calibrating computer - are Pro3 race > 700x23C?

neeb

Just got a new Polar cycle computer (it beeps! How cool is that! )

Usually when setting the wheel size on computers I am lazy and just enter the recommended number for the tyre size, but I thought I'd try measuring it this time with lines on the ground etc. Anyway, if I'm doing it right my Pro3 race 700x23C on Campagnolo Eurus are 2116mm, minus 4mm to account for weight = 2112mm. This is quite a bit more than the 2070mm standard for 700x23C and would put the tyre somewhere between the normal size for a 700x28 and a 700x32C! I really want to believe this as it would mean I'm going further and faster than I thought I was, but it seems such a big difference...

I did it twice and got the same answer, +/- 1mm.

Anyone else tried the same thing and found Pro3 race to be bigger than "standard" 700x23C?

It occurs to me that the same wheel/tyre combination should be a pretty consistent size, so it would be good to compare calibration values, maybe even have a thread where everyone posted their measured values for given tyres & wheels?

maddog 2

yes, always measure as the tables are inaccurate, as you say.

I think my PR3 was 2130mm IIRC

neeb

Cool, that means that my average speeds should go up by nearly 0.4mph!

The weird thing is that it feels like cheating to use the correct value...

verloren

Not that it will make a huge difference, but I'm not sure that 4mm is enough of an allowance for weight. That would mean that all your weight caused the tyre to compress by 0.6mm, which is pretty small. I would knock at least an extra 2mm off to be on the safe side

neeb

Hmmm... the 4mm came from the Polar manual. Because of the big discrepancy between the "standard" size for 700x23c (2070) and the length I measured for the Pro3 race (2116), I actually wondered at one point whether the 4mm was a misprint and they might mean 4cm. That would give 2116 - 40 = 2076, which is a lot closer to 2070. Presumably that would assume that the weight caused the tyre to compress by 6mm. Is that possible?

verloren

If my tyres deflected by that much I'd pump them up

neeb

63kg + 7.2kg of bike, tyres at 110psi.... Maybe not 6mm then?

To be honest I haven't a clue how much they squash. Fairly tricky to measure I suppose?

verloren

I guess you'd have to do the lines on the ground thing while on the bike, but that's pretty tricky An alternative if you have a helper would be to measure from the ground to the top of the tire, then sit on the bike and measure the same again. The difference between the two is the amount of deflection. Multiply that by 6.3 and you should be about right. Or you could knock 10mm off and call it good

alfablue

Okay, I am prepared to embarrass myself here, but I can't quite get why your weight compressing the tyre would reduce the circumference - if the tyre is compressed this patch revolves around the tyre as you ride, the displaced air would enlarge the tyre elsewhere (which is probably an irrelevance) , but surely the circumference of the tyre cannot actually shrink. . . . :?

neeb

Okay, I am prepared to embarrass myself here, but I can't quite get why your weight compressing the tyre would reduce the circumference - if the tyre is compressed this patch revolves around the tyre as you ride, the displaced air would enlarge the tyre elsewhere (which is probably an irrelevance) , but surely the circumference of the tyre cannot actually shrink. . . . Confused

It's a little counterintuitive (so no embarrassment required), but if you think about it, the wheel is always rolling on the compressed bit of the tyre. The total cicrcumference of the tyre at any given moment is not reduced, but there is a sort of "virtual tyre" that has the circumference of a hypothetical tyre that has the compressed bit going all around it, and that is what you are rolling on.

neeb

Or you could knock 10mm off and call it good Smile

Sounds like a good compromise..

verloren

In case an unrealistic example might help...

Imagine you have a wheel that's a yard across, and a huge tyre that makes the tyre+wheel 2 yards across. But let's say your tyre is totally flat, such that the rim of your wheel is basically touching the road. In one revolution your wheel is going to cover 3.14 yards, even though in theory the size of the tyre would make it cover 6.28 yards if it was inflated.

Now scale that down to normal wheels and tyres, but remember that your weight essentially makes your tyre go a little bit flat.

alfablue

Thanks guys, I think I get it, just it hurts my brain a little!

I can't get out of my mind the idea that the tyre tread remains a constant circumference (I mean, it doesn't skid or fold under) and you roll over the entire circumference each revolution (even if the diameter changes).

You're right, it is counter-intuitive :?

balthazar

The most precise method is to measure roll-out along the ground. the longer the distance you can manage (usually determined by the measuring tape you have), the more accurate your result.

Position the bike so that the front valve is nearest the ground. Mark the ground with tape at that point. Get on the bike, and roll as far ahead as you will be able to measure - one whole revolution at minimum, two or three if you can.

Either get a friend to mark on the ground where the valve arrives, or eyeball it against a detail (bit of gravel, etc) and remember that whilst you get off and tape it yourself. Then measure the distance and divide it by however many wheel revolutions you did.

On the other matter, I think it's easier to visualise the reduction in effective diameter, by picturing an old fashioned car wheel run flat, rather than a bike. It rides with the rim on the ground: nothing in the system cares what happens to the rest of the tyre, which is elastic and will take care of itself. As far as the wheel is concerned, there is no tyre; picture it running next to an entirely bare rim, and see how they turn at the same speed.

This reminds me of the periodical misconception that a bike computer magnet and sensor must be mounted at a certain radial distace from the wheel hub, a conviction that is tricky to displace in some. We all have difficulty visualising some systems and it is the good teacher who can explain them. That said, I think this could be explained much better than I've managed by somebody who really understands such geometry's.

alfablue

You explained it well!

I don't suffer from the other misconception, however 8)

neeb

Well, I found that if you do it indoors right next to a wall you can get on the bike and pedal very slowly with your shoulder against the wall. Measured that way it came out to 2102mm, which is 14mm less than when the bike wasn't weighted (still 32mm more than the recommended for 700x23C though).

on-yer-bike

neeb wrote:

Well, I found that if you do it indoors right next to a wall you can get on the bike and pedal very slowly with your shoulder against the wall. Measured that way it came out to 2102mm, which is 14mm less than when the bike wasn't weighted (still 32mm more than the recommended for 700x23C though).

I have the same tyres, how many PSI were in yours when you measured them sitting on the bike? assuming all rims are the same diameter I should be able to use your measurement.

COVEC

I do think that Michelin's come a bit big in the carcass, their 25's look massive and balloon like with a really tall aspect.

neeb

I have the same tyres, how many PSI were in yours when you measured them sitting on the bike? assuming all rims are the same diameter I should be able to use your measurement.

I think they were 110psi, could have been a tad less, but then I measured it again the next day just after checking the pressure (110psi) and got 2103mm with my weight on the bike. I'm pretty light though (63kg). Also, I suppose it's possible that different rims could slightly affect total wheel diameter. But if you set 2103 I'd imagine it would be pretty accurate..

neeb

Sorry, that should have been 2103!!

I'll edit the last post....

Wappygixer

I used to have my Garmin 705 and my Sigma 1606 DTS on the same bike.The Sigma was set using the instruction booklet ratings for tyre size and the Garmin got it from the gps sensor for satellite reception..
Speed wise they were within 1/2 mph all the time and I suspect the Garmin was the more accurate.