Knock Knock...

[halfmedley]

Thanks to the close attentions of 'White Van Man' I was forced into a pothole on my bike which resulted in a rather bone-jarring thwack. I thought I'd got away with it (no punctures, everything seemingly intact) until I came down a hill and applied the brakes. Now everytime I brake I get a 'knock-knock-knock' sound from the rear brake. It only occurs when braking. Anybody got any ideas what's causing this?

 

[flecc]

Sounds like the rim edge has been buckled against the edge of the pothole halfmedley. If you turn the bike over and run each edge of the rim round against your finger, you may feel or see a bump or depression at one point.
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[halfmedley]

Thanks Flecc, there was a metallic sound when I hit, I'll check the rims. I don't suppose that aluminium rims can be manipulated back into shape? I always tried to avoid potholes/kerbjumping too, as it isn't a very comfortable experience - something to do with the mass of the machine? It's no wonder people ride around on mountain bikes with roads as good as ours!

 

[flecc]

It's fairly unlikely one can be straightened, but occasionally one can be lucky enough to get just a top edge bit of damage knocking a pad that can be smoothed away with a file.

Our roads really are a problem now, and the drop handlebar & lycra brigade must ride on their nerves all the time with their skinny tyres and half inch rims.
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[Fat Girl]

Thanks Flecc, there was a metallic sound when I hit, I'll check the rims. I don't suppose that aluminium rims can be manipulated back into shape? I always tried to avoid potholes/kerbjumping too, as it isn't a very comfortable experience - something to do with the mass of the machine? It's no wonder people ride around on mountain bikes with roads as good as ours!

My new bike has thick tyres like a mountain bike that grip the tarmac too much - feels like riding through treacle and definately slows me down. I'd prefer touring tyres like on my old bicycle as they travel more freely over the tarmac.

 

[flecc]

I agree Mil. Many owners, including me, change to smoother tread tyres and the Schwalbe Marathon Plus tyres are the favourites since they are almost completely immune from punctures as well as being easier rolling. Very expensive though.
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[Haku]

....which resulted in a rather bone-jarring thwack.

I had a similar experience last night, going along a cyclepath at twilight so didn't have lights on, discovered (at speed) a broken off concrete bollard some [insert favourite expletive] kids had kindly left lying in the middle of the path...

Because the bollard was the same dark colour as the path I didn't see it until the last second, managed to swerve the front wheel out of the way just in time to stop a repeat performance of my superman impression, but caught the chainring on it which bounced me and the back end of the bike about a foot or two into the air. Luckily I didn't come off (or even slow down) but the chainring protector is well and truly smashed.



On the tyres subject, this is the first bike I've had in many years with a smooth center line, mostly I've had mountain bike tyres but I've really come to like these smoother tyres for road riding (just wish the roads didn't resemble the flatness of dirt tracks so much!).

 

[halfmedley]

Further inspection has revealed that the rim appears to have split on what looks like a seam. Are bike wheels made from an extruded strip which is then formed into a circle and then joined? Pardon my ignorance but I always assumed they were cast in one piece. Looks like I'll be needing a new rim anyway and just to make life harder it would have to be the wheel with the motor in wouldn't it!

 

[allotmenteer]

Yes indeed rims are made from a piece of metal bent round on itself and joined together. On good quality rims you have to look hard to see the join but on my cheapo bike you can see the join at twenty paces! (slight exaggeration there).

 

[halfmedley]

I have to say I find that amazing. Wouldn't the join then be a weak spot in the wheel? I thought aluminium was tricky to weld, and it's not as if there's a lot of surface area for a join to be made? Mind you, I wouldn't go back to steel rims, I quite like being able to stop when it's wet!

 

[flecc]

It's a cost issue, if moulded in one piece the cost would be extremely high, but formed and welded from strip the cost plunges. The welding isn't a problem though, and the best rims like those from Mavic who specialise in their manufacture are excellent.

Sorry to hear this has happened though, what make bike is it?
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[Ian]

Many cheaper rims are not welded at the seam, instead are simply secured by 2 pins inserted into ciricular grooves in the extrusion. The rim is placed under enormous compression by the spokes and even more when the tyre is inflated so there is little risk of the joint separating.

I have seen a rim failure at the seam on a high quality weinman rim, caused by the braking surfaces wearing too thin and no longer having the stregnth to resist the outwards force from the tyre.

 

[halfmedley]

Sorry for the long delay in updating, I've been away. It turns out that the rim wasn't cracked at all, just buckled. My friendly local bike dealer pointed out that the 'crack' was actually the seam. All that was needed was the wheel to be re-trued. I got both wheels done while I was at it: cost me a tenner. Also the hubs were adjusted so the bike rolls a lot easier now.

I had to cut wires to get the wheel completely off, which I did with some trepidation. But it was a simple job to re-connect them using a 'chocolate block', which I then velcro'd to the frame and waterproofed (wrapped in cling film!). This modification also enabled me to remove the rather heavy kickstand, saving a bit of weight. I also rewired the dynamo set up with better quality wire, so that I have reliable lights now the clocks have changed.

Whilst the wheels were off I degreased the chain, and then applied clean winter lubrication, so hopefully I'll be able to keep riding my Mistral now until the really bad weather sets in again i.e. next summer!

Incidentally, if you get the chance to visit the National Cycle Collection in Llandrindod Wells, it's well worth going and a bit of an eye opener. A lot of innovations seem to be a lot older than expected: a shaft-driven bike from 1908 for example!