sweeks

1) In both cases, the crack in the sidewall was continuous with the crack through the center. I know "correlation" and "causation" are two different things, but based on a limited study (n=2), there were no central cracks unless there was also severe rim wear (thickness <0.4mm).
2) The first failure was the original wheel, which was true but spoke tension was unknown. The second wheel was built by me. I used a Park Tools tensiometer, and all the spokes were around the middle of the range of acceptable tension (they also made a nice ring when tapped), and rim trueness ("truth"?) is within 0.5mm both axially and radially.
3) The rims are Kinetix Pro (ERTO 406-14). The same rim on the front has survived over 4,000 miles without any significant wear, much less cracking.
4) I am a heavier (210 pounds) rider, but I do not hit potholes or major road defects. That said, this is Chicago, and there are plenty of rough patches.
5) I have another folder, which is a dedicated "winter" bike. It has a roller brake and so no rim wear. This is a wheel I built several years ago, and it has no issues. Marathon Winter tires, 70 PSI.

I've been under the impression that smaller wheels are sturdier and *more* durable, all other things being equal. This may be from absorbing propaganda from folding bike makers (example: https://brompton.zendesk.com/hc/en-u...-small-wheels-.
The tires are rather wide for the rim: Schwalbe "Marathon Plus" 406-47. I inflate them to 70 PSI, the maximum rated pressure. I do think the pressure, while not excessively high, contributes tensile stress to the rim, and is related to the crack propagation down the center. At this time, I remain convinced the damage to the rim originates with brake surface rim wear.

What I am looking for is an explanation for the wear and a suggestion for how to stop it. 'Tis a puzzle!
Steve

gauvins

Wear comes from the grit/sand pressed on the rim by brake pads. As if you were power-sanding rims.

Since more weight sits on the rear wheel (usually 60% of total load, vs 40% for the front wheel), the rear rim is more likely to exhibit damage, and to eventually fail.

Suggestion to mitigate the problem are to: (1) purchased heavy duty rims, suck as Ryde Grizzly CSS, or, if you are patient and dutiful, (2) to regularly clean rims and brake pads, in particular after rain.

sweeks

That's a "Crown Thickness Gauge"... a very handy little tool. A good one is accurate to 0.1mm, and you can sometimes squeeze a little more accuracy out of it.

Another useful dental tool is the "Boley Gauge" (https://www.amazon.com/SurgicalExcel.../dp/B0074N397C), which is a simple "vernier" caliper. Also good to 0.1mm or a scosh better. (Disclosure: I'm a dentist. ^_^ )
Steve

sweeks

Understood. Why more wear on the rear though?

I'm not sure how increased radial loading causes more brake wear, which is in an axial direction. By this logic (as I understand it) the front rim should have about 2/3 as much wear as the rear, which is not observed.

I'm giving serious consideration to a steel rim for the rear, as suggested earlier (below). Meanwhile, I may try some softer pads on the rear brakes. Since the bike is a commuter, I prefer to keep maintenance requirements to a minimum... so extra service to the brake pads and rim, while not something I would absolutely rule out, is not my first choice.

Thanks!
Steve

70sSanO

I imagine no one makes or ever made a ceramic coated 20" rim. I think they are more resistant to abrasion, but I can't imagine any market exists for a BMX ceramic coated wheel. But who knows...

John

CliffordK

Many companies have moved away from ceramic coated brake tracks since the wear on the ceramic doesn't last that long, and many of them look ugly as they wear. Good, clean, square raised brake tracks are nice.

HTupolev

Rear wheels are grit/dirt/grime/whatever magnets. Nothing kicks stuff onto the front wheel, but the front wheel kicks stuff onto the rear wheel.

That's one of several reasons that many people recommend doing most braking up front.

70PSI is very high for your weight and that tire width.

sweeks

This is the most logical and satisfactory explanation I have to date. I have a nice front mudflap, so softer brake pads, a steel rear rim or more frequent maintenance seem like the only realistic options.

I've always used the front brake as my main stopper.

- It's rated at that pressure.
- The front tire is also at the same pressure.
- I'm not having tire problems.
Steve

CliffordK

There is something about pressure and tire size.

So, say one has a small and large tire at 70 psi,

The pressure at any single point is the same, 70 psi. But, I think the larger casing of the bigger tire apparently pulls out more on the sidewall and bead due to the larger area being exposed to that pressure.

So you are putting the equivalent of a much higher pressure on the bead.

It seems counter-intuitive, but that seems to be the conclusions I've read.

HillRider

If you install a steel rear rim you can eliminate brake track wear very easily. Since steel rims have such appallingly poor braking performance you could eliminate the rear brake entirely and not give up anything.

Seriously, your riding conditions and experience are one of the few good arguments for disc brakes. I don't know if any folding bikes are available with them but they would cure all of your rim woes.

sweeks

Well, they're not *that* bad. I just wish the rear hub (Alfine 11) had the option for a roller brake module like the Nexus 8 on my "winter" bike.

Actually, Tern (and others, I imagine, may also) offers a bike similar to mine with disc brakes and (drool!) belt drive (see: Verge S8i | Tern Folding Bikes | Worldwide)

Unfortunately, I already own the bike I own. Plus, the front disk hubs have an OLD* of 100mm, vs the hub I have which is 74mm. That makes a significant difference in the bike's folded width, which would have a negative impact on my commuting. I had a "loaner" with the wider hub (and disc brakes) and it would not fit on the train as well. Those brakes were to die for (so to speak... actually, they would probably *keep* you from dying!).

I can also do what I've been doing: replace the rim every 2 years. It just takes a couple hours and it's kind-of fun.
Steve

* Over-Locknut Dimension

sweeks

It *is* confusing, but I still come back to the fact that the rims don't fail except where there is severe sidewall wear, which suggests that the tire pressure is not the precipitating factor.

What I *would* like is a 20" rim with a significantly greater width. This wouldn't help with the wear, AFAIK, but it would make brake shoe adjustment a hell of a lot easier. With a rim as narrow as mine and a tire this wide, when the brake cable is released the pads contact the tire. This doesn't happen in operation, obviously(!), but it makes adjustment a PITA. Also, with "normal" pads, the curvature of the smaller rim doesn't match the pads so some of the middle of the pads is hanging in space *inside* the rim. To get that part of the pad onto the rim would mean the ends of the pads would touch the tire ("BOOM!"). I found some pads that are somewhat shorter than what I call normal (Clarks 55mm Threaded V-Brake Pads | Chain Reaction Cycles), so I can get more pad on the rim. It would be easier if the rim were a bit wider.
Steve

gauvins

Right.

I meant that since the rear wheel bears a bigger load, it will fail before the front wheel does.

FBinNY 

There's no single answer about how long rims may last. It depends on riding conditions, weather, number of stops per mile, and the speed from which you stop.

IMO - even under the worst conditions, urban riding in a wet climate (ie Seattle) with plenty of hills and emergency stops, I'd still expect rims to last at least 10,000km. Those on my commuter, ridden in all weather here in NYC lasted over 20,000km until totaled in a crash. On my road bike, I've never worn out a rim from braking, including a front that lasted well over 50,000 miles.

It's been my general experience, that except for urban commuters in wet climates, brake wear is not a major cause of rim failure. Rims tend to fail sooner from crashes, road hazards, metal fatigue at the spoke holes, or simply outlasting the bike which is sold off or mothballed.

personally, I've never worn out a rim brake track, though I was getting close with the commuter when the crash kept my record intact.

gl98115

And I would guess fenders makes it worse. Instead of the water and grit getting flung away, fenders funnel it back onto the tires and rims for another pass.

randomgear

Honestly a few of the above opinions are why you should have started a new thread; it's not that difficult. (says the guy who has only started a single thread...) Given the break location between the spoke holes it appears to have nothing to do with either brake wear or grit.

Methinks it's not brake pad wear but the spokes pulling the rim apart. Having only used a Park tensionometer once, I can confess that the meter markings were somewhat confusing (OK, just not completely straightforward) for a 26" wheel and wonder if your tesionometer works correctly for a 20" wheel. Unfortunately I don't know enough about wheel building to offer constructive advice.

2_i 

After 10k or so on a wheel you should start checking, on occasion, the rim thickness with a dental caliper. The thickness should not get below 0.5mm. Good quality and well maintained pads make the rims last longer. By now I wore out at least 2 wheels. I invest in high quality hand built wheels, hence their demise tends to come from either an accident or rim wear.

79pmooney

My city bike rims last two winters of commuting 20 miles round trip, 3 days a week plus some weekend use. Mileage? Perhaps 5000 miles? At that point, laying a straight edge across the rim and seeing the hollow is scary. I put a 32c tire on one of those rims, put perhaps 70 psi in and heard it blow like a gunshot from inside my house an hour later. Glad I was nowhere near.

Portland, OR is probably the worst place I've ridden for rim life in the winter. (Boston, Ann Arbor, Oakland and Seattle.) A function of both the hills, many stops at the bottoms, and the road grit. Up until this winter there was never salt. My experience says that grit is far worse for rim wear than salt. Now, my experience is a little skewed because my salt days were ridden on sewups where rim wear doesn't matter until your rim collapses. And by March those rims were so square that they got replaced as part of the spring ritual. I never cared how much rim wall was left, so I never looked.

Good thing for my wallet is that I am good at building wheels; good enough that my spokes last three rims. Makes re-rimming easy. Now if I could only get a bulk rate on rims ...

Ben

Darth_Firebolt

Unless you are riding with a 40-60 pound load on the rear rack, you should be down in the 55 psi range for your rear tire. If you aren't letting the air in the tires act as a cushion, you're sending stronger impulse forces through the rim, spokes, frame, and finally your butt and hands. The tire acts as a lightly damped spring. The overall force of whatever impact the wheel sees is the same no matter what PSI you use, but the deformation of the tire makes it take longer, so the wheel has more time to put that energy into its system instead of just breaking. Couple high pressure with the fact that the same bump appears larger to a smaller wheel than a larger wheel, and you have more easily broken rims. Here's the formula I used for your tire pressure, and I assumed 70% rear axle loading and a 25 pound bike:

P=((600*L)/(W^2))+(.75*W)-25

P is pressure in psi, L is wheel load in pounds, and W is tire width in mm. Most bikes have 60-65% of the total rider + bike weight on the rear wheel, so use the formula to set your rear tire psi, then use ~85% of that pressure for your front tire.

sweeks

Thanks! I will try the lower pressure (sometimes I go down to 60) and see what happens. But unless somehow lower tire pressure reduces brake-associated rim wear, I don't expect it will have any effect on my chief complaint.
Steve

FBinNY 

Changing tire pressure (in either direction) will have zero effect on brake track wear.

However it will have some effect near the end of the rim's life when the brake track thins.

The outward stress on the rim is equal to the product of tire width and pressure. So 100psi produces twice the stress in a 2" tire than it dies in a 1" tire. By the same token a 20% pressure drop in any given tire reduces stress by 20%. (yes the math is that simple, but there are subtle differences based on tire profile shape and construction --- search "hoop stress" for more on the subject).

So as the rim thins, it'll fail sooner when the hoop stress is higher. So the only benefit of lower pressure is that you can run your rims longer before they finally fail. of course there are other, better reasons to change tire pressure, mainly based on rolling resistance, traction, and handling.


For those running rims that you suspect are nearing the end of their life.

You don't want brake track failure while riding, especially in the front because the suddenly wider rim could bind in the closely adjusted brake and lock the wheel. Here's a way to raise the safety margin.

When pumping, go beyond riding pressure by 20% or so, then drop it back. This way you've done a stress test at 20% beyond where you ride, and greatly increased the odds that the rim will fail before you get on the bike. If you want a higher margin, stress test at a higher pressure. If you want the maximum safety margin, do the stress test at night, and wait until the morning to drop back to riding pressure.

I don't do this in the normal course of riding, and reserve it for rims I have reason not to trust. I also don't do it daily, only every week or two, figuring that the rate of thinning isn't all that fast.

gregf83 

This was my rear wheel from two weeks ago.

Rode home on a Fri night. Washed the bike on Sat expecting to take the rear wheel in to have it rebuilt (I knew it was close to end of life). Only after I cleaned the bike did I notice the broken rim. I suppose I got my money's worth...

FBinNY 

I'm surprised that you didn't feel significant pulsing when braking. That's a tip off that the rim is starting to spread under the stress. However, not pulsing isn't assurance that all is OK, because rims have been known to fail without notice.

But, yes, you got all it had to give.

sweeks

I've found (in two of two experiences!) that I get plenty of warning as the brake surface begins to push out. The rear brake begins to "pulse" when it's applied and after a few tens of miles it starts to really thump. The first time this happened, the brake was thumping for quite a while. I noticed the rim starting to separate when it was thumping *without* using the brake. At that point I was at work and had 3 miles to ride to get home, so I released the rear brake arms and lowered the tire pressure by 20 PSI. That rim is the one in the image I posted earlier where the rim sidewall is obviously separated.
Based on my experience, I think testing the rim by over-inflating the tire is over-cautious, but I suppose it's possible the rim could separate without warning. The best test I've seen is holding a straightedge to the rim's brake surface and looking for concavity; if it's more than about a half millimeter then rim replacement is in the foreseeable future.
Steve

FBinNY 

Yes, it is over cautious, which is why it's reserved for highly suspect rims known to be nearing the end. However, I consider it less over cautious than replacing a rim that might still have thousands of miles in it.

I'm also a big believer in Murphy's law, and it seems that once I decide that a rim, tire, or other part is on borrowed time, it seems to find a second wind. OTOH - a newly built wheel always seems to be looking for rain filled potholes.