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How much does mass matter - Hubs
I'm frequently asked technical questions via private message. Might as well respond to some of the more interesting ones as blog posts, so here we go.
This questions was about the impact of hub mass - 100 g, for example.
Considerations related to mass:
1. Linear inertia. All mass on the bike and rider contributes equally.
2. Rotational inertia. Pertains to perimeter wheel mass. Effects are trivial at the centre of the wheel and small to trivial effects at the perimeter of the cranks or elsewhere in the drivetrain.
3. Unsprung mass inertia. Pertains to the "unsuspended" elements, primarily the wheels, brake calipers and rotors, fork lowers, and a portion of the rear triangle and shock.
4. Moment of inertia (vehicle roll, pitch, yaw). Resistance of the bike to changing orientation about the three axes. This is a minor effect.
5. Weight. Effort required to pick up the bike, hop, etc. Related to #4 and not a big deal when you already have to get your body off the ground and fight the effects of suspension damping.
6. Non-linear effects. Most (intelligent) discussions of mass implicitly assume the effects scale linearly with the relative change. This is not entirely so, but that's a deep dive discussion, so let's treat the effects as linear with respect to the various parameters.
Let's consider hub mass with respect to points 1 - 3, ignoring 4 - 6 because they have minor contributions.
1. 0.1 kg out of a typical total mass of around 100 kg is about 0.1% change.
2. Essentially zero. Taking #1 & #2 together, 0.1% is the upper bound on how much it can slow you down, ignoring all other sources of loss (air resistance, drivetrain friction, tire rolling resistance, tire-to-ground friction, etc.), and that's only when climbing or accelerating. The actual number is closer to 0.05%, only when climbing or accelerating. On a half-hour climb, this puts the maximum possible difference at 1.8 seconds and a more conservative estimate around one second.
3. Difficult to accurately assess. Total mass below the hydraulic suspension is in the range of 4000 – 7000 g, but the tire contributes to the suspension, especially how it relates to rolling resistance. It’s a complex issue and the lack of trustworthy data on real-world rolling resistance makes it difficult to give a complete answer. If we ignore the tires’ suspension and look at the impact only on the hydraulic suspension, 100 g represents an increase of about 1.5% - 2.5%. Again, that’s ignoring the suspension contribution of the tire.
As with any question of weight on the bike, the greatest impact will be when it’s located at the perimeter of the wheels, followed by centrally in the wheels, distantly followed by the suspended portion of the chassis.
This questions was about the impact of hub mass - 100 g, for example.
Considerations related to mass:
1. Linear inertia. All mass on the bike and rider contributes equally.
2. Rotational inertia. Pertains to perimeter wheel mass. Effects are trivial at the centre of the wheel and small to trivial effects at the perimeter of the cranks or elsewhere in the drivetrain.
3. Unsprung mass inertia. Pertains to the "unsuspended" elements, primarily the wheels, brake calipers and rotors, fork lowers, and a portion of the rear triangle and shock.
4. Moment of inertia (vehicle roll, pitch, yaw). Resistance of the bike to changing orientation about the three axes. This is a minor effect.
5. Weight. Effort required to pick up the bike, hop, etc. Related to #4 and not a big deal when you already have to get your body off the ground and fight the effects of suspension damping.
6. Non-linear effects. Most (intelligent) discussions of mass implicitly assume the effects scale linearly with the relative change. This is not entirely so, but that's a deep dive discussion, so let's treat the effects as linear with respect to the various parameters.
Let's consider hub mass with respect to points 1 - 3, ignoring 4 - 6 because they have minor contributions.
1. 0.1 kg out of a typical total mass of around 100 kg is about 0.1% change.
2. Essentially zero. Taking #1 & #2 together, 0.1% is the upper bound on how much it can slow you down, ignoring all other sources of loss (air resistance, drivetrain friction, tire rolling resistance, tire-to-ground friction, etc.), and that's only when climbing or accelerating. The actual number is closer to 0.05%, only when climbing or accelerating. On a half-hour climb, this puts the maximum possible difference at 1.8 seconds and a more conservative estimate around one second.
3. Difficult to accurately assess. Total mass below the hydraulic suspension is in the range of 4000 – 7000 g, but the tire contributes to the suspension, especially how it relates to rolling resistance. It’s a complex issue and the lack of trustworthy data on real-world rolling resistance makes it difficult to give a complete answer. If we ignore the tires’ suspension and look at the impact only on the hydraulic suspension, 100 g represents an increase of about 1.5% - 2.5%. Again, that’s ignoring the suspension contribution of the tire.
As with any question of weight on the bike, the greatest impact will be when it’s located at the perimeter of the wheels, followed by centrally in the wheels, distantly followed by the suspended portion of the chassis.
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Member since Dec 15, 2009
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Only thing i'd mention is the front wheel hub weight % of unsprung mass of the wheel is higher.
So the same weight loss at the front has a bigger impact.
Tyres-rims-cassette-hubs is always my advice for people if wanting to get the most bang for buck in performance improvements.
Makes for a more responsive ride. ????
The differences between tires can be enormous. For example, I'm currently rotating five front tires, four of which are nominally 2.6" (actual range is 2.4" to 2.7") and one is nominally 3" (actually 2.8" with enormous lugs). All have "trail" level casings, and the weight range is 1058 g to 1234 g. Surprisingly, the 2.4" is the heaviest.
If we expanded the selection to include heavier casings, the difference could be a pound per tire, plus an enormous difference in rolling resistance. The weight spread amongst almost any other component type is tiny, compared to tires, plus the rolling resistance issue. Of course, the difference in traction and robustness between tires also varies a great deal.
This is one of the reasons I am kind of surprised about the surge in popularity of tyre inserts that add weight exactly where it's worst for performance.
I've been shocked at the weight differences on a lot of tyres recently, especially some of the Vee rubber ones I tested last year.
I'm a Schwalbe 2.4 man myself. Fat Albert Evo front (nice and light and great grip predictability), and Nobby Nic EVO E50 rear (bit sturder Ebike version, not heavy and great grip, just a bit more difficult to find).
Schwalbe tires seem to have gained weight in their update to the "Super" casing series. For example, the claimed weight of a Magic Mary 2.4" with Super Trail casing is the same as a Kenda Hellkat 2.4" with Kenda's DH casing.
(I do not have a Patreon account.)