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TECH NEWS
Paul Lew 1.jpgPaul Lew talks wheel dynamics
Paul Lew, Director of Technology & Innovation at Reynolds Cycling, has written a series of revealing articles on wheel dynamics that are available on the Reynolds website.
Part one deals with wheel stiffness.
“The qualifications for a ‘good’ wheel always incorporate stiffness and compliance, or more specifically, wheels that are laterally stiff and vertically compliant,” says Lew. “In engineering terms this means that the wheel has minimal deflection side-to-side and at the same time moderate to large deflection in the [up-and-] down direction.”
Lew says that the hub is responsible for 40% of a wheels stiffness and compliance.
“A well-designed and manufactured hub will not suffer from the static and dynamic forces of a high-tensioned wheel,” he adds. “A wide flange spacing decreases side-to-side deflection and at the same time increases vertical deflection, both contributing to a well-riding, ‘good’ wheel.”
According to Lew, spoke stiffness and spoke count are also responsible for 40% of wheel stiffness, but spoke crossing patterns and spoke tension don’t have a significant impact on this variable.
“Increasing the spoke count in a front wheel from 16 spokes to 20 spokes will increase the wheel's resistance to deflect laterally by approximately 30%. Increasing the spoke diameter and profile from a 2.0/1.8/1.5mm triple-butted spoke to a straight-gauge 2.0mm spoke will increase the wheel stiffness approximately 35%. The combination is additive, so increasing the spoke count from 16 to 20 spokes and the spoke diameter to a 2.0mm straight-gauge spoke will increase overall wheel stiffness by 65%.”
He says that the belief that a crossing spoke pattern will increase vertical deflection while minimising lateral deflection is incorrect; it will typically increase both.
The final component of the wheel, the rim, determines 20% of wheel stiffness, says Lew. The material used and the section aspect ratio both come into play here, the section aspect ratio being the more important.
A low aspect ratio rim – where the width dimension is greater than the height – will typically have minimal side-to-side deflection and moderate to high vertical deflection, but it won’t match a high aspect ratio rim – where the height is greater than the width – in terms of aerodynamics.
“Carbon fibre has the unique quality of low mass, stiffness tune-ability, high strength, and an unlimited fatigue life not possible to achieve with metal alloys which makes it an ideal material of choice for high performance rims and wheels,” says Lew.
In part two of the series, Lew again deals with spoking, comparing the stiffness of wheels with various spoke crossing patterns.
“The test revealed that a radial spoke pattern results in a less vertically compliant wheel than a crossing spoke pattern, but only by a very negligible amount,” Lew says. “I think most people would be surprised to see how little influence a crossing pattern has on vertical compliance and rider comfort. “
“Rider comfort and vertical deflection is best controlled by tyre pressure, not the spoke lacing pattern,” he concludes.
Lew deals with carbon-fibre variability in his third article, discussing the types of carbon-fibre and resins used in wheel manufacture.
“As a general rule, as the modulus increases, the strength decreases and the fibre is more brittle,” he says. “The grades of modulus that are associated with “best practices” for bicycle wheel production are Standard and Intermediate.”
He’ll be looking at the carbon-fibre moulding process in a future article so if you’re interested keep your eyes on www.reynoldscycling.com.
Mat has been in cycling media since 1996, on titles including BikeRadar, Total Bike, Total Mountain Bike, What Mountain Bike and Mountain Biking UK, and he has been editor of 220 Triathlon and Cycling Plus. Mat has been road.cc technical editor for over a decade, testing bikes, fettling the latest kit, and trying out the most up-to-the-minute clothing. He has won his category in Ironman UK 70.3 and finished on the podium in both marathons he has run. Mat is a Cambridge graduate who did a post-grad in magazine journalism, and he is a winner of the Cycling Media Award for Specialist Online Writer. Now over 50, he's riding road and gravel bikes most days for fun and fitness rather than training for competitions.
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13 comments
So if a shallow wheel gives better stiffness/compliance in both planes, but a deep wheel is more aero (but with little else going for it), why not have a shallow wheel with a lightweight, thin, deep-section fairing that, while attached, isn't rigid enough to contribute to a change in compliance?
Someone will correct me if I'm wrong but I don't think UCI rules allow this.
I believe you are correct. I have read the manufacturing regulations for several years from the UCI. The UCI specifically disallows non-structural elements for the purpose of aerodynamics.
If they did, there would be no point wasting time with a deep section aero fairing. A better solution would be to add a cover for the entire spoke area, ie a faux disc. Cheaper too.
This kind of idea is taken to an extreme with shell bikes such as are used for land speed records.
I suppose in an ideal world, a frame or wheel rim won't be subjected to the same level of flexing as an oar pulled by an elite rower.
In some studies, not in the bike industry, fatigue reduces the stiffness of a carbon fiber part by as much as 20%.
The fibers themselves are very resistant to fatigue failure, but the resin(s) that bind them are not, yet.
Nevertheless because the fibers do not fatigue, the degradation due to fatigue is mainly in the performance domain, not safety, unlike with metals which fail catastrophically.
Unlimited life span for carbon is simply not true. And I learned this from rowing, of all places.
Put carbon oars into the hands of the British heavyweight 8 and they will fatigue 2 sets a season. Fatigue in this instance being having them increasingly flex as the crew takes a stroke; at some point the oars will flex so much that the crew loses significant boat speed (significant being enough to lose a place in international competition).
Compare with the same oars given to British lightweight women; the much less stress they put on the oars is such that they can use a single set for 2 seasons.
The same goes for boats. An old hull will have significantly more flex around the riggers than a new hull.
Carbon is a wonderful material for racing or going fast but it will not last forever, maybe not for a season depending on your power/ mileage. It's a shame manufacturers tote out these mechanics that state otherwise; it really undermines their credibility.
Lew is comparing carbon fibre composites to aluminium. Al has poor fatigue properties because it has a very low elastic limit. Fatigue life is virtually unlimited at loads below the elastic limit, and limited at loads above that. Al structures have to be overdimensioned to compensate and give a reasonable life- remember the old Alan Al frames from the 80's that fatigued in a season. A composite structure can be designed to be loaded below the limit. The oars will be a compromise where the load is above that limit but the short life is accepted for the weight reduction. Also, the loading in the oars is a bending mode, that composites do less well than tensile loads- you can't oversize an oar shaft to keep the strain in tension as well as you can a bike frame.
That doesn't sound like a description of fatigue at all. It sounds like the heavyweight is able to go over the load limit. I'm a heavyweight for rowing and regularly ripped oar locks from their sockets, so I'd guess that all that you are seeing there is that the manufacturers are not planning for the loads that a seriously powerful back combined with long levers can put on a part.
If this was a fatigue issue only, both the heavyweight and the lightweight would be unable to exceed the load limit and both scenarios would experience the same performance dropoff with fatigue.
This is also characteristic of the difference between steel and aluminum. Steel also could be said to have a 'virtually unlimited fatigue life' if you stay below the load threshold, where alloy has a very low threshold, and tends to get more brittle over time just from age, so fatigue is a much bigger problem, even with low loads.
Fatigue is about how something gets weaker just from normal use, not how something becomes floppy after extremely heavy duty use.
"unlimited fatigue life"???
Really?
So if I set a piece up in a testing jig and set it running and kept it running and had my descendants and their's keep it running it would still be good thousands of years down the line?
but how will they report back to you?
You've hit the nail on the head there when using conventional methods for testing for unlimited life span - the test never ends!
You've hit the nail on the head there when using conventional methods for testing for unlimited life span - the test never ends!
Perhaps he should have qualified that statement by placing the word "virtually" before "unlimited".
The fatigue properties of CFRP are superior to all metals and particularly relevant to wheels is their superiority to Aluminium. I guess why he mentioned this was because if you are designing in CFRP, for all practical purposes, fatigue should not be a consideration, giving more freedom to address issues such as mass, aerodynamics etc.