Measuring Loaded Rim Height - Unassisted?

I going to try getting serious about the 15% rule: where a tire is
supposed to roll best when inflated so that the rim height is 15% less
than when unloaded.

Does anybody have a slick way to do this unassisted? Seems like a
rather fine measurement to me.
--
Pete Cresswell

On Apr 19, 5:40*pm, "(PeteCresswell)" wrote:
I going to try getting serious about the 15% rule: where a tire is
supposed to roll best when inflated so that the rim height is 15% less
than when unloaded.

Does anybody have a slick way to do this unassisted? * Seems like a
rather fine measurement to me.
--
Pete Cresswell
wassup, no cornflakes today? just tie a rule to a spoke with the end
touching the ground with the bike unweighted and read at the edge of
the rim. Get on bike in normal riding position and get off bike and
the rule will have slid up but the friction from the tyre wall will
have stopped it sliding back down. Read the rule again at the edge of
the rim. Alternatively use tubulars. You'll only worry about how
long you should leave the rim clouting the road when you get punctured.

Per thirty-six:
wassup, no cornflakes today? just tie a rule to a spoke with the end
touching the ground with the bike unweighted and read at the edge of
the rim. Get on bike in normal riding position and get off bike and
the rule will have slid up but the friction from the tyre wall will
have stopped it sliding back down. Read the rule again at the edge of
the rim. Alternatively use tubulars. You'll only worry about how
long you should leave the rim clouting the road when you get punctured.

Nice!..

I went into it thinking I was going to have to get around the parallax
error and the expected small distances.

But I just went and *did* it and parallax error seems tb moot.

To wit: I measured 40/35 at 70 PSI (5 more PSI than the tire is actually
rated for). 5/40 = 12.5%...... 5/35 = 14.3%....

Backed off to 65 PSI, got 42/34....== 20%

Mildly puzzled when I connected the pump the second time and the
pressure read more like 60 - not long after airing it up to 70... but
I'm rationalizing that as being due to air loss pressurizing the pump's
hose.

Allowing for measurement error (especially PSI error) and the fact that
the tire is going to lose air anyhow and I'm not going tb all that
compulsive about filling it before every ride... I think I'll just air
it up 70 whenever I think about it and call it a wrap.

Maybe some day when I've been drinking too much coffee, I'll try a
series of measurements using different pump pressures and see if one
consistently falls closer to the magical 15%
--
Pete Cresswell

On Apr 19, 9:40 am, "(PeteCresswell)" wrote:
I going to try getting serious about the 15% rule: where a tire is
supposed to roll best when inflated so that the rim height is 15% less
than when unloaded.

Does anybody have a slick way to do this unassisted? Seems like a
rather fine measurement to me.

Go to the butcher and have him cut you enough meat to equal your own
weight. Stuff a pair of your pants and shirt to make a meat scarecrow
and put it on the bike. Hold the bike with one hand to keep it from
tipping and measure with the other.

On Friday, April 19, 2013 12:40:18 PM UTC-4, (PeteCresswell) wrote:
I going to try getting serious about the 15% rule: where a tire is

supposed to roll best when inflated so that the rim height is 15% less

than when unloaded.



Does anybody have a slick way to do this unassisted? Seems like a

rather fine measurement to me.

I can't think of a really slick way to do it, to make it happen quickly, especially if you'll be sitting on the bike. Frank Berto talked about doing the measurements by using a hydraulic jack pushing down on the wheel, which was pressing on a bathroom scale. He apparently measured by hand, perhaps using calipers. (I assume he would first used the scale to measure the load on each tire when he was seated on the bike.)

You could try something like clamping a dial indicator to the rim or to a spoke, and arranging a mirror so you could see its reading while seated. Or something like arranging a battery, bulb, and some stiff wire so the bike tire, sitting on a conductive plate, sagged the proper amount, a stiff wire could complete the circuit and light a bulb.

With either method, you could overinflate the tires, then operate the valve to let air out until you got the indication of 15% drop. But you'd have to be sure the bike was vertical as you did it. And setting up either method would be a fair amount of trouble, maybe justifiable only if you intended to do lots of bikes.

The current issue of Bicycle Quarterly is mostly devoted to tire testing by various means. In it, Jan Heine claims that optimum pressure really isn't critical. It has some effect on rolling resistance, but the effect isn't strong, and that optimum pressure seems to vary with the tire's construction.

On Fri, 19 Apr 2013 12:40:18 -0400, "(PeteCresswell)"
wrote:

I going to try getting serious about the 15% rule: where a tire is
supposed to roll best when inflated so that the rim height is 15% less
than when unloaded.

Does anybody have a slick way to do this unassisted? Seems like a
rather fine measurement to me.

Re 15% rule I believe that you are referring to an article in Bicycle
Quarterly, Volume 5. Number 4. The method described was to weigh both
wheels with the bike loaded and rider aboard and than to use the graph
printed in the article to determine the optimum pressure for a 15%
drop, on both wheels.

I might comment that the graph only includes data for tires from 20mm
to 37mm, and wheel weights from 30 to 70 Kg.

--
Cheers,

John B.

https://www.google.com/webhp?hl=en&t...w=1152&bih=635

buy a laser, place bike on table. fix end plate on rim...rim is curved get out the math...drill hole in table...shoot laser from floor to rim end plate.

On Saturday, April 20, 2013 7:48:24 AM UTC-4, datakoll wrote:
buy a laser, place bike on table. fix end plate on rim...rim is curved get out the math...drill hole in table...shoot laser from floor to rim end plate.

___________

buy 2 you can reurn them

http://www.homedepot.com/p/Ryobi-Son...specifications

I ahve a digital vernier. ahhhhhh incroyable.

butbutbut using such for your prob involves intermediate exactnesses

afaik could be a method.

you could use a steel rule, average 10 measurements throwing out the 4 wroser

On Saturday, April 20, 2013 8:36:08 AM UTC-4, datakoll wrote:
On Saturday, April 20, 2013 7:48:24 AM UTC-4, datakoll wrote:

buy a laser, place bike on table. fix end plate on rim...rim is curved get out the math...drill hole in table...shoot laser from floor to rim end plate.



___________



buy 2 you can reurn them



http://www.homedepot.com/p/Ryobi-Son...specifications



I ahve a digital vernier. ahhhhhh incroyable.



butbutbut using such for your prob involves intermediate exactnesses



afaik could be a method.



you could use a steel rule, average 10 measurements throwing out the 4 wroser

.............

right. use the laser for squaring bike table/laser mount surface so the subtraction of laser to table from laser to rim point is accurate.

thats the hole deal...having a gravitationally/photonic accurate measuring table
no beer