Tyre Circumferences

[Grumpy Lee]

Could anybody out there let me know what the tyre circumferences are on the R1100 17" front wheel and the R1150 19" front wheel?

Thanks in advance

Lee

 

[richie]

Lee, put a dot of water on the tyre and wheel it along until you get two dots. Then measure the distance with a tape measure.

Are you setting up a push bike puter...

 

[Grumpy Lee]

Lee, put a dot of water on the tyre and wheel it along until you get two dots. Then measure the distance with a tape measure.

Are you setting up a push bike puter...

No, I've got 15" wheels on the bike and consequently the speedo runs a bit fast i.e. if the bike shows I've done 100 miles the following car had only done 83 miles.

The reason I'm asking is that I see there is a lower (or maybe higher - I get confused easily ) ratio speedo drive that is used on the 17" cast wheels. So if the circumference measurements are a lot nearer (and the speedo drive will fit) I might give it a go. Also it is a lot cheaper option that having a little gear box made up to go in-line (£100ish)

Lee

 

[Kropotkin]

Surely the simplest and quickest solution is a pushbike computer?

 

[Grumpy Lee]

Surely the simplest and quickest solution is a pushbike computer?

Speedo drive approx £29, not sure of pushbike 'pute about £25 I think - but then more wires another thing stuck on the bike & wheel , another clock to mount.

Mind you with that lump I've got stuck on the side

 

[Global Rider]

I started with the tire valve at the bottom, marked the road with a chalk line and then rolled the bike forward till the tire did 10 revolutions and marked another chalk line. The distance between the two chalk lines, divided by 10 is your circumference.

Yeah, I know the UK is pretty small, so just do 5 revolutions of your tire.

 

[Spout]

Lee,

I was expecting to see your bike in the "Enduro Sidecars" in this months TBM. Where was it hiding?

 

[LOLGEOFF]

I started with the tire valve at the bottom, marked the road with a chalk line and then rolled the bike forward till the tire did 10 revolutions and marked another chalk line. The distance between the two chalk lines, divided by 10 is your circumference.

Yeah, I know the UK is pretty small, so just do 5 revolutions of your tire.

Why not just measure it after 1 revolution? The country might be small but our brain power isn't

Note added smilie to stop an international incident

 

[richie]

Actually a pretty good idea. More acurate by distance. Easy to lose a CM with only one revolution.

Sure you figured that out by yourself though

 

[cookie]

er...why not just run a tape round the wheel?

is that too easy?

edit: hang on, i've got it now. you want to account for the reduction caused by the contact patch.

better sit on the bike too while rolling

 

[Grumpy Lee]

In case it has not occurred to all you very nice people out there, I do not have these wheels or I would have done all of this myself Indeed as I have done with the 15" one on the bike.

I was hoping a very nice person (or two - maybe one for each wheel) would be able to measure a wheel (or two) for me

I can then amuse myself with all sorts of calculations

Lee

 

[richie]

Doh,

I'd measure mine but it's drizzling and I can't be bothered...

Sooooooorrrrrrrryyyyyyy.....

 

[Paul Young]

R1100R wheel with 120/70 R17 tyre has a diameter of:

431.8mm (17"x25.4mm) plus 2x (120mm x 70%) = 599.8mm
so, the circumference is Pi (3.14159) x 599.8 = 1884.33mm

R1150GS wheel with 110/80 R19 tyre has a diameter of:

482.6mm (19"x25.4mm) plus 2x (110mm x 80%) = 658.6mm
so, the circumference is Pi (3.14159) x 658.6 = 2069.05mm

The smaller wheel has 91% of the circumference of the larger wheel so you need a speedo drive with the same reduction. The R1100R drive has a 2.6:1 ratio whereas the R1150GS is a 3:1 ratio but overreads by about 7-9%. Get the R1100R one & you will be more accurate.......

HTH,

 

[Grumpy Lee]

Just in case anybody is at all interested, fitted a 2.6 speedo drive today in place of the 3.0 one (eventually - why do I always need three hours to do a one hour job? Does everything only go wrong for me - or does it just seem that way?)

Been out for a drive round and according to the GPS, the speedo is now running at +5mph across the range and 4000rpm in E = 70mph (that should be handy to know if (when) the speedo fails

Thanks to everyone on various forums for their input (and of course Steptoe for the speedo drive )

Lee

 

[Mick_rw]

Could anybody out there let me know what the tyre circumferences are on the R1100 17" front wheel and the R1150 19" front wheel?

Thanks in advance

Lee

For any circle, the circumference is 3.141592....... x the diameter.