Speed / Revs

[grant81rs]

Gidday,

When cruising at 110kph on the speedo, the 78 RS shows 4500rpms on the tacho, is this normal?

The numbers stamped on the final drive are 33/11

It has been a while since a rode one of these earlier bikes and I cannot remember what my 81 RS used to do

Any thoughts!!

[SteveD]

100kph is around 3750 rpm Grant. 110 would be 4100-4150 or thereabouts I expect. 4500rpm might be more like 120. Careful! Maybe take your gps taped to the tacho and do a side by side comparison?
The 110kph showing on the speedo relates to the 33:11 and the W= 0.691 (?) number on your speedometer dial. This pic shows a number from a NA mph bike.

[barryh]

Here's the theory assuming standard tires.


https://www.dropbox.com/s/nelxh7v9e5jj0 ... s.png?dl=0

[Kurt in S.A.]

I agree with SteveD...I would be running around 75mph at 4500 RPMs on my /7 with a 33/11 FD and the 1.112 speedo (MPH) or 0.691 (KPH). Anton Largiader has a gearing comparison chart on his website:

http://www.largiader.com/articles/gearing/

It doesn't seem to work all that lately...maybe some of my software programs aren't enabled.

Kurt in S.A.

[grant81rs]

100kph is around 3750 rpm Grant. 110 would be 4100-4150 or thereabouts I expect. 4500rpm might be more like 120. Careful! Maybe take your gps taped to the tacho and do a side by side comparison?
The 110kph showing on the speedo relates to the 33:11 and the W= 0.691 (?) number on your speedometer dial. This pic shows a number from a NA mph bike.

The numbers/code on the bottom of my speedo is W-711 ??

[ME 109]

With a tall fifth, my RS runs 125 kph (indicated) for 4500 rpm.

[grant81rs]

Gidday,

Jeff, not to long ago I opened the speedo and fixed the trip meter which was not working, it now is but maybe, just maybe my clumsy big fingers refitted something wrong but I marked and took pictures as I went..

ya fix one thing and something else stuffs up!!

[SteveD]

These numbers are taken from Snowbums page http://bmwmotorcycletech.info/ringgears.htm Scroll half way down the page to the graph.
Grants 33:11 should be mated to a 0.691 speedo ratio. He has a 0.711 though.

In Oz if the speedo ratio number is 0.711.(1.144 in NA mph) it matches with FD stamp of 34/11.
At 55mph or 88kph rpm should be 3241.
At 70mph or 112kph rpm should be 4125.
At 122mph or 195.2 kph rpm should be 7200.

Grant will want to know if his setup shows a higher speed or a lower speed. His currently shows 4500rpm at 110kph.
Grant, test it again tomorrow and take the gps. Be certain you're in top (5th gear) when working it out. Your current numbers make more sense for 4 th gear at 4500, but that might also be a function of the mismatched ratios.

[grant81rs]

Gidday,

I just ran a test along the Hwy with the GPS on the tank,

Speedo is slightly lagging behind the actually speed of the GPS by about 5kms in the lower 40, 50 60 kph range

At 110kph on the speedo, the GPS shows 112kph @4500rpms - is this normal or not??

Any thoughts and the bike is a 6th Month 1978 R100RS

[Rob]

Gidday,

I just ran a test along the Hwy with the GPS on the tank,

Speedo is slightly lagging behind the actually speed of the GPS by about 5kms in the lower 40, 50 60 kph range

At 110kph on the speedo, the GPS shows 112kph @4500rpms - is this normal or not??

Any thoughts and the bike is a 6th Month 1978 R100RS

I always thought the civilian speedometers were on the optimistic side.
That is, they would show a reading higher than actual speed.
I think this is true of most manufacturer vehicles.