Speedodometer validity

S

SoSauty

1 kW
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Mar 17, 2009
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Until now, I referred to Sheldon's Gear Calculator site and relied on my semi-reliable 80rpm cadence to estimate speeds. On 1 occassion my wife drove beside me on a 2.5 mile 800' descent. Sitting up and C-lyte drag kept speed to around 30mph. Thought my cruise was 20mph. With my new Bafang (thank you J Holmes) I decided a speedodometer was in order. With motor wired, cross 4 spoke pattern mastered, and a sunny day I'm back out on the road.
:shock: The CatEye reads 20mph as I shift into hi gear!
This is what it tole' me:
Motor only: 22.5mph top speed

With lite pedalling: 24.8mph top speed
21.5mph cruise
19.5mph shift into hi
17mph up our local hill
16.2mph ave speed

Battery note: 3hr charge lite goes green (batt can take 4.5 hrs if drained to LVC)
Distance: 14.8 miles

I'm skeptical of this speedodometer. Have others found speedodometers to exagerate?
 
It all depends on where the magnet is on the spoke. If i use one on mine it would be almost near the motor before its close enough for the pickup to see the magnet. I would also measure theb actual tire diameter and instead of using whats in the cheap book. I had one to match my gps on speed and as far as distance it match me to less than 1/10th of a mile. My old CA used to match my gps my speedo. My new CA doesnt. I programmed the same settings and even tried to tweak them a little bit.
 
icecube57 said:
I would also measure theb actual tire diameter and instead of using whats in the cheap book.
Click to expand...

The most accurate way to do this IMHO is to mark a line on the ground, mark side of tire (or line up valve with mark) MOST IMPORTANTLY
sit on the bike, and wheel it forward one complete revolution until the valve has returned to bottom, make another mark, then measure the distance between the two. If you JUST measure the tire it will be incorrect without rider weight on it...Was a discussion about this on another forum i participated on recently.

KiM
 
I have found my bike speedometers to be quite accurate however the key is to input the correct tire circumference into the computer. The manual for bike computers generally list values for the various tire sizes however if in doubt simply measure your tires. Some folks do a roll-out measurement but I use a string to measure the circumference of the tire then I reduce that by 2% to account for compression under my weight. Over one of my favorite courses my computer tells me I've traveled 31.0 miles while using a mapping tool I get 30.7 miles, a difference of 1%.

As for your speeds they seem realistic though perhaps a tad high considering your 26" tires. With my 36V Bafang, 700Cx50mm tire and 48V LiFePO4 battery I get a top speed on the flats of approximately 23 mph with a total weight of 250 lbs.

-R
 
:shock: I never knew about the tire compression riddle, but makes perfect sense now that you guys mention it. All i did was wrap a measuring tape around the tire, and entered the value into my Cycle Analyst.

Thought this would be accurate enough, but you guys take it one step further.

My neighbour purchased a cheap plastic type electric bike, and her speedometer was out by 8 km/h.
 
recumbent said:
:shock: I never knew about the tire compression riddle, but makes perfect sense now that you guys mention it. All i did was wrap a measuring tape around the tire, and entered the value into my Cycle Analyst.

Thought this would be accurate enough, but you guys take it one step further.

My neighbour purchased a cheap plastic type electric bike, and her speedometer was out by 8 km/h.
Click to expand...

Optomistic speedo's are a classic marketing tool for slow vehicles. ;)
 
Yeah, the speedo on my Roketta Scooter reads about 15kph faster that it is at full speed of 55 mph.

I like to calibrate my speedo using mile markers on the highway. My commute follows the frontage, so I tweak the numbers on the speedo till it reads accurate mileage.
 
OK, maybe the speedodometer is close to valid. Insightful responses. I took AussieJester's idea and measured the roll. 70psi does allow a slight flat spot shortening the radius thus the circumference. The new measure, 1945mm is 78 mm less than the CatEye provided. As mentioned, the speeds were just a tad high. Also, it's realistic that I've been conservative with Sheldon Brown's gear calculations. Guess he took into account the tire compression factor for the MTB tires. So, gear calculation gets revised up and CatEye speeds edge down. I have confidence in these numbers:

21.5mph Bafang top speed no pedal
w/ lite pedalling;
23.5mph top speed
21mph cruise (flat terrain/no wind)
16mph sustained speed up 4% slope
15.9mph ave speed in easy stop/go
20mph shift into hi gear
18mph cruise 1 gear downshift

I traveled 'bout 16.5 miles this 2nd trip out. Very little sag from the V2.5Ping the latter miles. I used to ride a 23 mile loop. I think the little bugger a do it :!:
 
The ping won't sag much, till you get to the last mile, then it goes off a cliff and shuts down.
 
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