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Why do you need to do a rollout ot callibrate your computer(13 posts)

Why do you need to do a rollout ot callibrate your computerKristin
Aug 8, 2003 8:40 AM
When I set mine up I just selected 700 c wheels. How much can it be off?
re: Why do you need to do a rollout ot callibrate your computerKen of Fresno
Aug 8, 2003 8:45 AM
A 700x18c is a whole lot smaller diameter than a 700x35c tire. And tires labled the same size by dif makers usually have dif diameters. Also, dif rider weights affect the actual dia. A roll out is the only way to know the exact dia of your bike wheel.

do you trust authority?mohair_chair
Aug 8, 2003 8:48 AM
I do the roll out to get the highest accuracy possible, but I'm an idiot. Every time I do it on a road bike, I get the same number the manufacturer recommends. Still, I do it.

On a mountain bike it's a different story. Because of tread patterns, different tires can produce different results, so it makes sense to do the roll out if you want to be accurate.
I don't find the same result and here is what I do.maximum15
Aug 8, 2003 9:08 AM
I find large variations in 23C tires from various mfgs and brands within a mfg. To do rollout, I inflate both tires to normal riding pressure. Stretch a 16 foot tape measure and lay it on the driveway. Put a milk crate on either side of the bike, line up a mark on the tire with a point on the tape. Sit on the bike, feet on milk crates and roll forward 2 complete tire revolutions with tires very close to tape. Take measurement in inches, divide by two, multiply by 25.4, and you have the circumference in mm. Two wheel revolutions will be around 165 inches.
I always use the same tiresmohair_chair
Aug 8, 2003 9:40 AM
I very rarely use anything other than Conti GP 3000s, so I don't have to worry about variations.
after trying many, seem to like rubino pros for everyday (nm)maximum15
Aug 8, 2003 4:17 PM
If you don't do it. . .Mike P
Aug 8, 2003 9:03 AM
your average speed on a given ride could increase (or decrease), as if by magic, 2 miles per hour, like mine did. For a brief moment I thought, "holy cow! This training has really paid off." Then, only to come to the deflating realization it might be due to the new computer, I decided I am still to slow to shave my legs.

It just takes a few minutes and it gives you something to do with some of the nuts and screws laying around on the horizontal surfaces.

re: callibrate your computerChen2
Aug 8, 2003 9:11 AM
If you do a rollout it really needs to be with the full weight of the rider and equipment. It will make a significant difference. If you really want to get it accurate, ride some known accurate distances and then fine-tune the circumferance calibration.
How picky are you?Spoke Wrench
Aug 8, 2003 9:38 AM
If you are sitisfied with a 5% or so error, you don't have to do it. I don't on my own bikes (the ones that have computers that is) because I really don't care all that much. I just enter the numbers recommended for my tire size in the computer calibration instructions.

If you are really picky, find a speedometer calibration course, then ride your bike back and forth, adjusting your computer between runs, until it comes out spot on.

Doing a one revolution roll out fits somewhere between those two extreames.
cuz it's a shame to do a Century and only get 93 miles. (nm)terry b
Aug 8, 2003 9:53 AM
So you dont end up like Aarontoy!the bull
Aug 8, 2003 2:10 PM
Will not that far off but you get the picture!
But how accurate can a rollout be?gav
Aug 8, 2003 3:48 PM
I was just wondering because I have two computers that have different suggested circumferences for the same tire size.. I thought about doing a rollout but since the measurement is in mm's I'm wondering how accurate it would actually be. Any deviation in the course during the roll would throw the measurement off, and tire pressures, and any measuring inaccuracies.. As well as not stopping at exactly the right spot.. Is it really worthwhile? Thanks..

P.S. Does anybody know what the 'official' circumference of Conti Grand Prix 3000's are for 700 x 23 wheels?
But how accurate can a rollout be?Al1943
Aug 8, 2003 8:30 PM
The "official" tire circumferance is not what you want to put in your computer. You can easily measure the circumferance, but that number will be too big because it does not account for the weight of the rider, bike, & accessories. If you load the bike, measure the radius from center of axel to ground, multiply by 2 and by pi, the result would be a better effective circumferance calibration.
If you do a rollout you need a helper. Pump the tires to riding pressure, load the bike with full normal weight, roll the bike several full wheel rotations, mark the floor at each spot the valve stem reaches the 6 o'clock position, measure (metric) from the first to last spot and divide by the number of rotations.