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aerodynamic drag is hugely significant, even at relatively low speeds.
on a flat smooth trail, you'd be better of with tiny brompton wheels.
Ah, good point. I was completely ignoring aero, as it didn't seem all that relevant - the differences will be tiny compared to other factors
Depends how fast the rider is and what they're riding (the fireroads for example) maybe the pros should be on 26"? 🙂
Right, as an engineer who has done modelling of things, I'll weigh in with my serious opinion. Yes I think it would be possible to do, but it would be a lot of work to do properly - back of a fag packet stuff would be pretty much worthless. You would also need to do some real world testing of rolling resistance.
At which point you may as well just do proper testing rather than the maths.
[quote=ahwiles ]aerodynamic drag is hugely significant, even at relatively low speeds.
on a flat smooth trail, you'd be better of with tiny brompton wheels.
Maybe, but unless what you're actually interested is a flat smooth trail, then you're better off looking at other factors - aero drag certainly isn't a significant factor at MTB climbing speeds, whilst wheel rolling resistance is.
aracer - Member
Right, as an engineer who has done modelling of things
And that was the best you could come up with. Having an off day? 😉
ahwiles - Memberon flat ground, smaller wheels are faster - they're more 'aero'
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ahwiles - Memberaerodynamic drag is hugely significant, even at relatively low speeds.
At mtb speeds, with mtb tyre drag? Are xc racers using aero aids?
What about the DH and flats, aracer?
[quote=nemesis ]At which point you may as well just do proper testing rather than the maths.
No, not really, because as discussed there are too many variables which need to be eliminated in real world testing - even if you're taking my e-bike suggestion, just getting equal power outputs with that would be enough of a challenge.
Absolutely but then the same applies to trying to come up with a mathematical model of everything (or even just some stuff).
🙂
I think we're all agreeing FWIW...
[quote=nemesis ]What about the DH and flats, aracer?
Aero might be an issue, but the aero differences between different wheel sizes won't be (the wheels themselves are a small component of drag, the difference between wheel sizes even smaller).
Of course, the point isn't really that, it's that there are so many variables that in themselves are affected by other variables which come from whatever model you choose.
[quote=nemesis ]Absolutely but then the same applies to trying to come up with a mathematical model of everything (or even just some stuff).
The difference being that you can keep the variables constant when doing mathematical modelling. Sure there's lots of stuff you have to define, but there are good reasons people do mathematical modelling rather than just real world testing everything.
Are xc racers using aero aids?
Not so much on the bike, but lots of pros wearing skin suits and most seem to be adopting a more aero position on the bike these days too. Lots of aero tucking in DH too, and they'd all be wearing skin suits if they weren't banned.
Of course there are - you don't get muddy for a start 🙂 There are also situations where modelling just isn't that useful as there are too many variables. Designing a carbon fibre frame from a strength/etc perspective is great for modelling - generally the parameters are fairly well defined. Designing a frame to 'handle well' is not a great use of modelling.
Given the debate on here about what consititutes proper riding, good handling, goov bikes, whatever, it's clearly a minefield and that doesn't even start to consider the differences between people, conditions and so on.
just checking, you do realise that when we talk about a 'model' we're not talking about a CAD model, right?
No, I'm talking about a vehicle dynamics model like we use in F1 (notice the we).
The issue with this model is that in order to be accurate it will have to be complex. And that will require the modeller to make a lot of assumptions if they don't want to spend years testing and working out the individual effects of numerous small parameters first.
As with any simulation, it's output is only useful once correlated with the real world, up to that point it's just maths. For example 2+2 = 4 as everyone knows, but 1+2 = 3. Both are mathematically correct, but which formula represents best the real world? (you'd have to work that out for your particular case)
We do a lot of simulation work in the automotive/race world i work in, and it's all meaningless without correlation.
So, you'd have to test wheels for rolling resistance vs normal load, tyre type and pressure and bike geometery / set up on numerous types of terrain, then do the same for bike of different masses distributed in different places just to be able to say that "once a smooth surface gradient exceeds X, you are better off with a lighter 26" wheel" and "once a rough surface gradient exceeds Y you are better off with a 26" wheel" etc etc
Not worth the effort, just ride your bike and enjoy it!
That tells you very little about the actual trail though.
It would give you the macro terrain, which would be fairly important in assessing the effect of rider weight. I doubt that the micro "bumpiness" of a typical UK trail would make much difference when looking at the OP's question.
Isn't the micro "bumpiness" where the advantage of a 29eris assumed to lie though?
But for example is a trail with medium size roughness the same as a smooth trail with rocks of size y at x spacing?
@maxtorque - I don't think it would have to be that complicated a model to answer the OP's specific question. A lot of the variables (in fact all of them except wheel size) would be identical.
Isn't the micro "bumpiness" where the advantage of a 29eris assumed to lie though?
Not just that, they tend to ride faster on smooth trails too. But you could assess the bump rollover performance from a pretty simple square edged or ramped bump input on your trail. Again not that hard to model.
I'm with moshi here. It would be complicated, but definitely doable - they model more complex stuff.
But for example is a trail with medium size roughness the same as a smooth trail with rocks of size y at x spacing?
No, which is why you would need to develop a dynamic bike model to test different trails. The former doesn't appear to exist in the public domain, so it's a mute point.
OK, so a reminder of the actual question:
How many pounds lighter would a 26er need to be to offset the increased "rollability" of the same frame with 29" wheels?
So while we can come up with answers, there'll be as many answers as there are combinations of rider, terrain surface, elevation/twistiness profile, etc.
So yes, we can come up with an answer but does it really mean anything useful? IMO, no, not given the variability of what mtbers actually ride.
PS MOOT point 🙂
on a flat smooth trail, you'd be better of with tiny brompton wheels.
Road and Velodrome bikes tend not to have small wheels, so I reckon you are wrong there.
they got banned.
UCI reg's innit bro: 700c wheels or it doesn't count.
here you go:
[img] 
[/img]
photo taken roughly 5000 years ago, when innovation was still allowed.
although, it should be said, i think these bikes are Moultons - made in Bradford on Avon - where me Ma' was born.
(not Bromptons)
So while we can come up with answers, there'll be as many answers as there are combinations of rider, terrain surface, elevation/twistiness profile, etc.
That's true and why modelling is useful. Back to the F1 analogy, you have a vehicle model and lots of different tracks to get specific answers for specific tracks. But you also usually find common trends as you may well with 26 v 29.
I suspect 29 is theoretically faster than 26 for most real trails hence why XC racers are all on 29. When the trails get seriously gnarly it seems to swing in favour of the smaller wheels, but not sure for how long? In theory I would have thought 29ers should be killing the DH too, but maybe the practicalities e.g. wheel stiffness have not got there yet. Plus the current crop of DH riders are used to 26 of course.
photo taken roughly 5000 years ago, when innovation was still allowed.
Very interesting, but was it actually any good? Doesn't make sense to me as an engineer. If I was designing a velodrome bike without regulations I'd be looking at larger wheels, not smaller.
Edit: I guess I can see the advantage if you were sprinting from a low speed at the end
but larger wheels have a bigger frontal area, and that means more drag - and on a flat smooth track, drag is everything.
but was it actually any good?
yes, very, that's why they got banned, as opposed to simply being allowed to die out as a failed experiment.
How many pounds lighter would a 26er need to be to offset the increased "rollability" of the same frame with 29" wheels?
Obviously more than anyone (including world champions factory teams) is willing to spend to achieve.
End of.
going back to the e-bikes for a second...
This could actually give a fairly interesting opportunity for some real world testing.
26er and 29er versions of same bike (ie same material, as near as possible geometrically given the wheel sizes, ie: same intended use), standardise the tyres/pressures, if they are [i]entirely [/i]motor driven*, no pedals, and then use a large sample of test riders to go round the test loops numerous times on each bike and under different conditions.
This should give a big sample spread of rider ability, conditions, weights etc. and if no pedal input allowed you have a relatively easy way to measure and report on energy usage.
You could then start to vary individual elements, like tyre pressures, tread, biek weight, rider weight etc.
The key to it would be using a large enough sample size of riders and test tracks, but I bet you could build up a very interesting set of data.**
Would hopefully give you some indication of power output differences and energy expenditure but in close to real world riding conditions with real riders, but removing the variability of the human engine.
*might have to think carefully about things like a max instantaeous power cap, and possibly max duration at certain power to stop people literally just opening the throttle, but its all in the details.
**still might not prove anything but would be interesting either way!
but larger wheels have a bigger frontal area, and that means more drag - and on a flat smooth track, drag is everything.
I get that, but the total frontal area of the bike/rider doesn't look any different to me. I would have thought the pros of a small wheel are:-
low mass
low inertia - easier to accelerate
The smaller velodrome wheels' advantage wasn't actually the wheels. It was that you could ride much closer to the bike in front which reduced drag - so funnily enough, it was smaller wheels to allow the main aero drag generator (the riders) to generate less drag...
It should be added that in the pic, they aren't really making much use of it though!
How about we just arrange to get two 2014 Specialized Enduros, in 29er and 26er flavour and attach some gps gear to them before sending them around a short circuit. After the first lap you swap the riders around and then do two more laps with another two riders with a half time swap each?
It's scientific, innit...
And to F1, I don't think it's a very good analogy. F1 is actually a much simpler model despite the many tracks, etc. A closer equivalent would be rallying but with bigger jumps, logs rocks, etc
Come to think of it, that could be pretty thrilling! 🙂
Just to throw it into the mix I was curious so did some Strava segment comparisons in recent weeks - 'tested' my new 23 lb 120/100mm carbon full sus 29'er versus the times from my previous 21 lb 120mm carbon 26 HT.
The segment conditions were pretty much identical even though on different days - a local dry, dusty wooded and slightly rooty 1.5km singletrack section with a gentle rise and fall and 17m altitude difference. Legs were pretty fresh on both days and I managed to give both efforts what felt like 100% gas with no interuptions.
29'er - 3mins 08 secs - ave speed 29.5km/h
26 - 3mins 26 secs - ave speed 26.9km/h
Make of that what you will. 🙂
If you trust strava to be accurate to any usable degree, you may well be able to prove that a rigid shopper is faster that a top of the range bike 🙂
Make of that what you will
full sus is quicker?
the course favours one bike over the other?
you were less tired on one day than the other?
this is the problem with limited sample tests...
You'd need to do back to back, same day, and multi day, multiple runs, multiple riders etc before you start to get any thing that resembles data rather than anecdote.
FWIW, my fastest ever times on my local loops have been clocked up on the bike I would least expect to be fastest.
How about we just arrange to get two 2014 Specialized Enduros, in 29er and 26er flavour and attach some gps gear to them before sending them around a short circuit. After the first lap you swap the riders around and then do two more laps with another two riders with a half time swap each?
As Al alluded to earlier Matt Page did just that with 3 Focus bikes a couple of years ago, complete with power meters.
^ Njee, and that test was actually really interesting, would be waaaay better done with a lot more runs, and more riders, and on more trails though.
It's also still very hard to deal with the variable power aspect of the human though, even with a power meter and good self control, lots more you could do, but where do you stop in tests like this...
I don't think you could rule out rider input\feedback or normalise things like tyres pressures. The bikes interact with the riders in different ways and would effect the energy input as would what tyres and pressures complement the different wheel sizes.
This year I thought I'd get a 29er and see for myself the difference.
My unscientific thoughts...
I found I was able to lower pressures before feeling roots etc (rolling over easier rather than compressing on the square edge hits sooner?). Use tyres with a lower tread height for lower rolling resistance but with the longer contact patch and lower pressure I found I had the same grip with a less aggressive tread. A change in technique helped the issue of tight corners and accelerating out of them. Accelerating from a stop involved slightly more effort until somewhere between 5-10mph then the larger wheels gained momentum faster and retained more speed with the same effort as the surface got worse.
Overall I've found the 29er quicker as whole (on or off road) and more comfortable and less fatiguing on long rides (rider fatigue\comfort has to effect energy expenditure?, we are the power source after all).
The bike is great fun, which is the most important bit 🙂
Just to poop on the science, it just depends on what bike\wheels work for the rider\trail.
By the time the 26er lost enough weight it would be so fragile and skittish it would be unusable IMHO.
The doubters always harp on about wheel weight and the increased effort to accelerate them, but in my experience the 29er corners better on the same tyres than the 26er, thus needing less accelerating to be done post corner. Then the lower rolling resistance kicks in and bingo, Kryton buys a new bike.
I suspect 29 is theoretically faster than 26 for most real trails hence why XC racers are all on 29. When the trails get seriously gnarly it seems to swing in favour of the smaller wheels, but not sure for how long? In theory I would have thought 29ers should be killing the DH too, but maybe the practicalities e.g. wheel stiffness have not got there yet. Plus the current crop of DH riders are used to 26 of course.
As travel increases so does stability which is the main benefit of 29". The argument is 180mm plus bikes don't need the added stability of 29" BB drop. However, there is also lower rolling resistance of 29" so I think the wheelsize has huge untapped potential. Even bikes like the E29/TF01 are compromised to appease traditional views.