Rim thickness - worn out?

[Mick F]

My Moulton TSR rims are Sun CR18 406 in silver.
https://www.sjscycles.co.uk/rims-tape/2 ... im-silver/

They've done 7,200miles and showing signs or wear on the brake tracks.
Smaller wheels wear out faster than bigger wheels of course!

I bought a cheapo Swanson gauge a while back, and have just removed the back wheel/tyre/tube/rim tape and measured.

1.1mm of metal left. Not measured the front, but it seems to look similar.

Question:
What thickness were they to start with, and how long do I have left before replacing them?
Is 1.1mm ok for now?

[horizon]

AIUI .9 is the point at which you dump them. I have CR18s on my tourer. They have a small dimple which is a wear indicator on both sides.

[Mick F]

Thanks!
Seems like they have a stay of execution for now.

Can't see any dimples or anything that would be a tell-tale of wear.
Where are they?

[Brucey]

IIRC whether a CR18 has a wear indicator or not depends on when it was made; since they made in batches and some sizes don't sell well, purchase date is no real guide to this. In unmachined rims the thickness is mainly dependant on the wear state of the extrusion die. In machined rims it is dependant on how straight/well fixtured the rim was before it was machined.

IME most machined rims vary a lot more in thickness than unmachined rims, from rim to rim, side to side and around the circumference on a single side, i.e. whichever way you cut it, they are nearly always more variable (worse) which makes assessing the true wear state more difficult; if you take a single measurement it could be at a thin bit or a thick bit.

Whether a given thickness remaining is 'safe' or not depends on the strength of the material and of course the anticipated service loads. Very many rims will flare measurably before they actually split; dunno if that is very likely in your case or not, but I'd guess it is quite likely; CR18 rims are made of a relatively soft and ductile grade of aluminium and your tyre pressures will be high, making flaring more likely, I'd guess. Maybe other CR18 users would be able to comment?

cheers

[reohn2]

<1mm renew.

[horizon]

I thought it a good question as to rim width when new but a quick Google didn't produce any results. Of course you could measure this when you buy the rim but that wouldn't help in making your purchase. Is there a source of this information? Do rim makers provide it? Have I missed something? And of course, is it significant anyway?

e.g.

https://www.ryde.nl/edge-m-30

[Cyril Haearn]

Brucey, please to explain the difference between machined and not-machined rims, Diolch

[Mick F]

I can answer that, but as a layman.

Rims are extruded through a die and come out in one long long long long spiral.
They then are cut and joined into the correct circumference.

Some rims are then put on a machine and the brake surface is machined parallel. They do this (maybe) so they don't need such an accurate die method, leaving the accuracy to the milling machine. This could result in a "buckled" extrusion being trued up, and therefore the rim walls could have varying thickness.
Non-machined rims are probably parallel from the moment they are extruded and don't need machining.



As for the original thickness, when I buy new ones, I'll make a note, and then know how long I have before renewal.
Maybe my Moulton needs a Christmas present of new rims.

[Brucey]

Brucey, please to explain the difference between machined and not-machined rims, Diolch

aluminum rims have been made for about 90 years and (prior to about 1990?) were pretty much never machined, except to the extent required to dress the welded joint (if there was one) locally. The consequence of this was that most new rims were slightly uneven (for a few tens or hundred miles until they wore a bit) and very occasionally you would get a new rim that was unacceptable because this effect was so bad. [ Most wheelbuilders were in the habit of at least running their fingers over the rim joint and if there was a noticable step, making sure that the wheel rotated in one direction vs the step and perhaps using that rim on the rear where it would be unlikely to cause bad juddering.]

Some bright spark (or complete moron, as it turns out, depending how you look at it) noticed that new rims were a bit inconsistent and thought they could improve things by machining the brake track so that it was exactly parallel sided. It is even written into some euro-guff standard that you should do this, I think.

Indeed when you first use the brakes on such rims there is no judder. Job done then? Er, well, not quite; beware the law of unintended consequences....

a) the best (closest grained, hardest, most wear resistant) bit of the extrusion is machined off and thrown away. Duh.
b) the rim extrusions are often little changed from before when they were machined. The result is that the rim is a tiny bit lighter than before, but it is also very often more than half-way worn out vs the unmachined version of the rim. Double duh.
c) the machining is very often imperfect; often pieces of swarf are (invisibly) embedded in the surface, and then embed themselves in the brake blocks. The swarf-encrusted brake blocks will then wear the rims about ten times faster than normal. Triple duh.
d) it costs money to make a perfectly straight rim before machining. The manufacturers have cottoned on to the fact that when they machine a rim, they needn't bother to have it perfectly straight before they machine it; it is nearly always 'OK' afterwards.... This means that most machined rims have an uneven wall thickness, by the extent to which the rim wasn't straight before it was machined. Quadruple duh.
e) I harbour a lurking suspicion that they now favour rim materials/hardnesses which favour ease of machining over wear resistance. Quintuple duh.
f) the whole manufacturing process is now less efficient than before; maybe 10% of your expensive extrusion ends up as swarf; I'd sooner have that material stuck to the rim, thank you... Duh Duh Duh Duh Duh Duh.... and so on...

So it perhaps wasn't a 100% daft idea, but overall it has made the lives of most rims much, much, shorter than they used to be and it has made very many cyclists much more miserable than they ever could have been with a bit of brake judder on new rims. I think it has likewise caused many more accidents than it ever could have prevented; rims can wear so quickly and so unpredictably (e.g. because of the swarf effect and the uneven thickness) that rim failures appear to be about ten times more commonplace than they ever used to be, despite wear indicators and so forth. Even very experienced cyclists are caught out this way; you expect your rims to last longer than your tyres....? Don't bet on it, not any more....

FWIW it is a while since I built with any but CR18s are (I think) still an unmachined rim?

cheers

[Cyril Haearn]

Thanks for the explanation, I shall take a bit of time to read and understand, and I shall have a close look at rims and joints
Sticking to my back-pedal coaster brake

[PDQ Mobile]

Mick I really reckon Koolstops for improving rim wear.
The technique is to dry the rim by using on/off braking until the "paste" of dirt and bits of alloy that occurs in damp conditions is a powder which then sheds off.
Warming the rim and block achieves this.
The sound is the clue, as soon as any grating is heard, ease of that one and use 'tother until quiet reigns. Two or three firm applications at the beginning of a hill usually suffice.

This results with Koolstops in a nicely polished braking surface that lasts a very long time.

I have a (quality) 20" rim that has covered more miles than yours and shows but a little recession.

[andrew_s]

When I've had a rim actually fail (Mavic), it's been down to 0.6 to 0.7 mm.

I've also had worm rims that flared quite obviously when felt with the fingers, and once they reach that state they don't last very long due to the back and forth movement as the flare gets squeezed flat by the brake blocks.
You can check for flare by measuring the outside width just by the tyre with no air and with the tyre at full pressure, and calling it dead if the measurements are more than 0.3 mm different (guess)

I've also had rims lasting a lot longer with Koolstop than with Shimano blocks (6000 miles/9 months)

[Mick F]

Koolstop eh?

Moulton is on Swisstop for the past 1,000miles.
Ok, but nothing to write home about, even though they were highly recommended to me on here.

I wonder if a softer block would be better than a harder block?
I'd rather replace brake blocks than a rim.

Are Koolstops soft?

Found four spare Campag Record blocks and shoes this morning. Never had any issues whatsoever on Mercian with Campag.
I could fit the one's I've found onto Moulton perhaps.

Are Campag Record softer than Swisstop?
Have Swisstop worn out my rims sooner than Campag or Koolstop would have done?

[reohn2]

Mick
BBB Techstops are soft and give the best braking in adverse conditions,don't pick up bits of rim or road grit like some other makes do and they're very rim friendly.
The only downside is they don't last as long as other blocks/pads.
Koolstop Salmon would be my next best choice.

[Samuel D]

They’ll all wear the rim if you brake in the wet. Can’t you avoid that, Mick? Play Scrabble on rainy days. Or refrain from braking in the wet.

If you ride on dry days your rims (and pads) will last near enough forever. Essentially all wear seems to happen in the wet. I knocked over a millimetre off my pads in one memorable wet day. Spun the barrel adjuster twice on that ride.