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TECHNICAL MOTOBRICK WRENCHING In Remembrance of Inge K. => The Motobrick Workshop => Topic started by: scoobs on July 05, 2014, 11:15:29 AM
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Does anyone have a link to a chart comparing mph with rpm for a '92 K75RT ? I've got one for my GS that I downloaded from another site but can't find one for the k bike.
I've just installed a replacement engine and haven't ridden it for a couple of months- it feels like it should be going faster at the displayed rpm...
5000 rpm in top gear gives me around 60 mph
6000 rpm in top gear gives me around 70 mph,
does this sound about right ?
The situation is complicated further by my swapping back and forward between the K bike and my F650GS and they have completely different characters.
Cheers,
Ian.
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BMW's seem to be notorious for Speedometer error, usually reads 2-3 mph higher than what you are actually going. Yours seems to be off by about 10 mph.
I get around 70 mph at 5000 rpm on my 95 K75RT. Close to 80 at 6000 rpm. That's using a GPS to measure actual speed.
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http://www.largiader.com/articles/gearing/ (http://www.largiader.com/articles/gearing/)
This gives graphs that shows speeds at 1200 and 5000 RPMs for any given final drive ratio. Applies to K-bikes as well as R-bikes.
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I know that my speedometer reads about 3 mph lower than actual speed by comparing it with my GPS, my concern is that the replacement engine I've installed is not giving me as much top end power compared to the original unit. Unfortunately it's been about 3 months since I ran the old engine and I can't remember how it felt in top gear at a certain rpm. I feel it should be running faster than it is at 6000 rpm. Does that make sense ?
I'm taking it for another test ride this morning and will report back with more details.
Thanks,
Ian.
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I know that my speedometer reads about 3 mph lower than actual speed by comparing it with my GPS, my concern is that the replacement engine I've installed is not giving me as much top end power compared to the original unit. Unfortunately it's been about 3 months since I ran the old engine and I can't remember how it felt in top gear at a certain rpm. I feel it should be running faster than it is at 6000 rpm. Does that make sense ?
I'm taking it for another test ride this morning and will report back with more details.
Thanks,
Ian.
They are usually proportional. About 10% fast. So 80mph indicated is 72mph.
The butt dyno is notoriously inaccurate. Why did you need to put a new engine in?
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The original engine (135k miles) was losing compression on number 3 cylinder and had 30% leakdown into the crankcase, probably due to bad rings but I haven't dismantled it for examination yet. After doing the maths it was cheaper to install a used one than to do a rebuild- I'm looking forward to stripping the old one down to find out what the problem was.
I've replaced the fuel filter since my previous test ride and today's run seemed a lot smoother with better oomph at the top end. The approximate speed (GPS not speedometer) versus rpm in top gear came out as follows:
40 mph 3750 rpm
45 mph 4000 rpm
50 mph 4200 rpm
60 mph 5000 rpm
70 mph 5900 rpm
75 mph 6250 rpm
80 mph 6500 rpm
These figures seem reasonable, I did not go much faster as there were a lot of LEO's on the interstate this morning, if anyone would care to take their bike for a ride and compare these values I'd be grateful to see how things match up. The bike was a lot more responsive with the new fuel filter, which may have been the original unit- I've put 65k miles on the bike and hadn't changed it during that time.... but I'll pay closer attention in future.
Cheers,
Ian.
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I've seen a few of these 'rpm vs mph' threads recently in various places - every one seems to imply that it's a variable relationship that can be affected by how the engine is running...
It is not variable. Not on the same bike (whether you change the engine or not), not on different models of bike with the same spec and tyre wear and pressure.
It doesn't matter if your engine is running at absolute peak performance or whether it is running on one cylinder with no oil in - the rpm will ALWAYS be the same for a given wheel rotational speed in the same gear. It also doen't matter what fairing you have, what handlebars you have, how much you ate the night before or whether there are 1 or 7 people on the bike (see caveats at the end).
Basics....
The engine drives the gearbox input shaft through the clutch, this is a direct 1:1 drive.
Then the gearbox has a set of absolutely fixed ratios, let's use the 1.67:1 reduction of the K75 top gear for this.
The output from the gearbox is transmitted through the shaft to the final drive unit - this is again a direct 1:1 drive.
The final drive has another reduction - let's assume a 10/32 FD, or 3.2:1.
So, at 1000rpm engine speed:
Gearbox input shaft = 1000rpm
Gearbox output shaft/propshaft = 598.8rpm
FD output/rear wheel = 187.1rpm
This relationship cannot vary - 2k engine rpm = 374.2 wheel rpm / 7k engine rpm = 1309.7 wheel rpm. As I said above, it matters not a jot whether your engine is shoving out 371bhp running with nitrous or whether it's struggling to make 7bhp on one cylinder.
So now we get to speed... We have the wheel turning at 1309.7rpm (7k engine rpm in top) and a 130/70-17 lasertec tyre fitted, diameter 613mm/1925mm circumference. At this rpm you will be travelling 2521172.5mm every minute. That's ~94mph...
So, make a 'chart' off these figures.
1k rpm = 13.5mph
2k rpm = 27mph
3k rpm = 40.5mph
4k rpm = 54mph
5k rpm = 67.5mph
6k rpm = 81mph
This won't be 100% accurate - it will be affected by rounding errors plus the fact I'm working off a manufacturer published tyre specification, hardly a real-world measurement.
As always - caveats apply:
If the shaft splines are worn enough to skip or the clutch is slipping then you are inadvertantly altering one of the physical components in the same way as swapping out the FD for a different ratio.
Actual road speed (real speed, not what the speedo says) can be affected by the following:
Tyre wear - more wear = less speed
Tyre pressure - lower pressure = less speed
Tyre/wheel size - smaller tyre = less speed
Enough load to compress the tyre therefore affecting it's rolling diameter - more load = less speed
It's also worth noting that the tyre diameter can change with speed - the faster you go the bigger the tyre effectively is due to centrifugal force pushing the middle of the tyre away from the spindle/axle.
If anyone would like to dispute this (the theory that is, I'm open to the end result figures being slightly off) then I'd love to hear from you.
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So, make a 'chart' off these figures.
1k rpm = 13.5mph
2k rpm = 27mph
3k rpm = 40.5mph
4k rpm = 54mph
5k rpm = 67.5mph
6k rpm = 81mph
PDG's calculations are damn close to my real life experience. Enough so that I think his calculations are probably more accurate than my GPS. So, there's your requested chart.
Regarding the speedo error being proportional, I found that mine is a fairly constant error - off 2-3 mph (reads too high) at lower speeds and 3-4 mph off at speeds above 50 mph.
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pdg's logic is inescapable - nice job. As to what the speedometer actually indicates - very little logic on these bikes. Mine reads different values for a particular rpm depending on whether I have accelerated up to that point or decelerated down to it :dunno2: !
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Of course, that 'chart' is only valid if you have a standard K75 gearbox and a 10/32 final drive with a brand new 130/70-17 lasertec at it's design diameter...
It's nowhere near right for my particular K75S - I've got an 11/32 final drive running a 150/70-18 and it'll be even further out if/when I get the k11 gearbox in it (and don't even mention a paralever final changing it some more :hehehe )
That said - if anyone wants the same thing for their application I'd be happy to work it out if the follwing information is provided:
Gearbox ratios (or from which bike it was derived)
FD ratio (monolever usually has it stamped on the case - i.e. 10/32)
Rear tyre make, model and size (or, more accurately, the measurement from the centre of the rear axle to the level ground with the bike perfectly vertical and normally loaded...)
Working out this sort of 'rubbish' makes a nice change to my day job as a gardener :2thumbup:
As for the tacho and/or speedo giving inaccurate or variable readings, there are a few reasons for that and most of them are age related - fixable though, usually just got to strip, clean, lube and rebuild the instrument head, sometimes renew a few components - while you're in there you might as well calibrate the speedo and swap the backlighting for pretty coloured LEDs too.....
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PDG's numbers are spot on. I checked my speed today with the GPS and at 5k rpm I am doing 68mph(Garmin rounds to nearest mph) and at 5500 rpm am doing 74mph. Not sure how accurate the tach is, but it looks like about 13.5mph per 1000 rpm.