Speedo Accuracy
 

We filled both bikes up prior to our trip to Donington on sunday and zero'd the trips. When we got home my bike had done 53.5 miles and Lynns had done 51.3. I didnt expect them to be the same but i'm supprised how far out they are. Both bikes are running standard size front tyres as well.

Made me think for a moment until I remembered that the ST has a speedo drive on the front wheel. Can it be that tyre wear could make that difference? Just a thought, but maybe not a very good one. :puzzled:

Could be a whole host of things I guess, tyre pressures, wear on tyre, and I guess to a lesser extent actually riding and one taking a wider/longer line over said distance could add a few feet here and there.

It's about 5% from the figures above which would fall within government regs I guess.

Hmmm .. I've always meant to work out how different the rolling circumference is when you are leant over.
Sounds like lynn braked a lot and kept the bike upright, and you took the corners faster (more revolutions) :D

[Edited on 31-5-2005 by phil_h]

Well my bikes got a brand new front tyre while lynns M600 has a fairly worn one. Having said that they are running the correct sizes i must check the monster to be sure when i get home. I'll take the GPS out with us next time and try and see which one is closest to being right.

The only thing i have changed on my bike is the speedo drive as i managed to crack it when i changed the tyres. I'm assured the new one came from a 916 so it should be okay. Are they the same across the range? Did some 748's have a differnt profile front tyre as standard?

That might be true for most of our riding but from my house to Donington is motorway nearly all the way!

Tyres may be standard sizes, but what are the standard profiles? Could it be a difference between 60's and 70's?

All the speedo drives are the same, Ducati fitted the same thing across the range, but they're is bound to be some difference between bikes.

I'm more concerned about the innacuracies of the speedo to be honest. Although my speedo cable is still broken at the moment, when it does work the ST speedo seems to be wildy out on both my ST and Glyn's. Anything over 100 and it seems to make it's own mind up what to read. On our private test track the most I've ever seen on the clock is about 110-115 when my colleague test riders with 749/999 digital speedos reckon it should've been reading 135+

As I've said, Glyn's is the same.

I meant to say profile rather than size. Yes, my concern was speed accuaracy. I do use the gps sometimes but only if i'm going on a route i dont really know and i've never had it on the monster and i havent had it on mine recently.

Sounds like a good excuse for some road testing!

Like most, Ducati speedometers are set-up to read optimistically to avoid law enforcement issues. The speedometer error for a 998 with a 70-section was measured against a GPS standard by one owner to be:

40 998 = 37 GPS
50 998 = 46 GPS
60 998 = 56 GPS
70 998 = 65 GPS
80 998 = 73 GPS
90 998 = 82 GPS
100 998 = 89 GPS

So for example, after a one hour trip at an indicated 80mph he would have traveled only 73 miles but the odometer will read 80 miles. The odometer always records more milage than the actual mileage traveled. The greater the difference, the less accurate the clock.

So if your friend’s odometer reading is lower than yours for the same trip his odometer is more accurate than yours.

Looks like Lynn popped about 2 miles worth of wheelies Paul :lol: :sing:

Well she was behind me for mosr of the trip so she might have done that!

So if your friend’s odometer reading is lower than yours for the same trip his odometer is more accurate than yours. [/quote]

Lynn's a girl's name in this country Larry ;);)

Fascinating to see those statistics.

And Nelly, your explanation seemed equally valid - as always! :lol:

If you do the math you can see that a trip of that distance can easily have a reading difference of 1 or 2 miles with just a difference of 2/10 of an inch in tire diameter. All other things being equal, i would think the difference in tire wear accounts for most of the difference (except for those wheelies she may have been popping!). Play with the numbers yourselves and see what a difference a 10th of an inch makes over 50 miles.

Mad Dog, i'll leave the maths to someone else, its not my strong point which is why i asked the question on here!

So after 20,000 indicated miles, her bike might only have actually done 19,000. Thats quite significant isnt it??

With tire wear and other mechanical events taken into account, that (5%) would not be so unusual and in fact may be a fairly good accuracy for all those miles. Rally cars, even for amatuers are equipped with much more accurate speed and odometers because of the inherent problems in normal civilian stuff. Just think how much difference low tire pressure would make (again, plug in the numbers). Even my mountain bike has adjustments for actual tire circumference instead of tire size as over a few thousand kilos there is a noticeable difference. Ask Lynn to keep the front wheel on the ground!

Thinking about this made me realize that just small deviations in tire molding would make the speedos differ between similar bikes, but I think Shazzam needs to do another erudite post on the matter!

It could explain why the fork seals keep going!

I know the terratrip things on rally cars be be set with the rolling radius of the tyres there are using.

That is why even amatuers check their tire pressure with digital gauges and do a look at their actual tire circumference. Small differences can mean missed check points even over small differences.
As for the fork seals, just put an 18 tooth front sprocket on.......great for mileage and saves forks and frames as well.....heh, heh

Excep that over longer distances it will average out between you because you will have both been through several sets of tyres from new to worn.

No need to go through complicated maths or averages to get a big difference between indicated and actual mileage. My speedo cable broke at 14,827 miles and now, some 3,000 miles later it's still reading 14,827 :lol::lol::lol:

I bet you've never broken a speed limit since then either! Ideal! :eureka:

I never look at my speedo, until then orange light goes on, then it's time to fill up.

I only quickly glance at my tacho when the rockers start to growl around Ten grand.

Dr am i normal?