Need precise OD measurement of Rat Trap Pass tire urgently

[John Clay]

All,

Can someone provide me with an accurate measurement of the outside diameter of a mounted, fully inflated Rat Trap Pass tire? A measurement I can believe in, say within a mm or two, not just draping a tape across the thing but by standing it up on a hard, flat surface, against a wall and using a square to make a light mark on the wall, and measuring that distance. I ask because I've started a frame, won't have the tires for a little while and just discovered that there is a 10mm difference between the actual and calculated ODs of my Baby Shoe Pass tires. I can't build to such a disparity. The two tubes I've fitted up are OK regardless, but I can't fit any more without a reliable plus/minus 1 or 2 mm OD measurement.

Many thanks in advance to anyone who can provide the information.

John Clay

Tallahassee, FL

[DarinM]

I could get a measurement to you tonight. They are standard casing, 2-3 years old (non-TC), have probably 5-600 miles on them and I have them mounted on a 21mm inner width rim. All of those things could add up to a difference of 2mm or more compared to your actual dimensions. 

How long is "a while?" I would hate to provide a measurement that was wrong resulting in a frame that is not to your satisfaction (unless you then wanted to sell me said frame as it would fit my tires perfectly!). You might be best off waiting for your tires so they can be measured on your rims instead of relying on someone else for such a precise measurement.

Darin

[Hunter Ellis]

Depends on the rim width.

But I have it on good authority that a recent EL batch runs 55.3mm on dually rims

-H

On Tuesday, July 17, 2018 at 2:26:40 PM UTC-7, John Clay wrote:

[Reed Kennedy]

On Tue, Jul 17, 2018 at 2:26 PM John Clay <nice.c...@gmail.com> wrote:

...just discovered that there is a 10mm difference between the actual and calculated ODs of my Baby Shoe Pass tires.

Hey John, sorry to side-track, but could you tell us more about the Babyshoe Pass measurements?

I've run BSPs (both the new tubeless compatible version and older non-TC) on several rims (VO Diagonale, A23, HED Belgium+) set up both with tubes and tubeless, pressures from 60psi down to 30psi, and they've always come in between 40 mm and 43 mm measured carefully with my Mitutoyo calipers following Jan's instructions. The vast majority of my measurements came in between 41mm and 42mm. 

These are all relatively wider rims, so I can imagine the BSP measuring a bit less on a narrow rim, but a full 10mm difference surprises the heck out of me! 

Best,

Reed

[DarinM]

John is asking for OD measurements of the tire mounted on a rim, not the width of said tire.

Darin

[Hunter Ellis]

Oohhhhhhh, doh!

(that info I don't have)

Hunter Ellis

Cell: 707-583-6583

On Tue, Jul 17, 2018 at 3:13 PM, DarinM <darin...@gmail.com> wrote:

John is asking for OD measurements of the tire mounted on a rim, not the width of said tire.

Darin

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[John Clay]

Hi Darin,

I haven't ordered them yet! I find myself with the next few days available AND a high level of motivation (sometimes hard to muster in the hot & humid tropics). I'd trust your careful measurements so if it isn't too much trouble then it would help me get down the road. It's an old Mavic something or other; I'll check the inner width to make sure it's close to what you have. And I might delay the down tube fit-up. The front wheel is in SF (Middle Richmond) at my sis in law's place so I need to get her to send it to me too.

Reed,

My BSP measures 38 actual, so 38*2 + 584 = 660. Actual OD is 669. Until today I'd based the calculated OD on 42*2 + 584 = 668. Calculated the same way and the RTP came out to be 54*2 + 559 = 667 so, having not messed with my main frame fixture since removing the BSP frame I just used it as it was for the RTP one. Today I thought I ought to verify that calculated was close to actual. It wasn't so I hit the brakes after fitting the TT 'twixt the HT and ST.

Thanks to everybody!
John

[John Clay]

Eye candy...and if Compass has changed the mold or fabrication this could work well out for Darin. But I sure hope not. BTW, what is non-TC....oh, not tubeless compatible. 

Guess I'll go ahead and order but I could still make progress with the measurement if it's no trouble. Don't do it if it's a nosebleed.

I am very stoked to be building this!

Thanks much!

John

--

John

[Reed Kennedy]

On Tue, Jul 17, 2018 at 4:01 PM John Clay <nice.c...@gmail.com> wrote:

My BSP measures 38 actual, so 38*2 + 584 = 660. Actual OD is 669. Until today I'd based the calculated OD on 42*2 + 584 = 668. Calculated the same way and the RTP came out to be 54*2 + 559 = 667 so, having not messed with my main frame fixture since removing the BSP frame I just used it as it was for the RTP one. Today I thought I ought to verify that calculated was close to actual. It wasn't so I hit the brakes after fitting the TT 'twixt the HT and ST.

Ah! The total outer diameter of the whole tire-mounted-on-rim combo. Gotcha. I somehow first thought you were looking at width and then thought you were asking about the OD of the cross section of the tire (which I couldn't figure out how to measure reasonably). Just goes to show I should read more carefully! I've not got a RTP bike, so I'm of no help though. (Maybe next bike...)

Thanks for sharing the in-progress pictures, by the way! Looks like tremendous fun.

Best,

Reed

[DarinM]

Ha! Seeing the swoopy fork blades seals it, I will send you my measurements soon and hope for the worst (kiddding of course).

Darin

[DarinM]

Ok John, I took four measurements and came up with an average of 674.75mm for a set that measures 53mm wide. Using your theoretical method, the wheel's OD should be 665mm, so a very similar error to your wheels. What width are the rims with your BSPs? It seems obvious now, but I'm thinking that a narrower rim which forces a narrower tire likely results in a greater overall wheel OD. That rubber needs to go somewhere! I ran RTPs on a set of 17mm inside width rims and they measured 49mm if I remember correctly, so would those wheel's OD be greater than 674.75mm?

Anyways, hope this helps, or at least doesn't hurt.

Darin

[satanas]

Hmm. I've seen it written, and believe it to be true, that tyre circumference and thus diameter are NOT going to change with rim width. Instead, what happens is that the tyre's sidewalls "fold" in or out a bit when the rim width changes. This will affect the measured tyre width if the maximum width changes, but note that with knobby tyres the knobs or the sidewall may be the widest points on various width rims.

The *effective* diameter will vary with tyre sag, in turn affected by the inflation pressure, tyre volume and load applied, however I assume John is trying to figure out clearances for his frameset, and those won't vary with load or pressure.

FWIW, Surly publish a chart showing the widths of their tyres on a variety of rims, and the maximum widths of the casing and knobs do vary with rim width. They also list outside diameter, and this remains constant.

Re the RTPs: I have seen radii from 328-336mm mentioned in various places. A reliable figure would indeed be useful. Grand Bois list ODs for tyres on their site and IMHO it would be useful if Compass followed suit.

Later,
Stephen

[Harry Watson]

Hi John,

I came up with an average of right around the 665mm mark on my old mavic oxygen rims (30mm external width). They are maybe two years old, TC I think, and haven't done many kms. So seems like there could be significant variation depending on rim width.

Cheers,

Harry/NSW/Aus

[John Clay]

Darin,

It's a great help! Thank you very much!!

I'll get the tires ordered but this information will allow me to proceed; I'm headed out to the shop now.

Your rim comments make sense to me. I'll check mine.

But just now, Harry's info just popped up in the thread. Well, that makes things interesting....I'll go figure it out how to proceed in the shop. Maybe change the order of execution and do some other tasks while I wait for the tires.

Thanks for the additional info Harry!

This is the first time I've started a frame without the tires on hand and mounted. It will be the last one, too!

Thanks again everybody.
John

[John Clay]

Thanks again everybody, especially Darin and Harry for the measurements. Discretion being the better part of valor, I decided to order the tires and work on ancillary tasks rather than main tube fitup. This morning it was to slot the fork blades and fit the DOs. I also added an index on my frame fixture so I can start using setback instead of ST angle. Photos of this project will accumulate here: https://www.flickr.com/photos/21624415@N04/albums/72157671170583438

John Clay
Tallahassee, FL

[DarinM]

Woah! This is either a lesson in taking better measurements or a demonstration in the variability of wheel diameter. Glad you hadn't melted any metal John, but I am curious to see the OD you measure when you receive your tires.

Darin

[Jonathan Pearson]

I think it is also worth stating that I have ridden RTP tires for a couple of years now. The standard casing version stretched 4-5mm in width and height over its lifetime. So, don't mount the tire, inflate it, and assume that will be its width for life. I would build in some room for the tires to grow over time. I just (1 month ago?) put fresh RTP ELs on my rims and they make the fenderline look off, but slowly I am noticing that they're stretching to fit.

[John Clay]

All,

The RTPs arrived. I mounted one, inflated it to 45 psi (to get whatever new tire stretch might be available) and measured 668 x 49 on rims with a 17.5mm inside/inside measurement. My BSPs measure 669 x 38 on rims of essentially the same inside width. So they are the same OD as the BSP.

As an aside: The 650b x 38 Loup Loup Pass tire....anyone know what it's actual dimensions are on rims in the 17mm wide range? I'm hoping it runs narrow with a smaller OD that turns out to be 663 x 34 as that would make it a nice replacement for the pair of Col de la Vie that are on my fixed gear. It's fork doesn't have much surplus tire clearance in either dimension.

Thanks,
John 

On Tuesday, July 17, 2018 at 10:26:22 PM UTC-4, DarinM wrote:

[Stephen Poole]

On Sun, 22 Jul. 2018, 10:06 John Clay, <nice.c...@gmail.com> wrote:

All,

The RTPs arrived. I mounted one, inflated it to 45 psi (to get whatever new tire stretch might be available) and measured 668 x 49 on rims with a 17.5mm inside/inside measurement. My BSPs measure 669 x 38 on rims of essentially the same inside width. So they are the same OD as the BSP.

Very useful - thanks!

As an aside: The 650b x 38 Loup Loup Pass tire....anyone know what it's actual dimensions are on rims in the 17mm wide range? I'm hoping it runs narrow with a smaller OD that turns out to be 663 x 34 as that would make it a nice replacement for the pair of Col de la Vie that are on my fixed gear. It's fork doesn't have much surplus tire clearance in either dimension.

Not quite, but I have the 38mm Écureuil ELs on v1 PL23 rims (~i18mm; I'm not keen on taking the tyres off to measure to .01mm). They started off at 35mm wide but have now stretched to 36.5mm after a few weeks and rides; I'm using tubes, as per GB's instructions. Height above the rim edge is very close to 34mm, and OD is right around 660mm; GB specify 662mm online, and I wouldn't dispute that.

The BB appears to have dropped ~3-4mm compared with Hetres or Soma GLs; these probably have a similar profile to BSPs.

Please note that all these measurements are accurate to +/- 1mm or so, tyre width and height above the rim edge excepted - it's difficult to measure bike and tyre dimensions super accurately, and my vernier calipers only go up to 150mm anyway.  :-)

Hope this helps,

Stephen