I have had my 85 k100rt for about a year. On the interstate at about 70 mph the engine is turning around 4300 to 4500 rpm. Is this the norm? I feel like I need another gear.

My '84 RT does about 3800 rpm at 100k's or about 62 MPH so your's sounds about right. I have not done the maths for this in particular but if you work out the circumference of your rear wheel and multiply through by the appropriate ratios and rpm that might give you a rough idea if your speed is accurate. It is interesting though that the difference between 4th and 5th seems only about 500 revs. Also if you are lucky enough for the windscreen to be of any use and there is little wind noise it does seem like another gear is needed.

Regards,

Phil Mars

A footnote to the above post as I did a quick calculation after a rough measurement of my back tyre at 650 mm diameter translates to 98k's at 4000rpm so if you took into the fact the tyre expands a little as it gets hotter and a bit of centrifugal force (educated guesswork) it is about right. Anybody wishing to prove or disprove please feel free as I consider all theories are good until proven otherwise.

Regards,

Phil

i agree that the k100 feels like it needs another gear. However once you get above 4800 rpm or so, it smooths out like magic and that is where the torque sweet spot begins and the engine becomes silky smooth. you can zip from 70-100mph with no effort at all. and for some reason my suspension feels much better at 90mph than 60mph. also the fairing works better at 90mph and not so great at lower speeds.

Did anybody ever noticed that when in 5th gear, the Needle on the spedo moves at the same angle as the rpm?
This is on a speedometer in KM/h, but I don't know if it works with the speedo in miles.

Scottiesharpe,
Is so right, once one gets above that engine speed on the RS everything smooths out and as he says everything gets smooth.

Bearing in mind the public nature of the internet my comments refer to when I went last weekend with my bike to a section of German Autobahn where there are no official speed limits. (I would never exceed the speed limit in NZ), the bike is so smooth at speed, the fairing is so so good.

The effort they put in during development at the Pinin Farina wind tunnel was just amazing, the RS fairing looks deceptively simple but the way the air slips past at high speed is nothing short of engineering brilliance.

Also as our dear leader Crazy Frog says, the way the needles on the speedo move together to me at least is very aesthetically pleasing.
Regards,
K-BIKE

Hi,

If not too much trouble can someone demo this calculation end to end, from RPM, thru gears, final ratio and wheel circumference?

Thank you,
Roy

Roy, as I mentioned mine is more educated guesswork but it seems to be pretty close and is open to discussion.

Basically it started out as rule of thumb as a colleague has a very steep driveway with speed humps! and his R1200ST tends to stall if he has to slow down half way up so it was a bit of fun to work out which would be a more suitable mount. My K100 wasn't much joy either.

Anyway I mentioned above that my back wheel has a diameter of 650mm and therefore a circumference (multiply by PI) of 2.042M.

To work backwards if engine rpm is 4000 divide by primary drive or in this case by the 5th gear ratio of 1.666 (as it includes primary reduction ratio) which gives 2400.96.

Divide that by the final drive ratio. In my case 2.91 = 825

Multiply by the rear wheel circumference = 1684.650 M/min

Divide by 1000 to give KM and multiply by 60 and you have 101.1km/h

The only change in the calculation is if you have a separate primary gear ratio which then becomes the first division.

Hope this helps,

Phil

I have a similar question. Using the above math, my indicated speed is considerably different from the calculated speed.

The speedometer is correct (+/- 1 mph) at 40 mph.

The following speedometer / tachometer reading are provided:
70 mph - 5600
60 mph - 4600
50 mph - 3600
40 mph - 2900

Above engine speeds are within 100 rpm. Also, tachometer is not a sophisticated calibrated instrument, rather just the bike’s tach.

Bike is ‘87 RT with standard gearing.

Given the above numbers, the indicated speed is considerably different from the calculated speed.

Hopefully someone can shed some light on what is happening.

The clutch and rear seal were replaced ~20,000 miles ago, clutch is adjusted properly, and no apparent oil leak (at or near location of rear seal).

davemadse wrote:Bike is ‘87 RT with standard gearing.

You never know what you got with a used moto until you check what's stamped on the final drive. What's the number there?

Unless the number is on the side next to the brake rotor, I don’t see any markings at all.

It should be on the upper surface of the final drive near the edge where it joins the shaft drive housing.  

A number like 32/10.

davemadsen wrote:Unless the number is on the side next to the brake rotor, I don’t see any markings at all.

It's stamped on the topside of the rear drive housing extension that is bolted to the swing arm—i.e., where Tackler indicated.

Of course you are correct: 32/11

My final drive ratio is the same as yours, but at 100km/h (60mph) it's at 4,000RPM, and the needles are parallel. I think all manufacturers calibrate their speedos conservatively. There's also the tyre circumference to think about.

Tire is a 130/90 - 17
I considered my speedometer may be reading slow at 60 mph and faster, but dismissed this when I consider traffic flow which moves around 65 mph - not 85 mph - in our area.

I considered it might be a slipping clutch but at speed, releasing the throttle to the closed position should immediately bring the engine speed in sync with wheel speed. When I do this, there is no abrupt decrease in engine speed indicating (to me) there is no slipping of the clutch. Also, I would think clutch slippage - if present - would be at higher speeds, yet the rpm/wheel speed ratio is consistent 35 mph through 75 mph.

davemadsen wrote: Also, I would think clutch slippage - if present - . . .

One way to detect a slipping clutch is to cruise at 50mph then whack the throttle wide open. It'll usually slip then if there's a problem.

Ah yes, that takes me back to the days of the leaking rear seal. But alas, today there is no slippage to explain the higher engine speed.

Has to be clutch slip. Re-check the clutch cable and make sure it is set according to the book. I had one set slightly tight and the only indication was - as you - that the RPM was a bit higher than expected. Otherwise, I couldn't feel any real difference.

I checked the clutch setting before I made my initial post. I will back off the adjusting bolt at the back of the motor and insure the clutch handle has a bit more free play and report back.

I adjusted clutch...maybe even a bit lose. No appreciable change.

With the bike on the center stand and bike in 5th gear, I recorded virtually the same engine speed/real wheel speed as when driving. This exercise eliminates any clutch slippage given the bike is not pushing any load or incurring any wind resistance.

At this point, I seem to be dealing with either a gross speedometer error (although as stated previously, I have some confidence that at 40 mph the speedometer is within +/- 1 mph), or something going on with the tachometer (yet the tachometer needle moves smoothly at all speeds).

Can you postulate on another scenario which would explain this mismatch between rear wheel speed and engine speed?

Somebody's put a needle back on in the wrong place? Not joking. Do the needles rise 'in sync' in top gear?

davemadsen wrote:This exercise eliminates any clutch slippage given the bike is not pushing any load or incurring any wind resistance.

. . . Can you postulate of another scenario which would explain this mismatch between rear wheel speed and engine speed?

I'm not convinced that your clutch is ok.   Regarding other scenarios, is there an outside possibility that you've miscalculated?

Anyway, have you explored the use of this tool?

To correct my earlier post, 100km/h is about 62mph. Add to this a typical conservative factory setting. If your speedo has been calibrated for accuracy, there is possibly nothing wrong with it except that it is accurate.

Maybe check it against some GPS systems.

Dai wrote:Somebody's put a needle back on in the wrong place? Not joking. Do the needles rise 'in sync' in top gear?

The tach has never been apart in the past 15 years and this is a situation which i started noticing a year ago. O understand the “needles moving in sync”; in the past, the needles did exactly that.

Laitch wrote:

davemadsen wrote:This exercise eliminates any clutch slippage given the bike is not pushing any load or incurring any wind resistance.

. . . Can you postulate of another scenario which would explain this mismatch between rear wheel speed and engine speed?

I'm not convinced that your clutch is ok.   Regarding other scenarios, is there an outside possibility that you've miscalculated?

Anyway, have you explored the use of this tool?

I had my wife record the data and she is more anal than I am. Actually the data she recorded with the bike on the center stand is the same as I found while driving.

Re clutch: I’m with you on this and is one of the reasons I continue use to pursue this. This phenomenon started about a year ago. I am reluctant to split the bike in half again just to assess what is happening with the clutch. I thought with the bike on the center stand, any difference in engine speed due to slippage would have been nil.

Re: tool. I have not used it. I did graph my data points and the curves are different (duh!). I have used the calculation based on rpm/FD/gear ratio/wheel circumference and that gives me yet again another number. At this point I have a lot of numbers but no solution

davemadsen wrote:. . . and she is more anal than I am.. . .  At this point I have a lot of numbers but no solution.

That's a heck of a competition you two have going there. Social isolation has spurred a lot of family recreational innovation. Don't pull any muscles.

When's the last time you gave your moto a complete tuneup—the works, including a compression test and smudging—then went for a two-up ride into the sunset?

Crazy Frog wrote:Did anybody ever noticed that when in 5th gear, the Needle on the spedo moves at the same angle as the rpm?
This is on a speedometer in KM/h, but I don't know if it works with the speedo in miles.

All my cars have done the same......

Before you go further tell us the numbers stamped on your final drive. They can be 32/10, 33/11 32/11 or 31/11. 32/10 is for K75, the others are K100. They are interchangeable so you could have a different one installed at some point.

92KK 84WW Olaf wrote:Before you go further . . .

The Grand Reveal was in Reply #14. I don't think there was adequate fanfare.

Laitch wrote:

92KK 84WW Olaf wrote:Before you go further . . .

The Grand Reveal was in Reply #14. I don't think there was adequate fanfare.

I found it, 32/11. Correct for the bike in question.

But I must add, my 84 K100RT came having original clocks. But they started failing and one of the fails being that it started reading slow speed, noticed because the revs appeared higher than speed suggested and there are these road speed limit signs here that have speed indicators on them so very easy to check.

Replaced the rear sensor and made no difference and then got a replacement clocks and all solved.

As an aside the clocks read 62mph/100kmh. But checking against gps I found clocks read 70mph to an actual 100kph. Funny thing my Rover car [mostly BMW] also reads 70mph against actual 100/kmh off gps.

This is the point I was trying to make but didn't succeed too well. With the needles parallel and the speedo at 70mph,  the tacho would be in the ball park of 4600rpm. I should check mine with gps. I'll try to get it working on my phone.

10/86 K100RS with 31:11 or 2.81 ratio final drive after calibration using Karamba software, 120 km/h being a red pubic hair over 74 mph. Piccie taken near Lake Argyle in Western Australia, 17 September 2010.

I hooked up an inductive tachometer and if it is to believed, my issue is with the bike’s tachometer which is reading much faster (20%) than the inductive tachometer.

In a possibly unrelated issue, I have recently (last couple of months) noticed both the odometer and tripometer are unreliable - although the speedometer seems to work fine. The odometer gears were replaced a couple of years ago. The odometer and tripometer work intermittently. I am not that familiar with what happens behind the glass of the instrument panel.

Well, there's fourteen screws between you and finding out...

Dai wrote:Well, there's fourteen screws between you and finding out...

I usually misplace that many screws on a typical project!

Epilogue:
If found the potentiometer for the tachometer and lowered the resistance from ~22 to ~13 and now the speedo and tach track each other.

Yes, but do they indicate the correct speed?


 

(Runs away)

davemadsen wrote:If found the potentiometer for the tachometer and lowered the resistance from ~22 to ~13 and now the speedo and tach track each other.

Where is the potentiometer located and how is the resistance lowered?

Sorry, no picture. There is only a single potentiometer in the tach cluster and the pod is clearly visible - and accessible - without separating the odometer wheels from the assembly. It is located at the edge of the various circuits and components. With a small screwdriver resistant is lowered by turning the pod clockwise. The terminals of the pod are accessible, so I could put a voltmeter to the pod to view the changing resistance in real time. 

I had a donor instrument cluster whose tachometer was correct and I measured the resistance of the donor pod and found it to be 13.4 ohms. My tachometer - which reflected a too high of a value - showed 22 ohms. Lowering the resistance to ~13 ohms did the trick.

Here are views from above the tachometer of the pot adjuster (at black arrow) and of the points to which the multimeter is attached for testing (red arrows). In the second image, the tachometer needle is visible on the left.