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Road Wheel Stiffness

You picked a fine time to leave me, loose wheel…

It seems as if most road bicycles these days feature carbon frames, and stiffness is consistently high and no longer an issue. Steel bikes were suppler and typically not quite as stiff, aluminum was rigid and could be somewhat harsh, and titanium frames (I have owned five) were quite often a tad flexy. With universally stiff frames on almost all of today’s carbon bikes, the relative rigidity of wheels is far more of an issue. It’s also a subject that goes completely unnoticed.

When people buy road wheels these days, they seem to focus on the weight, price, aero qualities, and appearance. Rarely do they pay attention to a very important quality of a wheel: Stiffness. There are three basic planes of stiffness; radial, lateral, and torsional. These three planes of wheel movement under load could more simply be described as “up and down,” “side to side,” and “twist.” Torsional stiffness (twist) seems to be trivial in all higher quality bicycle wheels, so it isn’t an issue worth discussion. The two significant dynamic planes of wheel stiffness are “radial” and “lateral.”

Radial stiffness (up and down) determines to a great extent the comfort of a bike’s ride. Bicycle wheels should have some vertical compliance so minor pavement irregularities aren’t harshly transmitted to the rider through the bars, saddle, and pedals. Perceived radial stiffness is also determined by the type and size of tire, fork stiffness, wheel rim depth, and air pressure. For example, a high-profile rim with 19 mm clincher tires and 140 lbs. of pressure would yield a teeth-chattering, brutal ride. Yes, those sweet-looking high-profile aero rims don’t ride as comfortably, and they’re made mostly for smooth asphalt time trials. A deep section rim, perpendicular to the road, doesn’t deflect much and you feel all the bumps. Years ago, Cervelo road frames used a tall, bladed top tube that contributed to this bike’s skitterish, harsh ride. They have since gone to top tubes that are flatter, with more vertical compliance for a smoother ride. Both my Specialized Tarmacs have wider, flatish top tubes for a supple ride but with no lateral flex. To summarize, while you want some radial stiffness, you also want compliance for a better ride and more accurate tracking of pavement eccentricities.

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The type of deflection that you do not want in a bike frame or in a wheelset is the lateral type of deflection (side to side.) If you feel your rear wheel getting squirmy in a sprint or a climb, it may not have adequate lateral rigidity. If you feel or hear your rear wheel scrapping off your brake pads during an out of the saddle climb or in a sprint, your wheel is too soft. A soft front wheel can make high-speed descending an unwanted adventure. Years ago, I owned many Zipp aero wheels, and in a conversation with Zipp’s Bill Vance, I suggested they design a non-aero, low-profile lightweight climber’s wheel, and call it the 202. At the time, they had 303, 404, and disc wheels. Bill told me Zipp had no interest in producing a non-aero wheel. A year later, though, they had tubular and clincher versions of a new “202” wheel. Sheepishly, they sent me a set of 202s for suggesting the concept. They weighed a ridiculously scant 1075 grams and they were beautiful. But they had quite a bit of lateral flex, I couldn’t climb while standing, and descending Snow Canyon was terrifying. I sold them to a friend who weighs 20 lbs. less than I do, and he’s fine with them.

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If you’re a heavier rider, you are bound to get more deflection from a wheelset that’s under 1,400 grams. And if that isn’t bad enough, bigger riders tend to break spokes and rims on lightweight wheels. Once you start regularly breaking spokes, it never ends. Consider a rugged 1,800 gram wheelset, which adds only a pound or so over a more fragile lightweight set of wheels. You can make up that pound by cutting out ice cream for a week…

There are certain aspects of wheels that compromise lateral stiffness. Rim rigidity, especially with carbon rims, doesn’t seem to be much of a factor. Spoke tension doesn’t appear to be much of an issue regarding wheel stiffness, either. The main factor seems to be the spokes themselves; specifically, spoke thickness and spoke number. It’s not hard to build a 36-spoke rear wheel with thick, non-butted spokes and have the wheel be rigid. There is a considerable weight penalty, though. Also, thick spokes, especially box-shaped bladed ones, are not as efficient aerodynamically. If you can hear your wheels “singing” on a fast descent, what you’re hearing is turbulence. And drag.

 

specialized-rapide-clx-64-disc-scs-system-245249-1Because lateral stiffness in road bike wheels is more important for your rear (drive) wheel, it’s common to see wheels with a different amount of spokes; usually with more rear wheel spokes. A common configuration is 20 or 22 spokes in front and 22 to 28 spokes in back. I used to time trial on a 16-spoke front wheel, and during races, its potential imminent and sudden failure was always on my mind. I rode a disc in back, though, and nothing is stiffer (and uncomfortable) than a disc wheel. More spokes and more robust spokes seem to make a wheel more laterally stiff. High profile wheels have another unintentional advantage. Because the spokes are shorter, the wheel can be stiffer laterally. Mixing spoke patterns on the rear wheel can help keep it from deflecting under load, too. My rear wheels of choice use 24 special steel spokes laced radially on the non-drive side, and laced 2-cross on the drive side. They have excellent lateral stiffness, yet the wheelset weighs a svelte 1,231 grams.

Okay, so how do you choose a wheel that’s both comfortable yet has good lateral stiffness? Despite all my postulating, you have to physically engage the wheelset, and in two ways. One way to determine the lateral stiffness, and probably the best way, is to mount the rear wheel firmly onto a bicycle, and pinch the wheel rim near the brake pads. Move the wheel laterally, side to side. If you can move it enough that it touches the brake pads, that wheel is a noodle, and is to be avoided. The other way to test wheels is to ride them, and do some sprinting and climbing. Adjust the rear brake fairly tight, and if the wheel rubs during spirited riding or when you’re climbing hard out of the saddle, look for some other wheels. Flimsy wheels can be unnerving to ride on, and they waste energy.

I’ve experienced sturdy, heavy wheels that have excessive flex, and also very light wheels that have very little lateral flex. Despite analyzing wheel properties, it’s almost impossible to tell if a wheel will be laterally stiff just by looking at it. A high profile wheel will almost always have a bit too much radial stiffness, though. Test out wheels before you buy, and try to ride them for a few days. And select the wheels that are appropriate for your style of riding, budget, and weight. By the way, the biggest improvement you can make to a bicycle is to swap out the cloddish stock wheels that came on it for a new set of super-zoot, flashy upgraded wheels. Currently, Red Rock Bicycle Co. favors Roval and Enve wheelsets, based upon value, performance, looks, and most importantly, reliability.

Come to think of it, I could use a nice new set of Roval CLX 64 tubular wheels

Written by Paul Scarpelli

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One Response to “Road Wheel Stiffness”

  1. Stuart Holt on 18 Mar 2017 at 9:02 am

    Who the heck rides setups (these days) for general purpose riding and training? Even the great Al Stern out here in Chicagoland tells me that his tubular tire repair business has fallen flat (no pun intended) as no one generally rides setup/tubular whees and tires because of the expense and quality of the road. A good, top of the line clincher is the wheel set of choice out here (Chicago) these days. Just sold a pair of Mavic Ultimates (setups) and had to find a customer in Canada who was willing to use them. All the same your article is excellent. Keep writing (and riding!) Miss you.

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