Rims.. How wide is it worth going?

[ianganderton]

In this article
http://blog.artscyclery.com/science-behind-the-magic/science-behind-the-magic-wide-mtb-rims/
They have calculated it to be 10% increase in air volume

Therefore by moving from a 20mm id to a 30mm id you could move to a smaller tyre and keep similar volume (with less weight?)

But with added side wall support

Doing what I have done has just given me bigger tyres but I have gained volume and sidewall support

 

[Warp]

In this article
http://blog.artscyclery.com/science-behind-the-magic/science-behind-the-magic-wide-mtb-rims/
They have calculated it to be 10% increase in air volume

Therefore by moving from a 20mm id to a 30mm id you could move to a smaller tyre and keep similar volume (with less weight?)

But with added side wall support

Doing what I have done has just given me bigger tyres but I have gained volume and sidewall support

Only problem with that is that most 2.3" and under tyres come in harder compounds. Yeah, there is increased traction from volume, but there should be some decent grip coming from the compound too, particularly in wet conditions.

 

[mint355]

Does anyone have a list of tyres they've had issues with on wider rims? What rim ID, what volume tyre and what the issue was?

As I've written previously I've not experienced any squaring problems with the 38mm yet, that might very well be due to the tyre choice that matches the wheel to begin with. I did originally think the minion SS squared off a bit, until I saw the same tyre mounted to an ENVE M60. It was squarer, but not dramatically so, and not in a way that has affected performance negatively.

Lets clear something up. So your rims are 38mm ID? You keep refering to your rims as 38mm. Then you want a list stating peoples rim ID?

 

[Zaf]

Light Bicycle 38mm rims, abbreviated to 38mm's. Think ENVE M series just being called by their M#0.

Outer diameter 38mm
Inner Diameter 31.6mm

 

[No Skid Marks]

I wonder what the optimum volume is for tyres by use and by wheel size

I've moved from a 19mm id to a 30mm id rim but I'm still using the same 2.4 inch tyre. The circumference of the tyre and rim has increased by 11mm

Just to be clear.
Tyre diameter probably didnt change much. Tread even less.

 

[Boxer]

Glad others are finding the same, thought I was missing something. I fitted my latest build bike with 27mm ARC wheels and have 2.3 tyres on it. With the verniers can only measure a 1mm increase in tyre width compared to the same tyres on my 24mm rims.....and this increase is in the casing walls - tread width is ezzactly the same.

That said I am running a few less PSI and the tyres feel nice and grippy....so I agree the benefit may be in lower pressure, but I don't see any wider tyre tread benefit

 

[ianganderton]

Important point

Only the internal dimension of the rim matters

External measurements do not affect the tyre shape or volume at all

 

[ianganderton]

Just to be clear.
Tyre diameter probably didnt change much. Tread even less.

This depends where the diameter is measured. Construction has an influence on this

The volume would have changed if the rim id changed

 

[Boxer]

Important point

Only the internal dimension of the rim matters

External measurements do not affect the tyre shape or volume at all

Well that's elementary isn't it? I'm talking in internal width

This depends where the diameter is measured. Construction has an influence on this

The volume would have changed if the rim id changed

Umm ok I'll bite how has your rim circumference changed due to internal width change? And Diameter if measured as the total diameter should not increase with wider rims either - which will be largest at the centre and is the true tyre diameter......I'm assuming you may be referring to the outside edges of the tyre being taller?

But agree the volume has changed

And in all these measurements we are talking poofteenths

 

[ianganderton]

Interested to see what the actual calculations of this were

I've just measured the tyre casing of a 26" 2.4 racing Ralph from the cupboard and its 14.5 cm across

Add to this the 19mm ID of a mavic 719 gives a tyre cross section circumference of 16.4 cm and a rough area of 21.4 cm2

With a 30mm ID rim I'd get a circumference of 17.5 cm and this gives a cross section of 24.37cm2

I make that just over 12% difference in tyre volume

Now it starts to get mathematically super complicated at this point and I don't have the right numbers to hand to work it out properly

But in essence increasing volume means you can run less air pressure but get the same casing tension (what you feel when you squeeze the tyre)

Anyone with the extremes of a road bike, a mountain bike and a fatbike will tell you anecdotal evidence of this

My 23mm tyred road bike feels soft below 100psi where my 2.4" tyred mountain bike feels bullet hard at 50psi and my 4.7" tyred fatbike rock hard at 10psi

Additionally with increased tyre volume you have more air spring to absorb any trail unevenness giving greater compliance and grip

Now I could have achieved this 10% increase on my bike by using 2.5 tyres (guesstimate) on my 719's but they would have been more unstable in corners (think holding a balloon at the knot and wobbling it)

So by increasing the id of my rims by 11mm I've given myself the benefit of an extra 10% air volume but also gained increased sidewall support on the original tyres

Hope this makes sense. If my calculations are wrong then please correct me ASAP

Does anyone have the mathematical know how to work out the real world drop in tyre pressure that an increase in tyre volume of about 10% would give on the same casing tension?

 

[hifiandmtb]

Nicely summarised, Ian. That's pretty much how it works!

 

[EsPeGe]

Interested to see what the actual calculations of this were

I've just measured the tyre casing of a 26" 2.4 racing Ralph from the cupboard and its 14.5 cm across

Add to this the 19mm ID of a mavic 719 gives a tyre cross section circumference of 16.4 cm and a rough area of 21.4 cm2

With a 30mm ID rim I'd get a circumference of 17.5 cm and this gives a cross section of 24.37cm2

I make that just over 12% difference in tyre volume

Now it starts to get mathematically super complicated at this point and I don't have the right numbers to hand to work it out properly

But in essence increasing volume means you can run less air pressure but get the same casing tension (what you feel when you squeeze the tyre)

Anyone with the extremes of a road bike, a mountain bike and a fatbike will tell you anecdotal evidence of this

My 23mm tyred road bike feels soft below 100psi where my 2.4" tyred mountain bike feels bullet hard at 50psi and my 4.7" tyred fatbike rock hard at 10psi

Additionally with increased tyre volume you have more air spring to absorb any trail unevenness giving greater compliance and grip

Now I could have achieved this 10% increase on my bike by using 2.5 tyres (guesstimate) on my 719's but they would have been more unstable in corners (think holding a balloon at the knot and wobbling it)

So by increasing the id of my rims by 11mm I've given myself the benefit of an extra 10% air volume but also gained increased sidewall support on the original tyres

Hope this makes sense. If my calculations are wrong then please correct me ASAP

Does anyone have the mathematical know how to work out the real world drop in tyre pressure that an increase in tyre volume of about 10% would give on the same casing tension?

Nice mate that's a great explanation. Cheers Scott.

 

[The Muss]

35mm is too much

I'm running 35mm wtb rims that came stock on my Process.
Not sure weather I hate them because they are made of butter or that they are too wide.
Either way they ding easy and definitely seem sluggish on the way up. For the down though I do feel like I have the traction of a tractor. So each to their own.

 

[The Reverend]

Great post Ian, thanks for that and earlier ones. It seems to me at least that it's difficult to quantify in numbers but definitely something one can feel on the bike.
As and when I change they'll be quite a bit wider than the Arch Ex's I'm running now.

 

[fragile]

Does anyone have the mathematical know how to work out the real world drop in tyre pressure that an increase in tyre volume of about 10% would give on the same casing tension?

Not too difficult; for the same tyre/casing we can assume that tension is directly proportional to stress and treat the tyre as a thin-walled pressure vessel. The hoop stresses (ie sidewall tension) are then equal to (Pressure * radius) / casing thickness. Since we are conveniently using the same tyre, and aiming for the same stress/sidewall tension, we get the result that the pressure is scales linearly with radius. From this it follows that the change in pressure for a change in volume is proportional to the square root of the change in volume.

Less wordy more numbers, because numbers are good:
Lets take our control tyre and setup to be a real 59mm (around 2.3") in cross sectional diameter, running 30 psi.
The same casing in 19mm would need 59 / 19 * 30 = 93 psi.
In 64mm (2.5"), we can go as low as 59 / 64 * 30 = 27.6 psi
If we want to ride a plus bike (and hopefully we don't, but i won't judge) with a 71mm (2.8") casing we can run as low as 25psi!!!

Going off on a brief tangent, my previous experience (and biases, if I'm honest) made me think that plus bikes are stupid and there is no way you could run 15 psi in a light casing and not have it be a squirmy disaster, but this is the first time I've done the math or thought about tyre dynamics in terms of sidewall stress.

For your specific example of adding 10% volume- works out to be that 110% volume allows for 95% of the pressure. So for every 10psi you used before, you can use 0.5 less.

Of course, this is all idealised and the math gets so very much more complicated as soon as you introduce deflections from loads.

 

[Ivan]

1. No-one should ever (I'm mainly looking at you rim manufacturers!) discuss rim width in external measurements. The internal width is the only one that matters.
2. Wider rims were never about making tyres wider (unless your talking plus sized wheels and tyres). It was always about:
   • Increasing tyre volume which allows for increased contact patch due to lower acceptable pressures (at the same rider weight),and;
   • Moving the sidewall outwards to support the cornering knobs, especially now that the tyre has less pressure.

 

[ianganderton]

1. No-one should ever (I'm mainly looking at you rim manufacturers!) discuss rim width in external measurements. The internal width is the only one that matters.

1. 
   
   Agree
   
   

   [*]Wider rims were never about making tyres wider (unless your talking plus sized wheels and tyres). It was always about:
   

   • Increasing tyre volume which allows for increased contact patch due to lower acceptable pressures (at the same rider weight),and;
   • Moving the sidewall outwards to support the cornering knobs, especially now that the tyre has less pressure.

Isn't it also about moving the support point for the sidewall

 

[Boxer]

Interesting, so riddle me this - we are all running lower pressures on the wider rims (I'm using 20F and 24R on the 27mm ARCs), but as Ian says - casing tension is very much the same as I feel on the 24mm rims at higher pressure.......so whilst there is no argument the volume is increased.. if the casing tension is the same - does the tyre actually deflect anymore and provide any bigger footprint?

 

[redbruce]

Interested to see what the actual calculations of this were

I've just measured the tyre casing of a 26" 2.4 racing Ralph from the cupboard and its 14.5 cm across

Add to this the 19mm ID of a mavic 719 gives a tyre cross section circumference of 16.4 cm and a rough area of 21.4 cm2

With a 30mm ID rim I'd get a circumference of 17.5 cm and this gives a cross section of 24.37cm2

I make that just over 12% difference in tyre volume

Now it starts to get mathematically super complicated at this point and I don't have the right numbers to hand to work it out properly

But in essence increasing volume means you can run less air pressure but get the same casing tension (what you feel when you squeeze the tyre)

Anyone with the extremes of a road bike, a mountain bike and a fatbike will tell you anecdotal evidence of this

My 23mm tyred road bike feels soft below 100psi where my 2.4" tyred mountain bike feels bullet hard at 50psi and my 4.7" tyred fatbike rock hard at 10psi

Additionally with increased tyre volume you have more air spring to absorb any trail unevenness giving greater compliance and grip

Now I could have achieved this 10% increase on my bike by using 2.5 tyres (guesstimate) on my 719's but they would have been more unstable in corners (think holding a balloon at the knot and wobbling it)

So by increasing the id of my rims by 11mm I've given myself the benefit of an extra 10% air volume but also gained increased sidewall support on the original tyres

Hope this makes sense. If my calculations are wrong then please correct me ASAP

Does anyone have the mathematical know how to work out the real world drop in tyre pressure that an increase in tyre volume of about 10% would give on the same casing tension?

I had to do a search for my original post in the American Classic Wide Lightening thread:

My OEM Spesh rims (DT 450SL) are 18.3 internal and the WL's 29.3mm so I was interested to see the effect on my Schwalbe RoRo front (2.25) and RaRa rear (2.25), both snakeskin versions and tubed (as I would initially also run on the AC's for direct comparison). Pressures would be the same also (23psi front, 27psi rear; 95kg smooth xc only rider).

Tyre size:
On the old rims, the 2.25 RoRo measured 54mm across the tread and 53mm at the widest point of the carcas. On the AC's this grew to 56mm across the tread and 58mm across the carcass.
On the old rims, the 2.25 RaRa measured 55mm across the tread and 55mm at the widest point of the carcas. On the AC's this grew to 56mm across the tread and 59mm across the carcass.

The final carcass dimensions for the RoRo on the AC's match the 2.35 version of the tyre (on 18.3mm internal rims), so AC's claim that the wide fitment takes a tyre up one size seems to hold, albeit in volume only as the 2.35 tread width is 60mm (and a bit more aggressive).

Tyre diameter was unchanged (static, no load) between OEM wheel and WideLightning.

I actually recorded the static (unloaded) tyre crown shape as well out of curiosity and there was negligible difference as a function of rim ID for both tyres. In any case the fundamental form is more construction driven and so any change in the static (unloaded) shape is irrelevant to what occurs at the grip (loaded) interface.

Going tubeless made no difference to dimensions, but made noticeable improvement to performance and ride.

Tyre volume is one thing, tyre footprint and carcass stability is another. Pick one (given ID is a constant).

Interesting, so riddle me this - we are all running lower pressures on the wider rims (I'm using 20F and 24R on the 27mm ARCs), but as Ian says - casing tension is very much the same as I feel on the 24mm rims at higher pressure.......so whilst there is no argument the volume is increased.. if the casing tension is the same - does the tyre actually deflect anymore and provide any bigger footprint?

PSI is PSI. If you and bike weight constant, a given PSI will determine the force over a given area for a given tyre size.. Casing tension is a surrogate (it's static not dynamic for starters) and would require a calibrated finger to translate too, what exactly?

The universal gas equation tells us PV=nRT. For the layman, P = pressure and V= volume and in short means PxV equals a constant. Pressure affects footprint (pounds/square inch) and volume affects irregularity compliance. Rim ID influences stability.

What is the performance criteria you feel is critical? Grip (complex mixture of PSI, tyre size, stability (construction and rim ID - given design constant )), bump accommodation (casing volume)?

 

[ianganderton]

Interesting, so riddle me this - we are all running lower pressures on the wider rims (I'm using 20F and 24R on the 27mm ARCs), but as Ian says - casing tension is very much the same as I feel on the 24mm rims at higher pressure.......so whilst there is no argument the volume is increased.. if the casing tension is the same - does the tyre actually deflect anymore and provide any bigger footprint?

Good question and similar to something I've just been pondering to myself while cycling round the park

Here's what I'm thinking at the moment. I could be well off the mark though

Casing tension is a really good measurement of tyre pressure where as the PSI or Bar pressure we all use on a day to day basis is ofter comparing apples and oranges because we are comparing different wheels sizes and different tyre volumes

Casing tension is the universal test we all instinctively use when pumping up tyres, we pinch the sidewalls between our thumb and forefinger. Unfortunately this is difficult to quantify into a number and so we have mistakenly taken to using a pressure gauge (typically on a pump). At least this makes it very repeatable for our own tyres.

Now to your question of footprint

My thoughts are that the air inside a tyre is actually working as a spring. a higher volume tyre at lower pressure gives a different curve to the spring. I think it is much less sensitive to low volume 'intrusions' into the tyre shape. By intrusion I'm thinking of something like a pebble you might roll over. This means you dont feel small variations in trail surface as much and the tyre conforms much better to the shape of the trail which would significantly improve grip.

My understanding is that high volume, low pressure air springs ramp up more quickly with bigger hits so intrusions like square edges etc would be less prone to bottoming out to the rim

So the above would be for similar case tensions across the same tyre on 2 different width rims (eg my 2.4 high roller IIs on my 19mm ID and then on my 30mm ID)

But perhaps with the improved bottoming out resistance combines with the increased resistance to rolling/burping folks are running lower case tensions without running into problems there by further increasing tyre compliance to the trail and increased foot print. Thats a big ramping up in grip

I'm trying to work out how to generate a case tension calculator. We would then be able to see what folks are finding is working well and see what is actually going on in this respect