Understanding the Mechanics

[drgibson]

Ok I have a question, What influences the ratio of RPMs to speed?

Here's the Deal looking back on a post I made right after “M” was broke in and what her speed was for specific gears and at specific RPMs. Here’s a quote.

Originally Posted by drgibson

Performance:

Here is what I can tell you Red Line is 6,000rpms. That being said;

First gear top speed 45mph

Second gear top speed 64mph

Third gear top speed 82mph

Forth gear top speed I may never know

Fifth gear top speed I’ve heard is about 128mph but I have no intention of finding out.

3,000rpms in fifth gear is 65mph

3,500rpms in fifth gear is 75mph

For control purposes we’ll use 75 mph in 5th.

After changing Pipes, adding the Air kit, and TFI controller the RPMs at 75 mph was right 4,000. Well when I removed the TFI controller a couple weeks ago I noticed that the RPMs at 75 mph had dropped back to 3,500.
(But wait there’s more)
After summerizing “M” which consisted of removing the windshield they dropped yet again, 3,300 = 75 mph. Now I can understand the TFI affecting the RPMs but the windshield?!
[FONT='Verdana','sans-serif']If anything I’d have thought that they would have gone up, ya know more wind resistance.[/font]

[GregR1]

it's ONLY the transmission / drivetrain gears, which you didn't modify. basic performance mods wouldn't change the rpms AT ALL. there is NO WAY for them to do that - UNLESS you have an automatic transmission with a torque converter which allows for an rpm differential.

most likely, you were checking stuff that wasn't at steady-state, but under acceleration. or, the tach was receiving a wrong signal, or there was some mild fluctuation / inaccuracy going on.

[drgibson]

Nope I always check it at a steady speed, at 75 I'm usually done accelerating, usually.
There is no fluctuation on the gauges either.

[Easy Rider]

Originally Posted by drgibson;332598[COLOR=black

After changing Pipes, adding the Air kit, and TFI controller the RPMs at 75 mph was right 4,000. Well when I removed the TFI controller a couple weeks ago I noticed that the RPMs at 75 mph had dropped back to 3,500.[/color]

Wild guess: The tach is "computer" controlled, meaning it is not directly connected to the actual engine speed like a mechanical one is.
The TFI controller changed the calibration of the electronic tach. and the actual engine speed never really changed.

Windshield off: Maybe there is a tiny air leak in the tach housing and the wind is blowing the needle down a bit.

[GregR1]

i'm guessing that the speedo and tach information are taken from the transmission points - one pickup for the tach (maybe) and one pickup for teh speedo. so, that's all 100% controlled by gearing within the tranny and nothing else.

it's possible that the tach isn't accurate at all, or rather not very repeatable, or the way the ECU is sending spark signals is making the tach interpret things differently.

it doesn't matter if you make 40hp or 2400hp, that bike with that tranny (assuming it lasts) is gonna go a certain speed at a certain engine rpm. hell, you could even change the front and rear tires and most likely the gauges won't show anything different at all

[Clint]

I agree fully with Greg, and I think ER's theory is plausible. It almost has to be an electronic error.

[drgibson]

hmmm food for thought. I do doubt that the wind is affecting the tach thou since it is almost vertical at that point, and it would fluctuate depending on wind speed.

[Mydlyfkryzis]

Or your clutch was slipping....

[Navy_p503]

The bandit has a speed sensor near the shift lever. Takes a reading directly off the output shaft. Runs the tach, the speedometer has an input from one of the ignition coils. Just take a look at the wiring diagram in your repair manual and see where the inputs for the tach and speedo come off of.

[Aussie Steve]

Don't forget to factor in wind and load for your calculations... (I'm not talking about farts either Landry) There are other variables also. As the engine gets older it makes less horse power so you loose some mph per rpm. This is not very much but adds up. Rider position, road conditions and air quality/temp also affect this as do the fuel quality.

[Navy_p503]

Aussie, I think you mean the amount of power required to get to the same speed. But the speed at which the engine turns to the speed of the bike doesn't change, the only thing that might change it would be if your clutch is slipping.

[Easy Rider]

Originally Posted by Navy_p503

But the speed at which the engine turns to the speed of the bike doesn't change, the only thing that might change it would be if your clutch is slipping.

on that.....

However:

"The bandit has a speed sensor near the shift lever. Takes a reading directly off the output shaft. Runs the tach, the speedometer has an input from one of the ignition coils. "

Isn't this backwards, that is, seems like the tach would trigger off the coil and speedo off of the output shaft. That's the way it works on other "vehicles".

[Navy_p503]

That's what I thought and it makes more sense, but unless the wiring diagram is wrong the coil signal goes directly to the speedo and the sensor goes to the tach. I'll double check and try to trace the wires out later.

[EDIT] I checked the diagram again, the coil signal does go to the speedo. Which still doesn't make too much sense. I'm gonna try and trace the wires tomorrow because now i'm intrigued

[Aussie Steve]

Ahh!!!! this is all too convoluted for me to think it through.

If an engine makes twenty horse power and the gearbox is setup to produce a certain ratio to the rear wheel this should equate to a certain rpm producing a certain speed.

"Engine speed will increase as horspower drops to provide the same amount of power to propel a vehicle, ( Brintsons 7th proponent of "power to propulsion" theory)

[Easy Rider]

Originally Posted by Aussie Steve

"Engine speed will increase as horspower drops to provide the same amount of power to propel a vehicle, ( Brintsons 7th proponent of "power to propulsion" theory)

And the way you accomplish that increase in engine speed is by changing the gear ratio.

[Navy_p503]

Horsepower drops to provide the same amount of power??? I'm not trying to argue, i'm just trying to figure out what you're saying
Power is a measure of work/time. A dyno will measure the amount of torque the engine produces at different RPM.... HP= T x rpm/5252. So as the enginer wears out it won't be able to produce the same amount of torque, to produce the same amount of power it has to operate at a higher RPM. Still the only thing that will affect vehicle speed is RPM, and gear ratios. Acceleration will be dependent on the amount of torque the engine is able to produce.

[Aussie Steve]

Gotcha!!!! Hahahah I made that bit up.

You are of course right Navy the engine cannot go faster than the rpm. True too is the torque to acceleration. Time, power output, and fuel consumption are the only things left out of the equation. It will take more time and fuel for a knackered engine to reach the same speed as one that is not. Sadly the rpms will stay the same for both.