Define the virtual trail for us

A topographical map of a typical trail centre should be okay e.g. Afan Skyline

Once you have your bike model sorted, the trail would be a relatively easy parameter to change. You could run multiple trail maps as they do in F1.

A topographical map of a typical trail centre should be okay e.g. Afan Skyline

That tells you very little about the actual trail though.

moshimonster – Member

Define the virtual trail for us

A topographical map of a typical trail centre should be okay e.g. Afan Skyline

Once you have your bike model sorted, the trail would be a relatively easy parameter to change. You could run multiple trail maps as they do in F1.

just checking, you do realise that when we talk about a ‘model’ we’re not talking about a CAD model, right?

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.

Is that typical riding? Fireroad climbs, singletrack descents?

And the surface is generally pretty smooth at trail centres, with roots/rocks then thrown in. How does that compare to mud on natural trails?

I’m not trying to be awkward, just saying that there are so many variables that aren’t clearly defineable (in a way that people will agree on) meaning that the results are pretty meaningless beyond a bit of a mathematical exercise.

This is why F1 teams invest loads of money on computers but still have to do testing as reality often doesn’t match.

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.

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 – Member

on flat ground, smaller wheels are faster – they’re more ‘aero’
…
ahwiles – Member

aerodynamic 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?

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…

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.

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:

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 entirely 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…