RecordAceFromNew
Swinging Member
- Location
- West London
A number of recent threads have posed this question, here’s my 2 cents...
To put things in perspective it is probably worthwhile starting with a rough baseline for power requirement. For a 70kg rider riding a 10kg bike over good, level road on a windless day, approximate power requirements at 15kph, 30kph and 50kph are 34W, 173W and 700W respectively. If you happen to be Tinkerbell or Shrek try this.
A pretty comprehensive assessment of road wheel performance exists, covering aerodynamics, weight and stiffness.
It is clear that high profile aero wheels help if you have legs like Cav’s and if crosswind is of no concern to you. At 50kph, a difference of 15W (between a Zipp 808 and a run of the mill front wheel) is 2.2% of total power requirement. Since power required to combat air drag varies with speed to the power of 3, the corresponding differences at 30kph and 15kph would be like 3.2W and 0.4W, i.e. reducing to 1.9% and 1.2% of power requirement.
However, the implication for most of us mere mortals is more likely to be the difference between having, e.g., say a DuraAce c24 (23.5W at 50kph, the most aerodynamic wheel on the list with 37mm profile or below) and e.g. a Mavic Aksium (28.3W at 50kph). The difference would represent 0.7% of power requirement at 50kph, 0.6% at 30kph, and under 0.4% at 15kph. Not only is the DuraAce's advantage small, even very minor rider posture difference will dwarf such effects.
Wheel weight and inertia on the other hand while having little impact at constant speed affect acceleration (and therefore the feel of bike’s eagerness and responsiveness). As the 2nd article says for every additional 25g on a rim, accelerating from standing still to 30kph in 10s requires an additional 0.4W (actually 0.35W to be precise). It might not sound like much, but since some tyres such as Marathons are 900g a piece, and that heavier tubes double the weight of light tubes, an additional power requirement of over 10W can materialise during that 10s even without having heavy wheels. Imho, having relatively heavy tyre more than heavy wheel is why many mtbs, hybrids and cheaper bikes feel sluggish. Obviously having the right tyre pressure matters too for minimising rolling resistance (it needs to be fairly high for the road but low offroad), but that is a different subject.
By the way every 100g we put on the rim is equivalent to 200g in the drink bottle or around our waist that we have to haul along when we accelerate. The reason being that the power it takes to spin that mass up is the same as the power it takes to accelerate it forward. The issue though is unless one does a lot of accelerating and braking, the effect of added wheel mass and inertia is going to be limited - rims that are 250g heavier may require you to burn another 3.5W when you accelerate to 30kph, but that will only be for the 10s it takes you to get there. Ok that extra weight will slow you going uphill, but it will also speed your way down.
Finally, as the 3rd article points out wheel stiffness can have a significant impact if the wheel is flexible enough to cause brake rub. No less common ime is loose/binding cup&cone hub bearings. Both can sap energy even on an otherwise easy ride. Inadequately tensioned spokes, not uncommon even on wheels of new bikes costing over £500, is also the key reason why many stock wheels don’t last. For most riders, imho these, rather than aero or weight advantages, are the more compelling reasons for getting better wheels (or getting existing wheels better maintained).
If all one wants is to have a decent set of wheels for commuting or even club runs, then I think the science shows that in terms of speed, climbs or acceleration the advantage of anything else over a pair of good, basic c£150 Mavic/Fulcrum/Campag/Shimano factory or custom hand-built wheels is always going to be pretty marginal - by that I mean you will be lucky if you get a couple of percents beyond placebo effect. Further, often with alloy freehub, nipples etc., more expensive wheels may not be more durable.
So are lighter, more expensive wheels not worth having? I wanted my full-sized folder to be no heavier than the lightest Brompton – such wheels helped. My favourite mtb currently is a full susser that can behave like a rigid mtb or a hardtail via air shocks with lockout, at 11.5kg it also carries little if any weight penalty (and no carbon fibre) – such wheels helped. Sure it is wants rather than needs, but if one can and wants to, why not? Especially if we ask ourselves what performance advantages do higher groupsets really offer...
To put things in perspective it is probably worthwhile starting with a rough baseline for power requirement. For a 70kg rider riding a 10kg bike over good, level road on a windless day, approximate power requirements at 15kph, 30kph and 50kph are 34W, 173W and 700W respectively. If you happen to be Tinkerbell or Shrek try this.
A pretty comprehensive assessment of road wheel performance exists, covering aerodynamics, weight and stiffness.
It is clear that high profile aero wheels help if you have legs like Cav’s and if crosswind is of no concern to you. At 50kph, a difference of 15W (between a Zipp 808 and a run of the mill front wheel) is 2.2% of total power requirement. Since power required to combat air drag varies with speed to the power of 3, the corresponding differences at 30kph and 15kph would be like 3.2W and 0.4W, i.e. reducing to 1.9% and 1.2% of power requirement.
However, the implication for most of us mere mortals is more likely to be the difference between having, e.g., say a DuraAce c24 (23.5W at 50kph, the most aerodynamic wheel on the list with 37mm profile or below) and e.g. a Mavic Aksium (28.3W at 50kph). The difference would represent 0.7% of power requirement at 50kph, 0.6% at 30kph, and under 0.4% at 15kph. Not only is the DuraAce's advantage small, even very minor rider posture difference will dwarf such effects.
Wheel weight and inertia on the other hand while having little impact at constant speed affect acceleration (and therefore the feel of bike’s eagerness and responsiveness). As the 2nd article says for every additional 25g on a rim, accelerating from standing still to 30kph in 10s requires an additional 0.4W (actually 0.35W to be precise). It might not sound like much, but since some tyres such as Marathons are 900g a piece, and that heavier tubes double the weight of light tubes, an additional power requirement of over 10W can materialise during that 10s even without having heavy wheels. Imho, having relatively heavy tyre more than heavy wheel is why many mtbs, hybrids and cheaper bikes feel sluggish. Obviously having the right tyre pressure matters too for minimising rolling resistance (it needs to be fairly high for the road but low offroad), but that is a different subject.
By the way every 100g we put on the rim is equivalent to 200g in the drink bottle or around our waist that we have to haul along when we accelerate. The reason being that the power it takes to spin that mass up is the same as the power it takes to accelerate it forward. The issue though is unless one does a lot of accelerating and braking, the effect of added wheel mass and inertia is going to be limited - rims that are 250g heavier may require you to burn another 3.5W when you accelerate to 30kph, but that will only be for the 10s it takes you to get there. Ok that extra weight will slow you going uphill, but it will also speed your way down.
Finally, as the 3rd article points out wheel stiffness can have a significant impact if the wheel is flexible enough to cause brake rub. No less common ime is loose/binding cup&cone hub bearings. Both can sap energy even on an otherwise easy ride. Inadequately tensioned spokes, not uncommon even on wheels of new bikes costing over £500, is also the key reason why many stock wheels don’t last. For most riders, imho these, rather than aero or weight advantages, are the more compelling reasons for getting better wheels (or getting existing wheels better maintained).
If all one wants is to have a decent set of wheels for commuting or even club runs, then I think the science shows that in terms of speed, climbs or acceleration the advantage of anything else over a pair of good, basic c£150 Mavic/Fulcrum/Campag/Shimano factory or custom hand-built wheels is always going to be pretty marginal - by that I mean you will be lucky if you get a couple of percents beyond placebo effect. Further, often with alloy freehub, nipples etc., more expensive wheels may not be more durable.
So are lighter, more expensive wheels not worth having? I wanted my full-sized folder to be no heavier than the lightest Brompton – such wheels helped. My favourite mtb currently is a full susser that can behave like a rigid mtb or a hardtail via air shocks with lockout, at 11.5kg it also carries little if any weight penalty (and no carbon fibre) – such wheels helped. Sure it is wants rather than needs, but if one can and wants to, why not? Especially if we ask ourselves what performance advantages do higher groupsets really offer...
My 60mm Carbon Clinchers look the dogs though ....
