Drive Ratios for the RS.

[Jean]

When I ran a Metzler 120/90, I could not use 5th gear without severely lugging the engine. Revs dropped to mighty near 3K.
This with a 2,91 ratio rear end.
There were other things about the 120 I didn't like either...getting it OUT OF THE FRAME was a PITA.
Now running a Dunlop 110/90 and 5th gear = 3700 approximately at 100kph. Still a bit low for making the engine feel happy, I think. BUT good 4.00/18s are hard to come by now...
So, if you are running a *130 (!) on the back, and figuring a bit of slack for an optimistic speedo, you "could" be running close to 3K rpm and that is not good for your engine. Response is next to nothing from there and ,well, just not good...maybe OK going downhill!
(*This is the biggest tire size I've ever seen mounted on an Airhead...and I'd bet it rubbed the frame now and then)

[Chuey]

There were other things about the 120 I didn't like either...getting it OUT OF THE FRAME was a PITA.

I have a friend who uses a wider rear tire in the rear, says he really likes it. But then, he never works on his own bike. Also, I can't believe the restaurants he thinks have good food. I guess this falls under different strokes, but I can't feel any advantage at all to the wider tire. I like the stock size.

On my bike that is the same model as Sunbeem's except it is not a real Motorsport, I seldom run around 60mph. He's named The Captain and likes to run at least 85mph indicated. There's a 32/11 stamped on his rear drive.

Am I understanding this correctly? The 32/11 is slightly higher than the 33/11?

Chuey

Chuey

[Major Softie]

Am I understanding this correctly? The 32/11 is slightly higher than the 33/11?

Chuey

Yes.

The first (big) number is the ring gear: the driven gear attached to the rear wheel. The second (small) number is the pinion, the driving gear. This is EXACTLY the same as if you have an 11 tooth ring on your crank, and a 32 (or 33) ring on your cassette. You can always think of the first (big) number as your rear wheel (cassette) gear, and the second (small) number as your crank gear. As I know you are very aware, with the same gear on your crank, if you change to a smaller gear on your cassette, your gearing is higher. The situation is exactly the same on your motorcycle.

However, the difference is very slight. 2.9 vs. 3.0 is a difference of 3.3%. That means that at 5000 rpm, the 32/11 will slow the rpm 99 rpm.

[2valve]

hi jean, next time your looking for tires , have a look at the avonroadriders, comes in the correct tire sizes for the front and rear rims. i use to run a rear metz marathon 120 on my 90/6, had the rear rim offset ( the /6 was a wolf in sheep clothing , that thing was a missle , idiot me sold it ) anyway , i agree the 120 was a pain to remove , so i wouldnt go down that road again

[Sunbeem]

Sorry Jean, forgot to mention my tyres - Heidenau front and rear , (4 .00 / 18), and very satisfactory so far.

I'm not sure if the dual-plugging will make much difference, or whether you also have that mod, but at an indicated 3,000rpm, there's all the urge I need for the fairly small roads I usually frequent.
Perhaps that means the actual revs are higher than shown?

Having said that, I'm used to riding old single cylinder machines which demand gentle treatment of the valve train, so I tend to fret a bit at anything over 5,000, though I know she's well capable.
I like to drift along between acceleration and overrun, in just the way a two-stroke doesn't.

The speedo seems to match traffic speeds in 30 and 40mph zones, which is the only measure of its accuracy I have, and the bike will take off easily in second gear with minimal clutch slipping required.

So -- I'm thinking there is a higher-geared F/D which I could look out for, and that the tacho may be under-reading.

Any comparisons with my Transmission wear-test will be of interest, (as long as your results are worse than mine.)
Thanks again.

Sunbeem.

[Jean]

Went out today and did a "Scientific experiment" paying strict attention to the Tach AND the speedo.
Recall I have a 78 R100s, 2,91 FD....At 3000rpm, in 5th gear, I'm making very close to 80kph (50mph).
I like to run around 3900-4200 to get the best carb response...and fuel economy...
ANYHOW, from that reading, I'd say if you have a 3,00 FD at 60mph you ought to be turning a bit more than 3000rpm.
(say about 3700?)
It appears your tach is not doing well at higher rpms.

[Sunbeem]

"It appears your tach is not doing well at higher rpms."

Thanks for that confirmation Jean, it does seem to fit the clues -- I'll have to work out what it should be reading, and make some comparisons. Perhaps there's some extra Ohms in there that I can shake out somehow.

Anyone been into an electronic tacho ?

Sunbeem.

[Jean]

Sunbeam, it just might be in the pickup, not the instrument head.
I'll see what I can find out. (HOW it works, for one thing.)

[Jean]

Remind me what year your bike is...The tach for the R80GS gets it's signal from one of the coil primary terminals. Seems to work like a tach-dwell meter we use (used) on autos with no computers to run things.
If you can find such a wire and it's intact for its whole run, then the moving parts in the instrument head are suspect. Maybe some insect set-up housekeeping in there. Happens all the time.

[chasbmw]

Jean, the tacho pickup on the RS is same as on the GS. My multimeter has a tacho function, which
I sometimes use when adjusting the idle mixture. I reckon that my tacho is not that accurate, has 500 rpm swings. I think the only way to be absolutely sure what fd ratio you have is to turn the rear wheel whilst counting the turns of the drive shaft! It's a mechcanical thing!