Rims.. How wide is it worth going?

[Ivan]

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

That's what I said.

 

[ianganderton]

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).

The circumference must have changed though, by 11mm

And if the circumference has changed then the average diameter has too, they are inextricably linked. Its just it might not be very easy to measure

On the tyres I measured the 19mm ID gave a calculated circumference of 16.4cm. This gives a calculated diameter of 5.22cm. The 30mm ID rim has a calculated circumfrence of 17.5cm giving a calculated diameter of 5.57cm. An calculated average difference of 3.5mm. Where that would appear on the tyre I dont know, what I am sure of is that something there abouts is there somewhere.

Again anyone please check and correct my calculations if I've made a mistake. I'm one of the least educated people I know :happy:

 

[redbruce]

The circumference must have changed though, by 11mm

And if the circumference has changed then the average diameter has too, they are inextricably linked. Its just it might not be very easy to measure

On the tyres I measured the 19mm ID gave a calculated circumference of 16.4cm. This gives a calculated diameter of 5.22cm. The 30mm ID rim has a calculated circumfrence of 17.5cm giving a calculated diameter of 5.57cm. An calculated average difference of 3.5mm. Where that would appear on the tyre I don't know, what I am sure of is that something there abouts is there somewhere.

No, as my measurements indicate, the base of support just widened. Tyres are really pretty flexible in the width department, not so in the overall tyre circumfrential department..

There is numerous web data that explains how circumference doesn't (can't) change due to tyre (con)structural considerations.

My detailed measurements (habits of a lifetime as a professional scientist) also support this.

But, as such I'm happy to have challenge. I do need evidence though.

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.

Ivan gets it.

I would add, as twentnineinches.com has stated, all things equal allows you to run a narrower tyre for most applications, hence lower weight/specified performance level.

 

[ianganderton]

Not challenging just discussing to ensure I understand properly

For my purpose I wanted to work out air volume so I measured the internal cross section of the tyre like so

I can then take the ID of the rim and that gives me a 'good enough for my purpose' circumference of the tyres cross section to work out area and volume

The problem with measuring the external carcass is the rubber on the outside of the casing has a lot of shape to it

I worked things out from this blog post http://flocycling.blogspot.com.au/2014/11/flo-cycling-why-do-you-use-less-tire.html

It has some nice clear pics

So if you are using the same tyre on a wider rim then I can't see any way that the internal circumference can't have increased and as the pressure in the tyre (unweighted of course) will make the overall internal shape more or less round then with increased circumference comes increased diameter

 

[redbruce]

So if you are using the same tyre on a wider rim then I can't see any way that the internal circumference can't have increased and as the pressure in the tyre (unweighted of course) will make the overall internal shape more or less round then with increased circumference comes increased diameter

Yep, tyre fundamental dimensions are fixed so circumference change is function of rim ID (variable), just don't over think the rest of it.

 

[ianganderton]

Yep, just don't over think the rest of it.

So the diameter diameter had changed on your tyres?

Not quite sure what you mean

 

[redbruce]

So the diameter diameter had changed on your tyres?

Not quite sure what you mean

Tyre OD has not changed. Tyre construction dictates (or constrains) that (tyre size constant).

Effective radial circumference (ie cross section) has changed because rim ID (ie variable) has changed as you are adding additional arc to the circumference through additional rim ID.

Anyway for this level of detail and insight, Rotorburn is a poor choice and there is a shitload of Quantitative info on this subject available on the web anyway.

http://engineerstalk.mavic.com/the-right-tyre-width-on-the-right-rim-width-2/
http://www.pinkbike.com/news/Tech-Tuesday--Wider-Rims-Are-Better-and-Why-Tubeless-Tires-Burp-.html
http://blog.artscyclery.com/science-behind-the-magic/science-behind-the-magic-wide-mtb-rims/
http://www.opencycle.com/blogs/the-effect-of-wider-rims-on-tire-width
http://www.pinkbike.com/news/To-the-Point-Bill-Shook-on-Wheel-Building-and-Wide-Rims.html
http://www.schwalbetires.com/tech_info/tire_dimensions

 

[ianganderton]

OD has not changed. Tyre construction dictates (or constrains) that (tyre size constant).

Effective circumference has changed because rim ID (ie variable) has changed as you are adding additional arc to the circumference through additional rim ID.

OK, I disagree with this but its irrelevant to the discussion anyway so no worries. Just wanted to make sure I wasnt missing anything

 

[redbruce]

OK, I disagree with this but its irrelevant to the discussion anyway so no worries. Just wanted to make sure I wasnt missing anything

You are entitled to you opinion, but where is the evidence to support it?

Also hopefully we are actually not at odds over terms (or semantics) here.

I have stated the overall OD of the tyre does not change (within reasonable limits, eg 18.3->29.3mm rim ID, my only reference). I have stated the radial, or cross sectional area does change (and have edited post #65 to be, more explicit).

The measurements stated indicate no change in overall tyre OD. A reasonable web search supports this observation. Those measurements also confirm the increase in cross sectional area, and therefore volume of the tyre.

It is relevant to the increase in "bag" of the tyre. If you add to the arc of the tyre from the rim ID (ie, a component of the effective tyre radial circumference) that adds to the tyre x sectional dimension.

Assuming your point of contention is overall rolling OD, in any case given the tyre pressures typically run my MTB riders, any minor change in static overall OD is irrelevant as it is the loaded (dynamic) OD that is relevant an differences will be even smaller.

Assuming we are actually talking about radial circumference and therefore tyre volume, then we agree. If overall wheel and tyre circumference, then not.

 

[Boxer]

I think redbruce is taking Ians reference of 'circumference increase' to be that of the overall rolling circumference of the tyre. Whereas he is actually referring to the cross-sectional circumference cut through the rim/tyre combination.

I did the same thing a couple pages back and wondered what Ian was smoking to say the overall diameter and circumference of the rolling tyre had changed....then realised what he meant (and calcs look sound)

 

[ianganderton]

I think redbruce is taking Ians reference of 'circumference increase' to be that of the overall rolling circumference of the tyre. Whereas he is actually referring to the cross-sectional circumference cut through the rim/tyre combination.

This!!!

Literally just woke up and realised which diameter!!

If been so focused on cross sectional dimensions when thinking about calculating volume that I completely forgot there is another diameter and circumference in a wheel

DOH!

Sorry redbruce, I'm catching up now

And it's caused me to realise something else to

Because I was focused on the cross sectional diameter etc I was thinking of the internal carcass and rim forming circles but of course they don't necessarily do they!!!

Here's what I've been thinking.

I've recently noticed that Maxxis state on the side of their tyres rim ID they are designed to work with. On my high roller 2s it's 23mm I think from memory

Maxxis now do WT versions of a couple of their tyres and these are designed for wider rim IDs but I can't remember the actual mm off the top of my head

The chat I've read about these has all been about the tread pattern and how much it comes down the sides blah blah blah.

But I don't think this is only part of the story. I think they will have significantly changed the internal carcass shape too

When looking at the diagrams showing carcass tension it was showing nice even force arrows around a circle. Circles are perfect for even force distribution. But the forces in cycling aren't even. I wonder what shape Maxxis aim for in the carcass to balance out the varying forces against the carcass tensions

To make things easy let's assume a circular internal cross section is ideal. They will have aimed to have achieved that on a 23 mm id rim

Based in the fact that the overall wheel circumference doesn't chance this will cause some interesting shapes with other rim IDs

On my 19mm id rims they will be taller, more egg like in profile I'm thinking

On wide rims they will be more car tyre shaped, much flatter

But this is the shape of the internal carcass. On top of the carcass there is a rubber moulding with knobbles etc. This will only be as designed on the 23mm

I can now see why the cross sectional profile of the tyre can potentially change so much. Earlier today I was looking at 2 slightly different circumference circles (164mm and 175mm) and thinking that's not much if any difference in profile

But now I've factored in the squashing effect the fixed overall tyre diameter causes I can see it would be much more pronounced

A real lightbulb moment

If someone need to visualise this they could cut some strips of paper to 145mm long and as wide as the sellotape in their kitchen drawer

Then sellotape the ends of the various strips with a gap relating to various rim IDs. The curve of the paper will represent the cross section of the internal tyre carcass. Make sure you don't crease the paper so it curves nicely like a tyre carcass under pressure. I'd suggest 19mm, 23mm, 30mm and 35mm

Putting them between 2 parallel kitchen chopping boards, sellotape flat against one edge, would represent the fixed overall wheel circumference. Optimise the shape of the pieces of paper for the 23mm ID strip just like the Maxxis designers do. I think this would give an easy to see difference in the cross sectional shape of the carcass of a standard 2.4 Mtb tyre with different rim widths

 

[No Skid Marks]

Dimension/width of tyre will still change only a negligable amount. Tread width even less, but tread will be flatter. You are increasing air volume with wider rim. Overall width won't change much and is pretty much irrelevant anyway. The main things a wider rim will change is stiffer more stable side wall, stiffer footing as tyre beads are further apart, flatter tread pattern, potential for lower pressures, less pinch flats, less burping, less chance of rolling tyre off rim. That's about all I can think of precoffee.
This is all being taken too seriously and there's some serious semantics issues.
The most important things to consider by going wider rim are tread deformity and the bennefits mentioned above. Tyre deformative is the only negative to wider rims IMO any other negatives are far outweighed by the benefits. All within reason.

 

[Jeeves]

No Skid Marks has summed it up pretty good.

Check out these links to what ENVE Composites engineers found.



http://enve.com/journal/is-wider-always-better/

http://www.pinkbike.com/video/415665/

 

[Capt.Gumby]

I am running 28mm ID and constantly seem to flat spot my rims..... on my second rim now!

Running Maxxis High Roller 2 EXO TR, tyre pressure at 32psi, set up tubeless, shock sag set at 30% and doesn't bottom out on big hits. Had the same setup on narrower rims and never experienced this.

 

[Scotty99]

Wow, mine my rims are only 22.5 ID, that seems like a big difference...

Anyway, haven't damaged the rims yet and i only run around 22psi

 

[No Skid Marks]

I am running 28mm ID and constantly seem to flat spot my rims..... on my second rim now!

Running Maxxis High Roller 2 EXO TR, tyre pressure at 32psi, set up tubeless, shock sag set at 30% and doesn't bottom out on big hits. Had the same setup on narrower rims and never experienced this.

It would be the rims I'd imagine. What brands/models were they both?

 

[Scotty99]

.

My rims are only 22.7 ID, big difference but no problems yet even at 22psi tyre pressure

 

[spoozbucket]

I am running 28mm ID and constantly seem to flat spot my rims..... on my second rim now!

Running Maxxis High Roller 2 EXO TR, tyre pressure at 32psi, set up tubeless, shock sag set at 30% and doesn't bottom out on big hits. Had the same setup on narrower rims and never experienced this.

Yup, the rim is wider, it will hit more rocks and be more likely to slice the tyre than a narrower rim..
People also want wide and light so you have to accept that if you went that way you'll get more digs or cracks.

 

[bowtajzane]

im no expert but 32psi seems hi for a tubeless setup........
+1 on what N.S.M said......
also,are you tough on gear/ride very rocky trails.

 

[The Reverend]

im no expert but 32psi seems hi for a tubeless setup........
+1 on what N.S.M said......
also,are you tough on gear/ride very rocky trails.

I'd agree too. From what I've now read on this I believ that;
Rims, riding style, location, rider weight are all a factor there.
How much lower / different are your tyre pressures now compared to the previous rims?