Cateye Bike Computer Seems Off

[ReidRules18]

So I have a cateye bike computer and set up the tire size to actually be a bit smaller than the 2.4 tire I have (calibrated at 2326 for 2.3" tires). I would assume with this being a slightly smaller tire calibration than what I actually have I would be "shorted" miles which I would okay with. Anyways, when I ride a specific loop of a known distance it just seems still off. Case in point, I ride the CSGA which is around a 7 mile loop I recall, but I get 8.25 miles (crazy!). Anyways, I am going to even dial it down lower, but have any of you had the same problems (I am sure of it).

I assume too it may be due to the fact I am checking out what MTB project has which is GPS based versus the bike computer which likely would be more accurate (at least what I would think). Any of you have this problem too?

[c0nsumer]

I know this isn't a direct answer, but maybe these questions will help get you to one?

- How do you know what the loop distance is? GPS notoriously under-records. Aliasing errors are a thing.
- Did you roll out your wheel to get an actual circumference? If not, do so. And have it slightly underinflated to compensate for the flat spot as you roll.

[DynoDon]

I like to mark out a straight line mile on a road with my vehicle , measure the wheel size for myself, put that in the computer, psi, rider weight, new, or old tire, etc. all make it impossible to get anthing but close, ride the mile then recalibrate if not satisfied, until its close.

[Roy]

Gps milage numbers are always less then actual, unless you ride in a straight line.
A mountain bike trail GPS miliage should be at least 10% short, and without loss for hills,
This because GPS distances are point to point segments added up. (usually in 2D)
In math this is how pie was first calulcaed by usiing smaller and smaller segments of a circle,
Maybe and advanced computer could estiment the radius of a curve and use that radius to better estimate actual distance traveled,
I don't think I could write that equation. I would be a good project for a math major.

Roy

[DynoDon]

Useing a wheel sensoner a bike can even be close on a trail, I've used wheel sensor Planet Bike computer on all my bikes for years, I get 8 miles on my computer on Tron Shirt, I get 18.5 on Poto, I get 22.5 on DTE,, you can get close with a wheel sensor, but the wheel sensor is way more accurate than GPS,, now GPS with a wheel sensor has caught up to the wheel sensor computer.. I know the Garmin GPS wheel sensor works pretty close, no experience with anyone with the Cat Eye, but if it measures the wheel, so it should be close, and way better then GPS on the trails.. GPS on a trail has only one thing even close to accurate, the time. On the road/gravel GPS does pretty good,

[ReidRules18]

You guys are all confirming what I have been thinking - ha. I am going to try the wheel circumference roll out one more time, but this time with it having lower tire pressure and give it a go (I did this initially and for some reason doubted the results due to the mileage shown seeming "high"). Yankee Springs for example is roughly 2 miles on the warm up and I believe 11 or 11.2 for the main loop. I seemed to end at 13.9 which again seemed high to me, just given my past bike computer values on different bikes.

Keep you all posted, and I agree on all the comments on the GPS 100%. If I ever use my apple watch to record my bike rides, in the woods on a trail it can be off by up to 20% (yankee springs full 13 mile loop would display maybe 9 miles?!).

[c0nsumer]

You guys are all confirming what I have been thinking - ha. I am going to try the wheel circumference roll out one more time, but this time with it having lower tire pressure and give it a go (I did this initially and for some reason doubted the results due to the mileage shown seeming "high"). Yankee Springs for example is roughly 2 miles on the warm up and I believe 11 or 11.2 for the main loop. I seemed to end at 13.9 which again seemed high to me, just given my past bike computer values on different bikes.

Keep you all posted, and I agree on all the comments on the GPS 100%. If I ever use my apple watch to record my bike rides, in the woods on a trail it can be off by up to 20% (yankee springs full 13 mile loop would display maybe 9 miles?!).

I would do a test on a straight-line road with a GPS on your phone for double-checking. Anything curvy is suspect.

GPS, in a straight line under open skies is quite accurate for distance.

For me, when I'm allowing my Garmin to auto-calculate wheel circumference I use a neighborhood street near my house that's a mile long and straight as an arrow. Head out, clear the distance in the Garmin, set it back to auto, and go. It'll beep and say it's auto-calculated and I'm good to go for all routes.

I once measured it's calculation vs. a manual rollout and I think it was 2mm or 3mm difference.

[ReidRules18]

I performed the manual roll out with low pressure and rode the same distance I had before prior. Total distance I ended up being "off" was only .09 of a mile for the CSGA loop, therefore I feel much better about the actual value displayed (pretty much I really wasn't that far off to begin with). I won't trust a GPS as I don't have anything that fancy ha, but will check out the straight line route next, perhaps on the White Pine trail and measure from mile marker and the GPS on my phone.

Thanks all.