October 23, 2011 4 Comments
[Note: this is #12 of a series of 20]
Designing wheels seems deceptively simple. Pick parts that fit the bike, ride, and budget. Make sure hub and rim drilling match and spoke length is correct. All too often, the design process is kept this simple and amazing opportunities are missed.
An exception is the fixie world, where a wheel's cosmetic appearance gets special attention. Sure, beauty is only skin deep, but bravo to those designers who take the time to at least be fashionable. Got to love bikes where wheels make a visual statement.
However, there's sooo much more possible. Bikes are stately structures, even cute sometimes. But it's all about the ride. There would be little demand for bikes if we couldn't jump aboard and have a vivid, useful experience. And when you ride, you aren't watching your bike. Your ride is a combination of sensory images and the bike is responsible for many. In particular, the feel of the road is heavily affected by the wheels.
Greatest single influence is the tire. Size, pressure, sidewall thickness, tread shape, all combine and will overwhelm the feel of your frame and other components. A stiff, lifeless frame can be smooth and resilient with the right tire combination. Thankfully, this is increasingly well known and long distance riders and competitors are opting for generous size and moderate pressure over tiny, hard tires. As a result, their rides are faster, longer, and more enjoyable.
Next up for ride influence is rim. Material, cross section, and weight hugely affect ride. Lighter rims with less depth can absorb large amounts of vibration. Wood is the king of comfort. Deep V aluminum rims are visually fabulous but, for long miles, are unhelpful with fatiguing road shock. Most carbon rims are designed for maximum stiffness at the expense of comfort in order to be as light as possible. Every rim is a special blend of visuals and dynamic performance. Ride, keep notes, read, and talk about rims every chance you get. As a chef can tell you, olive oils (or any ingredient) are not created equal. As wheel designer, think about the consequence of your rim choice. It's not just visual and economic!
Next big effect on ride is spokes. They have four ways to change the feel:
(1) More spokes give the rim greater strength and rigidity. When a structure is close to being inadequate, often the case with weight-conscious bike parts, then strength and rigidity are valuable. Nothing reinforces a rim's strength like spoke number. However, if the rim is fully loaded with spokes (high number) then it is less able to move with loads. Liveliness, suppleness, the ability to dampen and modulate shock, all goes away. Nothing like an aluminum rim with deep section or lots of spokes to give a dead ride. Such a wheel is NOT a lost cause. A great, lively tire can hide much of that deficiency.
(2) Light gauge spokes are much more elastic. Compared to carbon fiber, for example, stainless steel is super elastic. Each spoke is a spring with a constant proportional to its thickness. 14g spokes have 23% greater cross section area than 15g and are that much less springy. Light, butted spokes may not be easier to build but they give the sweetest ride. Too bad most of the industry has collectively forgotten. You, as wheel designer, can employ this feature anytime.
(3) Spoke material is also a factor. Carbon fiber is potentially much less elastic than steel but can be configured otherwise. Aluminum spokes are best kept from moving due to fatigue life. So wheels with aluminum spokes are generally very stiff.
(4) Spoke tension affects the ride as surely as tire pressure. Most wheels are built to maximum tolerable tension these days. There are some stiffness and longevity gains available but, like so many dynamic situations, a benefit in one department is a cost for another. Stiff, tight wheels are less comfortable, sustain injury less well, and make greater demands on hub and rim. Don't forget that the magnitude of spoke tension is YOUR option. There is no universal right or wrong. Modify as needed.
Hubs are part of the mix, too, but I'm going to leave them out of this discussion since their influence is low.
Now that I've reminded you, please approach your next wheel set with all your options in mind. Don't be a mindless drone, building identical wheels like an iPod with only one tune. Wheels can be as diverse as music. After all, they're our instruments. Design them creatively, apply some rhythm and heavy breathing, and the result is bound to be worthwhile.
November 02, 2021
What a treat to hear from a true wheel guru, Dr OCTTO, the sage of Toronto (and elsewhere).
Re: spoke tension, he is exactly right, the spring rate of a spoke is not changed by tension but a permanent consequence of material. However, more tension (and/or more spokes) increases the load capacity of the wheel. Greater capacity means less often at peak load instants can the rim flex.
November 02, 2021
I remember reading about certain team mechanics building wheels for Paris-Roubaix with a lower spoke tension in order to soften and tune the ride a bit over the cobbles. I think one of the techniques was to use 14 gauge spokes and lower tension, and then lightly crimp the spoke nipples to keep them from vibrating loose(obviously this was before SpokePrep became available).<br
November 02, 2021
Regarding point (4), spoke tension (assuming a metallic wire spoke) cannot affect ride.
As you allude to in point (2) each spoke is a spring with a constant spring rate that depends on the spoke gauge (or minor spoke gauge on a butted spoke). This means that spoke stiffness cannot vary with spoke tension, because the stiffness (i.e. spring-rate) is constant.
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November 02, 2021
Thanks for your kind words. The Dr had some great mentorists in the bicycle wheel field (fanatyks, some might say)!
Aluminum rims are also elastic bodies with a spring-constant. In a wire-spoked bicycle wheel, the rim and spokes form a single structure with single spring-constant, due to friction forces between the spokes and rim and hub flanges [*] (and therefore