Wheel diameter with rim change

wishitwasallflat

Hi,

I was using Open Pro rims with Conti GP 4000s 25 tyres and had Garmin 500 set for wheel diameter of 2146.

I am now using H-plus son archetype rims, with same tyres and the diameter is noticeably bigger (cant get my sks raceblade longs to fit anymore ) - I wondered if anyone knows what the wheel diameter is for this rim tyre combination?

My commute used to always come up about 41.65 miles but now it comes out pretty consistently at 40.15 with no change so it seems something's defo wrong with using 2146 for these wheel/tyre.

Reckon its a pretty common set up so hoping someone may know. Not sure about using the auto wheel diameter setting option on the garmin as I'm not sure I trust it. Ive also tried before to do a roll our test and measure wheel directly myself but I just couldn't get a consistent value (reckon that's cause its so hard to ride a perfectly straight line for the measurements).

PS - Yes I do reckon the garmin diameter is too small and I am faster than it tells me I am!

markhewitt1978

Are you using a speed sensor with the Garmin? Doesn't it auto calibrate from GPS anyway?

zx6man

Push the bike and measure it on floor for one rotation, or am I missing the point here :-)

Garry H

Draw a straight line using chalk on the ground, then push the bike along the line for one rotation, or just use the GPS function on your Garmin!

rolf_f

[url=http://www.bikeradar.com/forums/viewtopic.php?p=19723179#p19723179]wishitwasallflat[/url] wrote:

Not sure about using the auto wheel diameter setting option on the garmin as I'm not sure I trust it.

You prefer to trust something you know to be wrong rather than something that might be right?! That's interesting logic..........

Just use the Garmin - it may make errors relating to GPS accuracy but they won't be cumulative. Any error based on wheel diameter estimates will be cumulative.

markhewitt1978

You don't trust the auto option? WTF, you're getting accurate GPS measurements from satellites which are being used to calibrate things and you don't trust it? Or you could just measure the roll out.

What's going on here lol

wishitwasallflat

interesting comments folks!

I am using the new garmin hub and crank magnetless sensors for cadence and speed but they don't auto calculate wheel diameter unless you set the head unit to do so. I tried a few times to do a roll out measurement before but I just couldn't get the same value to appear over repeats. I concluded I was sh&T at that so I got the value I set from a chart and left it in the unit not giving it much thought for the last few years using Open Pro rims. Now with the new rims:

I cant fit my mudguards any more cause the diameter is bigger and clearance is too small (tyre rubs on brake bridge fitting clip),
the distances look reduced now over my standard commuting route I've been doing for years, and
I'm convinced I'm a bit faster than that f8888ing Garmin says I am :!:

I confess I don't know why I don't trust the gps auto wheel diameter setting - I just don't - BUT - I will try it and I stand chastised (justifiably) for being both:
crap at doing roll out measurements and
being illogical

:oops:

PS - I am considering denying myself a second cup of coffee as punishment for this ...

andrewjoseph

to do a roll out:

find flat stretch, put wheel with valve at bottom, mark line on floor adjacent to valve, roll bike/wheel in straight line until valve is at bottom. mark line, measure distance between line.

the new wheels may have narrower rims which may cause the tyre to stand taller.

wishitwasallflat

[url=http://www.bikeradar.com/forums/viewtopic.php?p=19723357#p19723357]andrewjoseph[/url] wrote:

to do a roll out:

find flat stretch, put wheel with valve at bottom, mark line on floor adjacent to valve, roll bike/wheel in straight line until valve is at bottom. mark line, measure distance between line.

the new wheels may have narrower rims which may cause the tyre to stand taller.

Cheers but that's exactly what I did ... then repeated it but got a different value, repeated again ... different again ...

markhewitt1978

One thing is when doing the roll out, you need to be on the bike, so that you get the effect of your tyres deforming under your weight. But as long as the value is approximately correct it doesn't matter if it's a few mm out.

wishitwasallflat

[url=http://www.bikeradar.com/forums/viewtopic.php?p=19723379#p19723379]markhewitt1978[/url] wrote:

.... as long as the value is approximately correct it doesn't matter if it's a few mm out.

Unless your an anal ocd freak with illogical distrust of garmin gps and auto calculation software

markhewitt1978

[url=http://www.bikeradar.com/forums/viewtopic.php?p=19723385#p19723385]wishitwasallflat[/url] wrote:

[url=http://www.bikeradar.com/forums/viewtopic.php?p=19723379#p19723379]markhewitt1978[/url] wrote:

.... as long as the value is approximately correct it doesn't matter if it's a few mm out.

Unless your an anal ocd freak with illogical distrust of garmin gps and auto calculation software

I use a Cateye Micro Wireless computer, and I just use the default value for tyre size, which I think is for 23mm, even though I run 25mm, it's close enough, after a 60 mile ride I might be half a mile out, I can live with that.

wongataa

[url=http://www.bikeradar.com/forums/viewtopic.php?p=19723307#p19723307]wishitwasallflat[/url] wrote:

interesting comments folks!

I am using the new garmin hub and crank magnetless sensors for cadence and speed but they don't auto calculate wheel diameter unless you set the head unit to do so. I tried a few times to do a roll out measurement before but I just couldn't get the same value to appear over repeats.

Getting the same number over a number of tries is quite difficult due to measurement errors. It would better to measure several times and average the results to get the figure to plug into the Garmin.

andrewjoseph

if you're really ocd, you would measure the diameter before and after every ride with the wheels at the same temperature. This would allow you to account for rubber loss as the tyre wore down to a smaller diameter.