Just had a revelation! - SV650.org

northwind · 28-07-06, 12:18 PM

Stop me if I'm talking rubbish

I've always thought my fuel economy was a bit rubbish... Well, fair enough, 180 jets do drink fuel (same size as what goes in a Firestorm with a K&N!) but it's more than most people report... Why?

Well, I think I've got it. Odo runs from the speedo, my speedo runs from the rear wheel not the front. The height of one side of a "perfect" 120/60 is 72mm, while the height of one side of a 160/60 is 96mm. So, the radius of my wheel+tyre is, in theory, 24mm bigger.

Total radius of the front, then, is 289mm, while the rear is 313. It won't really be that, that's just a mathematically perfect tyre without deformation, but it'll be somewhere in that ballpark.

So, the rolling circumference of the front is 1815mm. The circumference of the rear is 1966mm, an increase of 151mm or approximately 1/12.

So, my miles are 1/12 longer than yours! My average tank range of 120 miles to the flashing light is actually the same as about 130 for everyone else!

(also, the total on the clock is about 1/24th lower than it would normally be, as I've had the rear speedo conversion on for about half the bike's total miles.)

Funnily enough, the "error" caused by running on the rear wheel is almost the same as the error built into the speedo, making it more accurate now than when it left the factory.

Well. I thought it was interesting anyway

If you have a 120/70 front tyre (quick maths here) you fall exactly between the 2.

28-07-06, 12:35 PM

R u bored

I was bored enough to confirm your calcs. lol
well done

28-07-06, 12:36 PM

Don't worry Northy, I thought it was interesting too.

Not so much for the techie side, but it sounded very much like one of the rambling maths problems I end up doing on long journeys to keep my brain awake.
That one would probably last me most of the way to Scotland...

... I'm crap at mental arithmatic.

28-07-06, 12:41 PM

I thought all that was totally obvious?

.

jambo · 28-07-06, 12:44 PM

That's cunning that is.

northwind · 28-07-06, 12:47 PM

Quote:

Originally Posted by jonboy

I thought all that was totally obvious?

I knew the speedo was corrected, never occurred to me that the odo was too

Sid Squid · 28-07-06, 01:59 PM

northwind · 28-07-06, 02:26 PM

Ooh, good photo! Thanks Mr Squid! Cunningly taken from teh side with the undamaged mirror too

28-07-06, 06:20 PM

why does your spedo drive come from the rear wheel?

28-07-06, 06:24 PM

Quote:

Originally Posted by weazelz

why does your spedo drive come from the rear wheel?

'cos northy has a GSXR front end

28-07-06, 12:18 PM	#1
northwind Moderator Mega Poster Join Date: Jan 2004 Location: In the garage where I belong Posts: 17,082	Just had a revelation! Stop me if I'm talking rubbish I've always thought my fuel economy was a bit rubbish... Well, fair enough, 180 jets do drink fuel (same size as what goes in a Firestorm with a K&N!) but it's more than most people report... Why? Well, I think I've got it. Odo runs from the speedo, my speedo runs from the rear wheel not the front. The height of one side of a "perfect" 120/60 is 72mm, while the height of one side of a 160/60 is 96mm. So, the radius of my wheel+tyre is, in theory, 24mm bigger. Total radius of the front, then, is 289mm, while the rear is 313. It won't really be that, that's just a mathematically perfect tyre without deformation, but it'll be somewhere in that ballpark. So, the rolling circumference of the front is 1815mm. The circumference of the rear is 1966mm, an increase of 151mm or approximately 1/12. So, my miles are 1/12 longer than yours! My average tank range of 120 miles to the flashing light is actually the same as about 130 for everyone else! (also, the total on the clock is about 1/24th lower than it would normally be, as I've had the rear speedo conversion on for about half the bike's total miles.) Funnily enough, the "error" caused by running on the rear wheel is almost the same as the error built into the speedo, making it more accurate now than when it left the factory. Well. I thought it was interesting anyway If you have a 120/70 front tyre (quick maths here) you fall exactly between the 2. __________________ "We are the angry mob, we read the papers every day We like what we like, we hate what we hate But we're oh so easily swayed"

28-07-06, 12:44 PM	#5
jambo Member Mega Poster Join Date: Aug 2003 Location: Croydonia Posts: 5,376	That's cunning that is. __________________ Modern motorcycles are bloody brilliant, enjoy it while we can

28-07-06, 01:59 PM	#7
Sid Squid No, I don't lend tools. Mega Poster Join Date: Jul 2003 Location: Skunk Works, Nth London Posts: 8,680	__________________ If an SV650 has a flat tyre in the forest and no-one is there to blow it up, how long will it be 'til someone posts that the reg/rec is duff and the world will end unless a CBR unit is fitted? A little bit of knowledge = a dangerous thing. "a deathless anthem of nuclear-strength romantic angst"

28-07-06, 02:26 PM	#8
northwind Moderator Mega Poster Join Date: Jan 2004 Location: In the garage where I belong Posts: 17,082	Ooh, good photo! Thanks Mr Squid! Cunningly taken from teh side with the undamaged mirror too __________________ "We are the angry mob, we read the papers every day We like what we like, we hate what we hate But we're oh so easily swayed"

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28-07-06, 12:35 PM	#2
Kilted Ginger Guest Posts: n/a	R u bored I was bored enough to confirm your calcs. lol well done

28-07-06, 12:36 PM	#3
K Guest Posts: n/a	Don't worry Northy, I thought it was interesting too. Not so much for the techie side, but it sounded very much like one of the rambling maths problems I end up doing on long journeys to keep my brain awake. That one would probably last me most of the way to Scotland... ... I'm crap at mental arithmatic.

28-07-06, 12:41 PM	#4
jonboy Guest Posts: n/a	I thought all that was totally obvious? .

28-07-06, 06:20 PM	#9
weazelz Guest Posts: n/a	why does your spedo drive come from the rear wheel?