Tyre pressures for different width tyres using Silca

[SwiftyDoesIt]

Recently watched a video from NORCAL cycling using SILCA pressure calc to test differing tyre widths over a '"rough" road surface.
5.21 mile course
28mm (actual 28mm) @ 73/75psi
32mm (31mm actual) @ 59/60psi
34mm (33mm actual) @ 53.5/55psi
Watts for riders were 226w and 301w respectively for all rides
I made assumptions that they'd entered into the calc of 48/52 weight distrib, Cat1/2/3 speeds, rough road, high to medium tyre quality and bike+rider weight about 78kg so that I could match the psi that they came out with to use for the 28mm tyre but I could be wrong on that (I was curious as to the rider+ bike weight)

Results they gave were:
Rider 1 at 226watts for each 28/32/34mm: 19.8/20.2/19.9mph
Rider 2 at 301watts for each 28/32/34mm: 22.3/22.9/22.8mph


Their results 'prove' that wider was better, however I'm thinking that the pressures for these lightweight riders (I guessed that rider+bike weight was around 78kg give take) was far too high for the 28mm tyres compared to that used for the wider and the 'rough' pavement.

Reckon if the 28mm were dropped by 9 or 10 psi then the 28mm tyre would be as fast if not faster.
What do folk think, it feels to me that the Silca calc isn't right for the 28mm comparative to the pressures given for the bigger tyres.
Also mentioned in the comments was rim width to tyre width and clearance of the frame influencing things.

Here's the vid for reference

[Jdsk]

Results they gave were:
Rider 1 at 226watts for each 28/32/34mm: 19.8/20.2/19.9mph
Rider 2 at 301watts for each 28/32/34mm: 22.3/22.9/22.8mph

Their results 'prove' that wider was better...

Within-rider those all look about the same to me.

(This sort of study design needs an estimate of within-rider within-width variation in order to show that there's a genuine effect of width.)

Jonathan

[Nearholmer]

I thought that.

Try as they might, in real-world conditions there is so much variability that this doesn’t illustrate any significant difference.

What it might do is illustrate that “thin and hard” isn’t “fastest” in real-world conditions, and if the wider tyre was more comfortable and less-tiring to ride, that might suggest an ability to go quickly for longer on it.

My main ‘takeaway’ was that they have a very strange idea of “rough” - it looked like the sort of surface where you can have a bit of a rest because it’s so lovely and smooth!

[853]

I'd love to have 'rough' roads that smooth.

And how is it possible for a rider to have three rides, all at an average power rating of 226W, whilst the other rider had three rides all at 301W?

[SwiftyDoesIt]

I'd love to have 'rough' roads that smooth.

And how is it possible for a rider to have three rides, all at an average power rating of 226W, whilst the other rider had three rides all at 301W?

I think they ride to an average watts for the ride using their computer/watt meter to maintain it. People who do this regularl seem to be able to keep to within a watt or so on these rides to be able to compare.
Obviousl doesn't take into accout an wind variables or passing motor traffic in either direction which can also influence things as well.

[Brucey]

other suspension systems are characterised by their spring rate and available travel. Using these quantities, it is fairly easy to compare different setups. If bicycle tyres were to be considered in the same kind of a way (perhaps with the addition of 'footprint size', being the contact area between tyre and road) I think it might be easier to understand what is really going on.

[esasjl]

There is a recent publication relevant to this but it's not open access unfortunately.
Cycling on rough roads: a model for resistance and vibration
https://www.tandfonline.com/doi/full/10 ... ccess=true

[Nearholmer]

Looks like an interesting read.

Roughness resistance can be mitigated by reducing the vertical stiffness of the bicycle.

…. Is one of the things it says in the abstract that is visible.

Which I think absolutely anyone who regularly rides off-road knows already through experience. It’s why most off-roaders use fattish through to immense, lowish to very-low pressure, these days mostly tubeless, tyres.

I was watching a video by a bearded US cycle builder and rider of non-suspension off-road bikes, talking about this sort of thing yesterday evening. He went through the various parts of a couple of his personal bikes, looking at stiffness/compliance in key areas, and his big lesson (and this from a frame designer/builder) was that although frame design and material selection, and bars, seat and seat-post, were influential, tyre selection and pressure are utterly dominant.

Those long-go French paysans, on their pneus-ballon were no fools.

[rareposter]

Their results 'prove' that wider was better, however I'm thinking that the pressures for these lightweight riders (I guessed that rider+bike weight was around 78kg give take) was far too high for the 28mm tyres compared to that used for the wider and the 'rough' pavement.

That pressure sounds about right.
It's an interesting video - they've done their best to isolate the external factors and make it all about the tyres and at least it's a controlled test in terms of pressure, they're using the same source and the only thing that has changed is the tyre size. That said, I'm not entirely convinced by those tyre calculators - my view is that at best they give you a ballpark figure to start with and most people, even the most fanatical of perfectionists, are not going to be measuring their tyre pressure to the nearest half a psi.

Obviousl doesn't take into accout an wind variables or passing motor traffic in either direction which can also influence things as well.

Power is a constant measurement. If you're putting 300W through the pedals, that's 300W regardless of anything else. Speed, gradient, heart rate etc can all vary greatly but it doesn't matter if you're doing 8mph up a steep hill in a low gear or if you're pushing on a bit on the flat - so long as the power meter says 300W, that's the power you're putting through the pedals so in terms of effort, it's constant. And on a circuit like that one with undulations etc and possible changes in wind direction, the only way to achieve a constant effort throughout is to ride to power. If you change one variable (in this case the tyre) the more efficient tyre will be faster at the same effort. It also proves that the rider wasn't (consciously or not) "pushing on a bit" or "holding back a bit" which would skew the results.

You could use it in the same way to measure any other single variable. Simon Richardson on GCN YouTube channel does similar tests to measure "old vs new" or "this wheel vs that wheel" tests that they do a fair bit of.

[esasjl]

Those long-go French paysans, on their pneus-ballon were no fools.

Indeed, very little is actually new. It seems sensible to have vertical compliance in the tyres and minimise vibration making its way into the rest of the bicycle. This paper is consistent with physical testing and experience but it was nice to see that dashpots and springs can be combined to produce a plausible result. I guess the equations in this paper could be used to inform tyre choice given known road roughness conditions.
https://deepblue.lib.umich.edu/bitstrea ... /72764.pdf

[Miles_Turner]

Apologies for arriving late. Although the published text of my paper is indeed paywalled (I'm sorry to say), for those interested there is a nearly identical preprint text here: https://www.researchgate.net/publicatio ... _vibration

Of course, I agree that the significance of vertical compliance in this context is widely known at this stage. What the paper offers is a simple formula giving a number that can be compared with aerodynamic drag and conventional rolling resistance. This builds on a result that the road engineers have been establishing for decades, that there exists a (relatively) simple and universal mathematical description of road roughness characterised by a single number, the roughness index. When this is used with a model of a bicyclist incorporating various physiological parameters of the rider, there follows the astonishing result that only the vertical compliance and the roughness index matter. I emphasise "only". Everything else cancels out. This is a surprising result, in my view (to put it very mildly).

I'd also like to comment on the claim above that the tyres always dominate the vertical compliance. IMHO, this is false in general (although frequently true). Tour Magazine sometimes publishes vertical compliance numbers, and some recent frames have a vertical compliance so large that this claim is invalidated. Larger tyres impose an aerodynamic drag penalty, and for some people, this is important. For such people, building the compliance into the frame (including the seat post) is preferable to using wider tyres. Clearly, for many, wider tyres are wanted to improve handling on soft or loose surfaces, and others just don't travel fast enough for aerodynamic drag to be a dominant concern. (I'm in the second category.)

[Jdsk]

...
Although the published text of my paper is indeed paywalled (I'm sorry to say), for those interested there is a nearly identical preprint text here: https://www.researchgate.net/publicatio ... _vibration
...

Thanks

Very interesting.

Jonathan

[Cyclothesist]

Apologies for arriving late. Although the published text of my paper is indeed paywalled (I'm sorry to say), for those interested there is a nearly identical preprint text here: https://www.researchgate.net/publicatio ... _vibration

Of course, I agree that the significance of vertical compliance in this context is widely known at this stage. What the paper offers is a simple formula giving a number that can be compared with aerodynamic drag and conventional rolling resistance. This builds on a result that the road engineers have been establishing for decades, that there exists a (relatively) simple and universal mathematical description of road roughness characterised by a single number, the roughness index. When this is used with a model of a bicyclist incorporating various physiological parameters of the rider, there follows the astonishing result that only the vertical compliance and the roughness index matter. I emphasise "only". Everything else cancels out. This is a surprising result, in my view (to put it very mildly).

I'd also like to comment on the claim above that the tyres always dominate the vertical compliance. IMHO, this is false in general (although frequently true). Tour Magazine sometimes publishes vertical compliance numbers, and some recent frames have a vertical compliance so large that this claim is invalidated. Larger tyres impose an aerodynamic drag penalty, and for some people, this is important. For such people, building the compliance into the frame (including the seat post) is preferable to using wider tyres. Clearly, for many, wider tyres are wanted to improve handling on soft or loose surfaces, and others just don't travel fast enough for aerodynamic drag to be a dominant concern. (I'm in the second category.)

Thanks for the link Miles. Fascinating. Thinking of the contact points I'm not exactly enamoured to read "almost any cycling activity will breach public health guidelines relating to Vibration Dose Value". A potential underlying cause for many cycling related ailments.

[Nearholmer]

Fascinating paper; thanks..

It would be interesting to know how the vibration guidelines were set, and whether riding a bike actually sets-up the same vibrations, in the same bits of the anatomy, as what led to them. I’m thinking of “white finger”, for instance, never having heard of anyone suffer that from cycling, but I don’t know whether there are frequencies, or accelerations, or amplitudes, or durations that set-up trouble elsewhere in the body, or whether anyone has instrumented cyclists sufficiently to measure all the various bits wobbling about(!), because I think we might be a bit more complicated than one mass, one spring, and one dash-pot.

[Brucey]

Apologies for arriving late. Although the published text of my paper is indeed paywalled (I'm sorry to say), for those interested there is a nearly identical preprint text here: https://www.researchgate.net/publicatio ... _vibration

That is very interesting and tallies quite well with what I know of the real world. However in the real world I also note the following (which may or may not be important);

1) real bicycles are not massless (unfortunately)

2) real bicycles (and riders) exhibit very different stiffnesses at the front and rear wheels. A fork of a different stiffness is often one of the few options available to the rider.

3) engineers will soon tell you that any vehicle with suspension is liable to be faster over rough ground for a constant power, In reality however, the power is far from constant, because it is the human machine here. Anyone who has ridden full suspension knows that it doesn't suit everyone, and even those who like it are forced to pedal differently very often. So 'bobbing' (or other losses associated with a 'pulsy' torque delivery) are a real possibility with any suspension system, including fat tyres. Also there might be a substantial difference in the effects of stiffness changes between the saddle and pedals vs elsewhere. Even once you have come to grips with the likely physiological changes relating to power output, there is the no less important matter of 'what is going on between the ears?' to consider also.