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Mistral De-mystified – the AM 17-inch rim

23/06/2012

Doug Milliken writes:
This discussion strand started when a number of people in the bicycle industry were asked to comment on an undergraduate thesis on tire-rim design & the possibility of run flat tires for bicycles. The thesis was done in Spring 2002 by Rory Pheiffer at MIT, for Prof. David Gordon Wilson, well known as the co-author of Bicycling Science, MIT Press.

Out of the original discussion, CTC Technical Officer Chris Juden and I had some additional correspondence on the origins of hooked edge rims and the AM rim design – which, due to good design and controlled tire-rim fit, does have good run-flat characteristics. While the rim is mentioned briefly in The Spaceframe Moultons, there should be enough detail below to satisfy just about anyone – short of an actual rim designer!


Doug wrote about the AM rim:
I don’t know why this small hook is there. It’s clear when mounting an AM Moulton tire that the “bead seat” (“bead shoulder”) keeps things concentric.

Chris replied:
I can tell you, because I designed that rim! I’m assuming that you’re talking about the newer, more rounded profile Moulton use, which is the Mistral 217. The older 117 I did not design, but both are Raleigh cum Sturmey cum Mistral designed extrusion, squirted by SAPA at Tibshelf, then and perhaps to this day. Both are “straight sided” designs, since at that time folding tyres didn’t come in wide enough sizes for a 17mm rim and the theory that you need a hook (sorry Mr Michelin, a “crotchet”) to retain high pressures even with a steel bead, had not been born.

Another respondent questions whether the industry has even used the term crotchet. I can tell you that the Michelin company certainly did draw a distinction between crotchets and hooks. They were very keen to have the English term crotchet used for the special kind of hooked rim edge they deemed necessary to retain the unprecendentedly narrow and high-pressure wired-on racing/training tyres they’d invented in the 1970s. I well remember Ken Shufflebotham (who could forget a name like that) expousing on this at BSI meetings in the 80s. His firm especially liked the way the English language could distinguish this kind of hook from the hooks on mountain-bike rims, by using an alternative word for hook that was borrowed from French and hence the same word as Michelin used in France. French being French, has only one word for hook, like it uses “resistance” for everything from stiffness to electrical properties of a metal. That lack of words didn’t matter then, because the mountain-bike hadn’t arrived yet in France. (And whilst we’re talking about how useful it is to have different words for subtly different things, can you Americans and Australians do anything to correct your countrymen from using the term “bicycle” to include cycles that have other numbers of wheels than two?)

The hooks on mountain-bike rims are vestiges of the real hooks that retained the beads of clincher tires. Americans should know more about this (we never had these in UK) but they had a flap of rubber that interlocked with a definite groove in the edge of the rim. All the tire sizes that go N x something-with-a-decimal-point are wired-on sizes derived from clinchers. However that’s all water under the bridge. The genuine hook-edged rim is extinct and the little hooks on mountain-bike and road rims, though of different origin, are similar or not almost at random. And it doesn’t seem to make a jot of difference!

I’ve never liked these hooks however, since they produce a sharper radius of curvature on the rim edge at the point where the tyre is flexing as it comes into the contact zone and transferring road-load to the rim by friction at the rim edge. (It is useful at this point to visualise the mechanism by which a tyre actually carries your weight.) Since rims sprouted hooks we have observed an entirely new kind of tire failure, where the casing chafes at the rim edge. The problem is greatest on heavily laden bicycles (e.g. tandems) where the internal tire pressure is perhaps not high enough to produce enough static frictional force to stop the tyre shifting up and down as it flexes through the contact patch. However we didn’t ever get this problem in the past and so I believe that this very sharp radius of curvature is largely to blame.

So I designed the Mistral 217 with a compound curve on the rim edge, having a large radius of curvature where the tyre rolls on and off the cross-section as it flexes under load, and a sharper radius away from where it touches. And although they are not hooked, I deemed it desirable to thicken the rim edges for several reasons. The rim edge is vulnerable to denting when you run over a rock in the road or pothole with an inadequately inflated tire, and a thicker edge may resist that better. Also a rim is a beam and that beam is stiffer with material distributed to it’s upper and lower chords. The spokeface is thick enough already, so my objective was to make the tops of the flanges as thick as possible without making a hook edged rim, or the rim any heavier, or any other part too thin.

The resulting rim had a higher radial stiffness per unit weight than any other of its day. Some modern rims are stiffer and just as light, but have thinner sides that give only half to two-thirds the braking life. The nasty way that hooked rims explode without warning at the end of that life is another story.

In response to a question about Mistral, Chris Juden wrote:
After one of Raleighs earlier downsizing and outsourcing phases, myself and a couple of other redundant engineers bought and ran Raleigh’s in-house rim manufacture briefly as Mistral Rims, then the ex Raleigh Technical Director (who’d put all the money in) sold it all to Sun Metal of Warsaw Indiana. Although I’d rather be designing bits and bikes, there didn’t seem much demand for such talents and so I now make a safer sort of living from merely writing about them.

And finally, something on the difference between the 117 and 217:
Probably that other rim is the original Raleigh rim section that we subsequently called 117. It was only marginally lighter – by no more than 5% – because another design restraint upon my 217 design was that it must not cost more. (It’s true, weight-obsessed cyclists will pay a premium for stuff it costs less to make – a fool and his money are easily parted!)

The 117 had sharper corners between the spokeface and brake tracks and a marked transition from curve to flat at the edges of the spokeface, a few mm from those corners. It was pinned with 3mm instead of 4mm pins. A great many AMs were made with Raleigh 117 material, and Moulton did not change over to the 217 until a year or two after its introduction. I kept weight down on the stiffer 217 by reducing the inner thickness of the pinning hollows, keeping the outer thick. This also caused the pins to make a visible bulge on the inside of the rim, so you could see at a glance that they were there and centered, but with no effect on the outside. By virtue of the larger pin I maintained the same pullout force.

Chris Juden
CTC Technical Officer

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