32mm tyres on a 13.2mm (internal width) rim?

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deliquium
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32mm tyres on a 13.2mm (internal width) rim?

Postby deliquium » 20 Oct 2017, 10:46am

All my bikes have 17mm internal width rims and are mounted with various tyres which actually measure between 27mm and 32mm. To me that's just fine and dandy - the sweet spot?

However I'm tempted to buy a pair of 27" Velo Orange 'PBP' rims* which measure 13.2mm internally. Ideally I'd like to fit 32mm (1¼") tyres (although 28mm are available from Conti). I have an imaginary feeling/projection :roll: 32mm are going to look rather balloon like and perhaps feel very squidgy/wobbly at the pressures I normally run (~50/60 on 700C and ~65/75 on 20").

The bike in question (to be built up) is a newly acquired 1980 Dawes Super Galaxy and will never be used for touring - just fine weather day ride posing - and I'm a bit of a lightweight :)

Does anyone have any experience of 32mm tyres on narrow rims?

Dear Sheldon touched upon it http://www.sheldonbrown.com/tire-sizing.html#width

*VO suggest up to 32mm tyres.
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Re: 32mm tyres on a 13.2mm (internal width) rim?

Postby Brucey » 20 Oct 2017, 10:53am

I have used a claimed 35mm tyre on such a rim. It came up to 32mm actual size and I found that the thing that limited the use of lower tyre pressures was the lateral instability of the front tyre. You may or may not be troubled in the same way.

VO do another rim called 'Raid' or something that is a touch wider (but not in 27" perhaps). On a Galaxy I'd say it is a more appropriate rim to use, both for the bike and tyres.

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Re: 32mm tyres on a 13.2mm (internal width) rim?

Postby meic » 20 Oct 2017, 10:56am

I have always thought it a bit odd how the width of acceptable tyres is quite a narrow band up until 19mm rims and then it rockets out.
I suspect due to everybody ignoring the recommendations and using fat tyres on these rims, so the advice followed practice.

I used to ride with 28mm on 13mm rims and it was common practice, hard to see how the next size up will be lethal. However why should somebody set off using a combination which is non-ideal when instead they could start off with well matched rims and tyres? Rather than a case of making do with what you have.

Along with the fashion of using wider tyres there is also an upcoming fashion of using wider rims for a tyre than previously was the norm.
I seem to recall Keezx on this forum talking of 17mm rims being his ideal for 23mm tyres!
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deliquium
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Re: 32mm tyres on a 13.2mm (internal width) rim?

Postby deliquium » 20 Oct 2017, 11:47am

Brucey wrote:I have used a claimed 35mm tyre on such a rim. It came up to 32mm actual size and I found that the thing that limited the use of lower tyre pressures was the lateral instability of the front tyre. You may or may not be troubled in the same way.


That's what I wondered about :(

meic wrote:I used to ride with 28mm on 13mm rims and it was common practice, hard to see how the next size up will be lethal. However why should somebody set off using a combination which is non-ideal when instead they could start off with well matched rims and tyres? Rather than a case of making do with what you have.


The possible compromise introduced here is wheel size and brake drop. Which I readily admit could be mostly my imagination :roll:

---------------------------------------------------------------------------------------------------------------------

I have two bikes with the excellent VO 700C 'Raid' rims. They're superb quality, easy to build, with much appreciated deeper braking surfaces than highly polished rims of old and would be my no second thoughts default choice . . .

. . . what got me thinking about the VO 27" (630) 'PBP' rims* are the brake drops on the Dawes - 65mm front and 70mm rear with 700C. The rear being my concern. Happily running Tektro 60-73mm drop R559 dual pivots on a 1948 Percy Stallard 'Montlhéry' (with aforementiond VO 'Raid' rims) - but both front and rear calipers are max 65mm drop - and braking is perfectly satisfactory for my style of riding. I guess I'm 'projecting' (again :roll: ) a 70mm rear brake drop will be noticeably different?

Plus the PBP rims are slightly cheaper and it's re-assuring to see quite a selection of decent 27" tyre availability.

I think though, I'll take both Brucey and meic's advice and ignore the rear 70mm brake drop as it's probably more of an imaginary issue and stick with rims and tyres that match perfectly and have proved so in the real world - thank you both for helping align my thoughts on the matter :)

As mentioned, it's not as if the bike is going to be put anywhere near to its limits and I ain't no dare devil speed merchant who thinks braking is only for wimps :wink:
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Re: 32mm tyres on a 13.2mm (internal width) rim?

Postby Samuel D » 20 Oct 2017, 12:48pm

If you’re going with 700C anyway, there are more choices for the rim, some of which may be cheaper (not that you asked for a cheaper option). For example, the Sun CR18. A bit narrower are the Sun M13 II and Araya RC-540.

Since the force tending to blow the tyre off the rim is proportional to the internal width of the rim, I like a fairly narrow rim. Seems safer to me.

Someday, someone should make a list of all rims on the market in descending order of likeness to the Mavic MA2. I wish I had bought a stack of those cheap, light, durable, polished rims back when they were still available. But I knew next to nothing about rims back then.

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Re: 32mm tyres on a 13.2mm (internal width) rim?

Postby Brucey » 20 Oct 2017, 2:18pm

Samuel D wrote: ...Since the force tending to blow the tyre off the rim is proportional to the internal width of the rim, I like a fairly narrow rim. Seems safer to me...


the tension in the tyre carcass is solely dependant on the carcass width and the tyre pressure (assuming the bias angle is almost constant). That is the main force that is trying to tear the tyre beads from the rim, and to burst the rim wide open; the reaction load due to pressure x exposed rim area is (unless the rim is wider inside than the tyre) somewhat smaller than this, I would have said.

Re +4mm of brake drop; this will lower the brake MA between about 5 and 8 percent, depending on the DP design. If you want to use 27" rims, there are plenty of good super champion rims, weinmann (alesa) rims and rigida rims knocking about. These are not as strong as a modern rim like a 'raid' but they are strong enough and they weigh about 500g, so they are not heavy or anything.

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Re: 32mm tyres on a 13.2mm (internal width) rim?

Postby amediasatex » 20 Oct 2017, 8:09pm

Does anyone have any experience of 32mm tyres on narrow rims?


yes, several thousand miles worth, and about 3000miles on 38mm tyres on Chrinas (13.5mm internal), in summary... "It's fine"

Brucey is right though, the limiting factor is lateral stability on the front, but IME it's not a practical problem for normal riding at normal pressures. It you want to go hooning into downhill switchbacks from 40+mph with the brakes on at <40psi yes you're going to have issues. For normal*, even quite brisk riding at sensible pressures (for me that means 40+psi) you're unlikely to have a problem.

*ie: I've been fine on pootling commutes, fine on long audaxes at 15mph avg, and fine on the occasional 20mph avg. club runs too. I'd not want to have any great load on though but you said you're not intending to tour with them so that's a non-issue.

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Re: 32mm tyres on a 13.2mm (internal width) rim?

Postby Samuel D » 20 Oct 2017, 10:58pm

Brucey wrote:the tension in the tyre carcass is solely dependant on the carcass width and the tyre pressure (assuming the bias angle is almost constant). That is the main force that is trying to tear the tyre beads from the rim, and to burst the rim wide open; the reaction load due to pressure x exposed rim area is (unless the rim is wider inside than the tyre) somewhat smaller than this, I would have said.

Aren’t we at risk of counting the same force twice?

On page 14 of Jobst Brandt’s book it says:

“For clincher tyres (used on most bicycles), air pressure in the tube pushes inward on the bed of the rim while the tire bead pulls outward on the hooked rim edge. These two forces cancel each other exactly; only the effect of cord angle, as in tubular tires, causes a net inward force. That is, the bead does not pull out as much as it would if the cord angle force were absent, so the inflation force is not entirely cancelled by the bead pulling outward.”

I know the bias angle in combination with air pressure tries to shrink the tyre in circumference, which gives the bead less work to do to resist the pressure × exposed rim area force. This circumferential shrinking force must increase with tyre width, assuming the air pressure and bias angle stay the same. I guess the interesting question is which effect matters more for tyre retention as tyre width goes up: (1) the good shrinking force from the bias angle, or (2) the bad casing tension in the hoop-stress orientation.

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Re: 32mm tyres on a 13.2mm (internal width) rim?

Postby Brucey » 20 Oct 2017, 11:41pm

thinking about tubular tyres is not at all helpful in this context. In conventional beaded tyres the carcass tension is trying to tear the rim apart and tear the tyre off the rim. If you do the sums these are by far the largest forces acting on the rim (spoke forces asides).

Practical experience tells you that this is the case too; tyres blow off rims and rims burst from the stresses applied by the tyre, but the pressure loading on the rim well is low enough that even a relatively flimsy rim tape is enough to withstand it over the spoke drillings.

If the pressure force on the rim well were more substantial there would be a more significant drop in spoke tensions when the tyre is inflated. As it is there is some change in spoke tension, but not that much.

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Re: 32mm tyres on a 13.2mm (internal width) rim?

Postby The utility cyclist » 21 Oct 2017, 6:40am

have used 32mm tyre on a Mavic Open 4 (narrower than an Open pro/Open SUP) at 13.1mm, looks a bit bulbous but never had issues even at reasonable speed around tight corners and with a load.
Aesthetically looks a bit pants though on such a narrow rim

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Re: 32mm tyres on a 13.2mm (internal width) rim?

Postby deliquium » 21 Oct 2017, 10:30am

Thanks everyone for the useful information and experiences :D

Just found a pair of little used polished sliver 700C Mavic Module 4 rims in the bike bunker - forgot I still had them from 1990 :roll: Being 18.8mm internal width, they'll be ideal for 32mm tyres and will suit the 1980 Super Galaxy a treat, with Tektro R559 DP brakes (thanks Brucey for the 5-8% reduction in MA figure, differntiating the 27" and 700C 4mm brake drop difference - they should still be better brakes than the old Weinmann's and thier 66mm drop on the rear? )
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Re: 32mm tyres on a 13.2mm (internal width) rim?

Postby Samuel D » 21 Oct 2017, 10:34am

With apologies to deliquium if he has no interest in this…

Brucey wrote:In conventional beaded tyres the carcass tension is trying to tear the rim apart and tear the tyre off the rim. If you do the sums these are by far the largest forces acting on the rim (spoke forces asides).

I did some sums with these assumptions:

  • 25 mm tyre (pretending it’s a cylinder of infinite length rather than a ~2 m torus, but that doesn’t matter much for narrow tyres)
  • about 109 PSI air pressure (0.75 MPa)
  • internal rim width of 13.5 mm (i.e. a narrow rim)
The hoop stress causes each bead to be pulled away from the rim with a force of 187.5 N per 10 mm of length.

The pressure × exposed rim area force is 101.25 N per 10 mm length of tyre, shared by two beads, so about 50 N per 10 mm of bead. This is indeed a good deal less than the hoop-stress effect. But it’s still a significant part of the overall load on the bead and worth reducing with a narrow rim, it seems to me. Or at any rate, if you choose a narrow rim, you get a useful margin of additional safety.

For a 23 mm (or even a 25 mm) tyre on a much wider rim, as is often used nowadays, the pressure × exposed rim area force contributes relatively more to the load on the bead.

I’ve ignored tubular-like constriction from the bias angle here, though I’m not sure I’m right to do that.

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Re: 32mm tyres on a 13.2mm (internal width) rim?

Postby Brucey » 21 Oct 2017, 12:22pm

the numbers sound about right and if you went to a 17mm internal width rim the force would only increase by about 15 N per 10mm length; a little over 5% of the total or something like that. It would of course be a smaller fraction of the whole with a wider tyre.

Re effect of bias angle. The hoop stress in the tyre sidewall remains the same at any given pressure so the tension in the carcass cords varies with bias angle. I think the net tension in the tyre beads isn't affected by changes in the bias angle in the same way as the load in a tubular tyre is, because of the self-contained nature of the hoop stresses in a tubular tyre.

One point that does emerge from this is that there is an effect concerning hiked beads and/or friction in beads. Whilst the bead is unable to move (by virtue of a hook or friction) the pressure load on the tyre (through rim width) is arguably reduced to that which exists because of the gap between the tyre beads not the rim; whilst they cannot move, the tyre beads might as well be part of the rim. However once the bead starts to move, it sees more load due to pressure effects and may run away with itself. This may help to explain why badly fitted tyres don't always fly off the rim immediately, but sometimes blow off minutes or hours after inflation.

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Re: 32mm tyres on a 13.2mm (internal width) rim?

Postby Brucey » 21 Oct 2017, 12:24pm

the numbers sound about right and if you went to a 17mm internal width rim the force would only increase by about 15 N per 10mm length; a little over 5% of the total or something like that. It would of course be a smaller fraction of the whole with a wider tyre.

Re effect of bias angle. The hoop stress in the tyre sidewall remains the same at any given pressure so the tension in the carcass cords varies with bias angle. I think the net tension in the tyre beads isn't affected by changes in the bias angle in the same way as the load in a tubular tyre is, because of the self-contained nature of the hoop stresses in a tubular tyre.

One point that does emerge from this is that there is an effect concerning hooked beads and/or friction in beads. Whilst the bead is unable to move (by virtue of a hook or friction) the pressure load on the tyre (through rim width) is arguably reduced to that which exists because of the gap between the tyre beads not the rim; whilst they cannot move, the tyre beads might as well be part of the rim. However once the bead starts to move, it sees more load due to pressure effects and may run away with itself. This may help to explain why badly fitted tyres don't always fly off the rim immediately, but sometimes blow off minutes or hours after inflation.

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Re: 32mm tyres on a 13.2mm (internal width) rim?

Postby breakwellmz » 23 Oct 2017, 9:40pm

I`ve just put a Vittoria Hyper 37/35(?) onto a front Mavic Module E2 (Green/red label) rimed (36h/LF hub) wheel which i want to put on my recently acquired Bridgestone.
The tyre, inflated up to about 60psi measures in at just under 35mm width; what does anyone think of this combination, am i pushing it? (bike virtually unloaded apart from my near 90kg! :roll: )
I was surprised that the sides of these rims are only just under 2mm thick despite having had use.