post protruding, and cut your slots and put the clamp on it, the shim, instead of trying to clamp the seat tube (which is too darn thick and large a diameter for practical clamping).

 

[econrad]

I reckon I've got exactly the same frame. Yes, 34.9mm I.D. seat tube.

I've used all sorts of posts and homemade shims with it. Much experience with this annoying beast.

It's an unusually thick-walled seat tube. Shortening it puts the clamping pressure too close to the welds, which will resist the force.

It's a bad idea, you'll need so much clamping pressure that you'll strip threads or snap the clamp.

You'd only just get away with a very burly clamp using M7 or M8 bolt to secure it. That's with an actual 35.9mm post. With a shim, which effectively increases the post wall thickness and requires even higher clamping force ... forget about it.

If you don't want to use a 34.9 post, and must shim a narrower post, use your 34.9 post as the shim. Insert it well into the seat tube and rivet it in place. Or hammer a nail all the way through the seat tube if you're that way inclined. Leave at least 50mm of the shim/post protruding, and cut your slots and put the clamp on it, the shim, instead of trying to clamp the seat tube (which is too darn thick and large a diameter for practical clamping).

This seems different than yours.

35 is the outer diameter, its a 30mm inner diameter. I have ordered a 30mm seat tube, and a 35mm outer collar in hopes of saving the frame.

 

[2old]

IMO, it was very intelligent to ensure the slit didn't infringe on the weld, and if you drill a hole, it shouldn't infringe.

 

[Chalo]

35 is the outer diameter, its a 30mm inner diameter. I have ordered a 30mm seat tube, and a 35mm outer collar in hopes of saving the frame.

I think it's a smart repair, and it has a better shot at being a permanent fix than if you replaced the frame with one of the same.

One of my e-bikes has been repaired by running a 27" long thick-walled 1" chromoly tube all the way to the bottom of the seatpost bore, then sleeving a close-fitting seatpost over the outside of it. I drilled and slotted the 30.4mm seatpost and fitted a 30mm binder clamp on the bottom end.

 

[derekderek]

https://www.amazon.com/BESNIN-Seatp...ocphy=9012321&hvtargid=pla-2281435177578&th=1 nice thing about amazon. if 30.4 is too big to fit, send it back...