hi,

I recently replaced my 32:10 to a 32:11 rear drive. I have also replaced instrument cluster with one from a 1992 K75. So I now have a working odometer and can finaly calculate my fuel consumption. The calculated result seems too high I get slightly less then 13.6 KPL (34.3MPG) with mostly city driving 15.8 KPL (37.1 MPG) with fun driving outside of city (many twisting roads, speeds changing from 80-140KPH).

Do you agree that this is too high? so my 1st Q is if this is real or perhaps an artifact of "new" final drive?
If not an artifact then what can be done to improve this? - any adjustments I can play with?
I should add that that naked bike likely has more air drag and that I use the most common fuel available - the 95 octane kind.
perhaps others can comment on what they get per what kind of driving? If enough respond we can get some meaningful statistic.

Thank you,
Roy

There should not be a difference with the change of the final drive, but there will be one if the tyre sizes has been changed from original. Basically any different diameter than original will give you a wrong indication for speed/distance. Easy ways to check is driving a known distance (lots of countries have road markings at regular intervals) or use a SatNav/Bike computer to compare with. You could even drive a distance with a friend and compare distance on both bikes.
There are quite different fuel figures posted, but a lot has to do with a bike that is well adjusted or not. For instance a TPS that isn't set up right can result in a high consumption or a slightly off-set coolingwater temp signal (thee is more!).
I would first make sure the speedo indicates the right speed/distance.

Hi Rene,

Wheel /tyre are same and std - speed agrees with theoretical speed as per http://cid-a5226878da38d80a.skydrive.live.com/self.aspx/K100%20shared/Documents/Speed%20to%20RPM%20Calculator%203.0.xlsx
So I will than assume odo signal is correct as well.

In the past I have mistakenly (ok...stupidely) touched the adjustment in the air volume meter - can that make a difference? which way should I turn it to make mixture LESS rich?

tnx
Roy

Hi Roy, come to think of it you still might have a non-original speedy. Various models were fitted with different rear tires and the adjustment was made in the speedo, so I would always check it anyway.
Re making the mixture less rich, think of it like this - the further you turn in the bolt you will close the opening that bypasses the airmassmeter. This will then ensure all air goes through the meter and fuel mixture is accordingly. The more air that bypasses the AMM the more lean the mixture. To answer your question directly - turn it out (anti-clockwise) to get the mixture less rich.
Just a note of caution - if a bike runs too lean you will get more expensive damages than when it runs too rich!
On an American site is a method explained how to more or less set it if you don't have a CO measuring possibility, but I haven't got the address handy. If you do a search on 'how to k100' I think you get all the info from that site.

Hi everyone
ReneZ you are incorrect to suggest the tyres influence speedometer readings.
there are different instrument clusters for different final drive ratios. The speed is measured by the speedometer sensor which is attached to the final drive. there is a component inside the final drive unit which revolves and allows the speedometer sensor to register a signal.
changing tyres , in particular the diameter of a tyre has no effect on the speedometer, because the final drive and it's components is what the sensor depends on for speed calculations. when you change a tyre, you don't change the final drive. difference in diameter when changing tyres is minimal.
if you decide to change your final drive it's important that you also install the correct instrumment cluster.
Motobins catalogue list a few of the different types of instrument clusters (speedometers) available, depending on the final drive ratio.
Good luck!

Gabriel, understand what you say, but don't know if you're correct. The signal is generated by a ring with gaps in its circumference that passes underneath a proximity sensor (like a Hall sensor) and every revolution of the outgoing shaft of the FD it gives a certain amount of pulses which are being translated in the speedo unit to a certain speed. Therefore the FD ratio is not really important for the amount of pulses per revolution. What I mean is that a different FD ratio will give you a different amount of revolutions of the outgoing shaft relative to the ingoing shaft.
AFAIK all FD's us the same gapped (?) ring and therefore the same amount of pulses per revolution of the outgoing shaft. The speed is directly linked to the circumference of the rear wheel. So fitting a different tyre (120/80 instead of 130/90) will give you a different speed reading.
The Germans on the Flyingbrick site made a little chip-card that could be fitted in the unit so it could be adjusted if you fitted a different tyre. I don't know if they are still available??

Rene, I was wrong to state that the tyre size does not influence speedometer readings. I also think you are incorrrect to suggest that the final drive ratio does not affect the speedometer. speed is measured taking into account differential ratios (final drive) and the circumference of the tyre. all tyres of the same size have the same circumference, example, 130/90. don't know why you would fit a different size tyre.
to answer Roys question, changing the final drive ratio affects the speedo. bmw recommends the correct size tyre for it's motorcycles. therefore, the only variable is the final drive.
it is important that you check your speedo to ensure you have the correct one for your particular final drive.Motobins catalogue lists the available speedos and final drive ratios.
Roy I suggest you check your speedometer to ensure it's the correct unit for your final drive otherwise you
will have incorrect speed calculations

hope this helps.

Gabriel, you are incorrect re FD. The speed signal comes from the outgoing shaft of the FD, therefore whatever happens before the FD or the FD gear ratio itself is not relevant for the speed signal. Off course all the ratios between the ending and the outgoing shaft of the FD are relevant for how fast the bike can go, but are all irelevant for the actual 'speed signal'.
Re Tire size; some wanteed as wide a tyre as possible. Also later K's were fitted with other tyres (i.e. 120/80) from original and that's why there are different speedo's. (I'm not sure but think the original K75's with the brakes before the disk brake might have had another circumference as well).
I had a look at the MotoBins site, but they don't list speedo's with FD sizes, but I could have missed that. Could you list the link? Your story could only be right if there was still a ratio applied after the speed signal was obtained, which is not the case (AFAIK).

I think I have been quite clear that the FD does not affect the speedo. If you think different, get us some technical proof. Happy to be shown wrong if it helps others, but I'm quite certain.

If I may give my input, the final drive ratio DOESN'T affect the speedometer.
The pickup sensor for the speedo is on the outside diameter of the crown gear (the big gear). The crown gear is rotating at the same rpm as the back wheel.
Even if you change the differential and replace it with another ratio, the pickup sensor will still count the rotation of the wheel per minute. The speedometer is a tachometer where the marking is km or mph in stead of RPM.
The differential ratio is changed by modifying the number of teeth on the pinion and crown gear.
Early K75 with drum brake have an 18" wheel. The K100 as a 17". This is why BMW list different speedometers.
Yes the size of the tire will affect the accuracy of the speedo.
What 130/90 means?
the width of the tire is 130mm and the height of the tire is 90% of the width or 117mm. Now, a 17" wheel with this type of tire have a circumference of: ((17 x 25.4) + (117 x2)) x 3,14 = 2090.612mm.

If we take another tire as for example 140/90 the height of the tire will be 126mm.
On the same 17" wheel the circumference will be:
((17 x 25.4) + (126 x2)) x 3,14 =2147.132mm

You will travel an extra 57mm per rotation of the tire and the speedometer will not correct it.
Imagine the difference if you replace a 17" wheel with an 18"

Don't ask me why the tire sizes is a mix of inches and millimeters

Bert

Drum brake - Jeez couldn't remember the word! Wife no help as wel.

Re tyre sizes - "If you can't convince them, confuse them!"

Don't you have a drum brake on your CB750K?
Mine had one on the back.
Today I was at a rally in Digby (NS) and it was at least 15,000 bikes. With a friend we were commenting that we didn't see any CB750 or CB500. The only old Honda that we saw was a 500CX (V-Twin).



I had the opportunity to seat on a Ducati 750ss (had one when I was a Duke bike dealer in France)



I saw few interesting bikes:

That must have been a great day! Indeed the Honda has a drum brake aft.



Here it is, before I started to work on it, in good old NS.

Here is a drawing of a FD, which I hope helps to clarify.



Who ever has it, please send us some sun? Nice please???

Given the huge amount of twiddling that went on with Roy's bike plus the attentions of the guy Roy called the Ogre? I think it would be a good idea to get a gas analyser into his exhaust which can be done at any garage that has one it does not have to be a bike garage. Next check is to test the sensor with a soldering iron held next to the pick-up to see what the speedo reads.
Regards,
K-BIKE

hi everyone.
i am confused. the Motobins catalogue states that for crown and pinion gear sets 1970-84 ''crown and pinion gears are only available in matcheed sets. ratios are stamped near the driveshaft oil filler and are also indicated by the speedometer code. all gear sets can be interchanged to alter final gearing but if doing so you should also make a corresponding change to the speedometer."
The motobins catalogue lists two different speedos for the r100/7, 34.11 and 33.11. "the correct speedo for your bmw is defined by giving the final drive ratoi stamped near the fill plug of your rear bevel drive unit."
can somebody please clarify this situation. is there a bmw engineer who can give us an explanation for this situation?
Thanks.
i think that the final drive does not affect the speedometer, but don't understand why there are different speedos for different final drive ratios. rpm?

Mate, we're discussing K's here, not R-types, I thought. Why are you looking at 'R' information for your 'K'? I have no idea where an R-type gets its speed signal from.

Hi Renez and hopefully this will give a clue and put it to rest. The early R series do not have an electronic pick-up and from memory the speedos on mine were gear driven off the rear of the gearbox.

Regards,

Phil