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Which would pick up speed quicker and carry speed, a lighter aluminium wheelset or a heavier but stiffer carbon wheelset?
Stronger legs
Stronger legs
Where can I buy them?
They're diametrically opposed.
A lighter wheelset picks up speed quicker as there's less inertia to fight, a heavier wheelset maintains momentum better because of the heavier spinning mass (assuming the higher mass is in the rim/spokes), but as it's heavier, takes longer to accelerate due to having greater inertia to fight.
Any aeros to take into account?
My roadie is noticeably faster on its 50mm carbon wheels which are a fair bit heavier than the 'normal' aluminium training wheels.
You can't buy stronger legs, you have to earn them 🙂
I'd go stiff over light: light as a gnat, but wobbly like jelly would be useless - stiff as John Holmes but the weight of Buddha would be a pain to get moving, but marvellous once up to speed.
Rob does physics 
Carbon Wheelsets can be overly stiff...It's a fine balance.
Thats a bit odd to me
Is this a roadie thing? My experience is with MTB wheels
My carbon wheels are wayyy wider than the ally rims they replaced much stiffer and no increase in weight. I can only assume that if i had repalced the alloy wheels with the same width in carbon they would probably have been lighter and stiffer.
Given that i weight nothing and decend like a grandma, i'll take light every time as i have no need for stiff.
An overly stiff wheelset will tend to "ping" its way through a rock garden, constantly knocking you offline, a slightly less stiff wheelset would displace a little more and thus follow the line you were intending.
This is obviously all tyre and pressure dependent ...
While I'm waiting for a pot of soup to simmer...
I'm also a skinny lightweight with grandma's descending speed. I went through a phase of riding light wheels, because I'm a 12stone mincer, I shouldnt be able to break [i]anything[/i].
I thought it was great, and then I tried a pair of heavier wheels (from mavic 317, to mavic 519, slightly bigger tyres too), and more or less everything felt easier, smoother, calmer. Even (especially?) jumping was easier, so that rules out the effect of different tyres - there's no grip in the air.
Light wheels are great, partly (mostly?) because they feel different, but 'different' doesn't necessarily mean 'better'... I can easily believe that wheels can be 'too stiff'
Anyway, dinner's ready...
I'm thinking of a set of DT Swiss EX 1501 wheels which are 30mm internal front and 25mm internal rear (on the set I'm looking at buying). These will be about 1700g compared to a carbon set of rims with 32mm internal width and with Hope Pro 4 hubs, weighing in at approx 1800g.
Just wondering if it's the stiffness that will accelerate and keep the speed to help me make a decision!
I have been working on the stronger legs but after 4 years of riding 3 times a week, I'm as strong as I'm going to be with the time I can offer my riding!
Stiffness in MTB wheels does not (in most cases) = speed...Even in racing, you'll get more speed through compliance/smoothness than through stiffness. What you will get from stiffness is accuracy when cornering.
Bigger tyres on a wider rim will make you faster down and around, but not up. lighter/stiffer wheels will only really help you on the climbs as you'll get better power transfer on a stiffer wheel and faster acceleration on a lighter one.
Bigger tyres = more grip and ability to put more power down, particularly on sketchy uphill terrain on anything but hardpack, provided the legs and lungs can give it, a wider tyre allows you to climb quicker. Gimme a stiffer, wider rim and if happens to be lighter, all the better. A flexy wheel is horrible, push it really hard an it 'gives'.
I doubt the EX1501s would be 1700g, more like 1750g Im guessing, and thats closing the gap between the hope and the EX. Have you considered/Are you going 28h and alu nips on the Hopes? If you did that you'd probably find them a touch lighter than the EXs for the same basic build style. Of course if you've already done either/both of those then ignore that.
i'm an idiot.
mavic 519? - never heard of them.
it was of course 521's, best. rim. ever.
according to the available test data from a number of studies, wheel weight makes very little difference to actual bike speed - its negligible, speed that bicycles travel at is low, accelerations are small and not as constant as many think
perception of course is very different and the "feel" of a wheel can lead one to assume performance advantages that may not actually be realised in the real world
aerodynamics plays a much bigger role in wheel speed.
a slightly heavier wheel that is more aerodynamic than a slightly lighter wheel makes a measurable difference
TBH I'm pretty unfussed about wheel flex or otherwise (my trusty rovals were quite noodly) but I like how a light wheel rides. I'm unfussed as to whether or not the wheel itself is faster- when I'm riding a bike I like the feel of, [i]I[/i] ride faster. I think possibly the human element is going to overrule everything else.
mavic 519? - never heard of them.
Mavic 519's were great rims, loads lighter that 521's, but much stronger that a 517.
I have very light wheels on my 26er XC race bike.
The bike accelerates astonishingly quickly compared to my others (on the same legs). On rockly steep climbs I miss the momentum though.
So I would say it's horses for courses.
according to the available test data from a number of studies,
Road or off? Some twisty XC race courses involve a lot of acceleration and deceleration.
but only over 15mph.a slightly heavier wheel that is more aerodynamic than a slightly lighter wheel makes a measurable difference