Worn rims...

Raph

The rims on my mtb are getting thin - they don't have wear indicators but the surfaces have gone very concave. If I measure them with calipers, is there a recognised minimum thickness beyond which they're considered dangerous?

Also - it's on a 15yr-old carbon frame (GiantCFM), with steel forks, no disk caliper mounts whatsoever - is it possible to fit disks?

Simplest would be to rebuild on similar rims and carry on with the V-brakes but I'd love to build new wheels from scratch and get disks on it if possible.

supersonic

Depends how thick they were to start with. But from what you say, they are on their last legs!

Discs will be nigh on impossible for the frame and rear unless it has flat alloy dropouts that can take an adaptor, and even then the stress may knacker the carbon stays.

Front can be done - but would need new fork, brake, hub and maybe shifters if they are combined with your brake levers.

My advice? Stick with Vs, and pick up a cheap pair of wheels from Merlin like this:

http://www.merlincycles.co.uk/?fn=produ ... egoryId=99

Check your frame has 135mm dropouts though, and uses a cassette.

Raph

OK there goes my dream of getting disks onto this bike - I'll just carry on with those grinding noises on wet off-road rides, and the habit of pumping the brakes to clear the crud. Changing forks is venturing into "might-as-well-get-a-new-bike" territory, which I will eventually do when funds allow, even though this one's an old friend. And I did wonder about carbon taking the stress of a disk brake, so thanks for confirming it isn't a good idea to bodge it.

I'll pass on the cheap wheels, I've got a Ti-glide on there I'm quite fond of, so I've just ordered a Mavic XM719 for it, and a S/H fleabay front wheel - Mavic 521 on Hope pro 3.

Thanks for the reply.

supersonic

You'll probably need new spokes as well!

The 521 rim is half a pound more than the 717 - the cheap wheel I pointed out will be lighter!

supersonic

You'll probably need new spokes as well!

The 521 rim is half a pound more than the 717 - the cheap wheel I pointed out will be lighter!

Raph

You can say that again

The 521 was because it came as a wheel, I would have preferred the rims as a pair, but fancied the Hope front hub that it comes with. I did check the weights, but cheap and light usually means weak. It's going on a 15 yr-old frame, so despite being carbon it's not the lightest so no point saving weight on the wheels unless it's going to be a lot, and then only if it's not at the expense of strength. And it's a rigid frame so piling into tree stumps in the undergrowth could well need a strong rim - having babbled all that, I don't know for a fact that the 521 is necessarily stronger! For a start though, the 117 is pinned, the 521 and 719 are welded, for what it's worth.

I've reused spokes in the past and never yet broken one, so if they're the right length for the new rim I'll just reuse them. Though now you've got me thinking about it, I might just use new ones for the gear side...

Also - cheap wheels are usually cheaply built - I used to build wheels for a couple of shops, I insisted on getting them absolutely right so I finally got ousted by kids building twice as many pairs per day but the standard was shockingly bad. Rims nice and straight, but spoke tensions up the creek. I suppose I could buy cheap wheels and re-tension them... but if I'm going to do that it's easier to start from scratch.

Anyway, thanks for the advice - it did help!

supersonic

Youre right, the 521 is stronger! But is a DH rim, so you'd expect it ;-)

Merlin handbuild all the wheels, and are great quality - but it is a budget rim at the end of the day. Decent buy for those on a budget, but can see this is a well loved machine that deserves more lol

Hopefully the spokes are the right length!

John Stevenson

Chris Juden of the CTC recommends a proof test for rims: inflate the tyres to 2X their rated maximum. That will load the sidewalls and if they are worn, they'll fail.

nicklouse

if doing the above please use a track pump, as you do nt really want to be too close if it does want to fail.

It can hurt.

supersonic

Goes with a right bang too!

John Stevenson

Good points, chaps thanks.

Raph

"Merlin handbuild all the wheels, and are great quality - but it is a budget rim at the end of the day."

OK, I thought the build would be the weak point.

With the rim test - sounds like a good idea - my ha'pennyworth would be put a sheet or blanket over it so if it does explode and send slivers of cracked aluminium out they won't lodge themselves in your eyeball.

Raph

Oh, and "Hopefully the spokes are the right length!"

No guarantee of that, the rim profile is probably quite different to what I've got now!

Noclue

I've just ordered those wheels from merlin to replace a wheel that failed on me whilst riding ( the twisted aluminum also wrecked a tyre, inner tube and set of brake pads, so if you think your rim is on its last legs best replace the wheel before it dies!) they should be with me today or tomorrow so i can let you know what they are like.

artillerydave

Raph wrote:

the rim test - sounds like a good idea - my ha'pennyworth would be put a sheet or blanket over it so if it does explode and send slivers of cracked aluminium out they won't lodge themselves in your eyeball.

Good idea :roll: when I was in Iraq we used to use sheet to protect against shrapnel all the time, same way the quilt can protect you from the boogie man!

I would not wish to stand near potentailly exploding metal with or without a sheet. Believe me shrapnel hurts, I have the war pension to prove it!

Raph

"same way the quilt can protect you from the boogie man! "

Really - that effective? Wow!

The idea's from winching vehicles, a blanket on the rope/cable kills the energy of it whiplashing if it breaks or just gets caught on something then works loose.

"( the twisted aluminum also wrecked a tyre, inner tube and set of brake pads, so if you think your rim is on its last legs best replace the wheel before it dies!)"

Yes, that bike's laid up till the new bits arrive. I'll be interested to know how those cheap wheels are - a mate of mine was angling for my wheels when I said I might replace them - I didn't flog them to him cos of keeping the rear hub and the fact they're on their last legs- - if I can point him to something cheap and good then the problem's solved (except I don't get to rip off my mate...damn!).

Raph

Mildly amusing (though not from my point of view) - I got the XM719 rim in the post, took the tyre off the rear wheel, measured how thick the metal is at the thinnest point, where the tyre bead sits, and it's about the same thickness as the new rim, namely about 1.3mm.

So... perhaps the old worn rim was a lot thicker to start with - I don't remember it ever being concave in section so it must have been... but then... does that mean it was a) a lot stronger than the new one to start with, or b) made of crappier metal therefore needed to be thicker therefore is now weaker even though it's the same thickness?

Just wondering whether to carry on and rebuild the wheel or stash the rim away as spare and carry on with what I've got.

Haven't checked the front yet.

supersonic

They do vary in thickness. New regulations were introduced a few years back where all rims had to have a safety line indicator.

Raph

I'm getting a newish front wheel in the post any day now so if the front's any more worn than the rear I'll use the new one, but I think for the rear I'll put the new rim away and leave the rebuild to another time.

And in answer to "hope the spokes are the right length" - the new rim has a deeper profile so it needs shorter spokes, though the existing ones would just about do if they had to (meaning they won't poke through into the tube, at least that diameter's the same!) - anyway not rebuilding it now gives me time to get a set of exactly the right lengths.

Those cheap wheels are looking better by the minute! I do like the hubs I've got though (desperately looking on the bright side)