RPMs running higher than usual. - Page 2 - Honda CBR250R Forum : Honda CBR 250 Forums
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post #11 of 36 Old 05-22-2016, 11:34 AM
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Quote:
Originally Posted by Schroeder View Post
That's what I said. It changes the road speed but not the ratio of RPM to indicated speed. Or did I get that wrong?
Yep, backwards. Think of it like this: If you changed the rear wheel with a wheel that's 6" in diameter. The tire will rotate MANY times faster to achieve the same speed, effectively gearing the bike UP. This is why when rednecks lift their trucks and put 31" wheels on them, they complain their truck's speedo is off and it's slow, because they effectively geared the truck down making it a slug.

So if tire pressure is low, or the wheel is effectively smaller, RPM's will be higher and the speed will be under-represented, as engine RPM is measured in the motor and speed is measured from the back wheel.

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post #12 of 36 Old 05-22-2016, 01:28 PM
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Originally Posted by cbrlocal View Post
.... speed is measured from the back wheel.
Nope, the sensor for the speedometer is in the crankcase!
The tire pressure or the size of the tires or the sprockets have nothing to do, with the rpm and the speed on the speedometer.
I am still ridin 100km/h at 6000rpm in 6. with a 14/36 chainset, now.
Only 5km (gps) quicker, as before.

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post #13 of 36 Old 05-22-2016, 05:28 PM
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Originally Posted by CBR-250-R View Post
Nope, the sensor for the speedometer is in the crankcase!
The tire pressure or the size of the tires or the sprockets have nothing to do, with the rpm and the speed on the speedometer.
I am still ridin 100km/h at 6000rpm in 6. with a 14/36 chainset, now.
Only 5km (gps) quicker, as before.
That's how I thought it works I just wasn't sure anymore where exactly the speed was measured but I was pretty sure it wasn't at the wheel and therefore a certain gearbox RPM would always result in a certain indicated speed regardless of the condition of the wheels.

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post #14 of 36 Old 05-23-2016, 12:31 PM
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Originally Posted by CBR-250-R View Post
Nope, the sensor for the speedometer is in the crankcase!
The tire pressure or the size of the tires or the sprockets have nothing to do, with the rpm and the speed on the speedometer.
I am still ridin 100km/h at 6000rpm in 6. with a 14/36 chainset, now.
Only 5km (gps) quicker, as before.
No, the wheel/tire size and sprockets have everything to do with indicated speed. Speed is measured INDIRECTLY from the back wheel, because the back wheel rotational speed dictates the gear speed inside the crankcase transaxle. So yes, the back wheel does influence indicated speed, but not RPM. They are measured differently.

This is why when someone changes the sprockets in the chain drive, the speedometer needs to be corrected with a speedo healer. Think of the back wheel, in a sense, as another sprocket.

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post #15 of 36 Old 05-23-2016, 12:33 PM
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Originally Posted by Schroeder View Post
That's how I thought it works I just wasn't sure anymore where exactly the speed was measured but I was pretty sure it wasn't at the wheel and therefore a certain gearbox RPM would always result in a certain indicated speed regardless of the condition of the wheels.
Speed is measured in the crankcase, but the wheel/tire size influences the rate at which the internal gears turn. So it can alter indicated speed, which can change the RPM seen at a particular velocity.

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post #16 of 36 Old 05-23-2016, 06:35 PM
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so, again, first check memory - of specific rpm and mph numbers -
[eg, if you wrote it down immediately etc]
then if thats correct, check tyre pressure...

while not typically a major factor, pressures are adjusted by
some riders according to intended riding and real effects,
not limited to actual radius of tyres, which can and do
effect speed, power required for speed, thus rpm..

then, if pressures are about the same, check clutch..
going from easiest factors, memory, then tyre pressure,
then clutch adjustment, to other possible clutch effects
[wrong oil etc] then to mechanical inspection etc..

interesting how assumptions can seem to be
realities.. until the veil is lifted..
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post #17 of 36 Old 05-23-2016, 10:29 PM
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And make sure the oil is not energy conserving or normal car oil (under 10w30 viscosity). Anything with moly can have dire consequences on a wet clutch.

Tis better to be thought a fool and remain silent, than to open one's mouth and remove all doubt.
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post #18 of 36 Old 05-24-2016, 11:30 AM
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Originally Posted by cbrlocal View Post
Speed is measured in the crankcase, but the wheel/tire size influences the rate at which the internal gears turn.So it can alter indicated speed, which can change the RPM seen at a particular velocity.
Do you read what you write?

Yes, you have to (should mostly) adjust the speedo, after chainging [sic] the chainset to a different ratio or a 17'' tire to a 15''.

".. but the wheel/tire size influences the rate at which the internal gears turn.."

No, that is absolutely not changing the ratio from the rpm of the crankschaft (thanks Cbrlocal;-) to the rpm of the driveshaft and "assumed" (programmed) speed, Honda want the speedo to show.
In gear one, gear 2, gear 3, ...., gear 6, at a specific rpm!
As long you don't change the gear ratio itself, 6000 rpm in 6. is always the same rpm at the driveshaft and about 100km/h on the speedo. In Reality it is maybe 94, 100, 106, whatever. But as long the clutch is not slipping, even a flat tire will show ~100km/h on 6000 rpm in 6.
Because in case the clutch is good, there is always (I rehearse here ;-)the same ratio between crankshaft rpm and driveshaft rpm.
Set by the gearing, the chosen gear.
Thats how a gearbox works.
And as long the speedo takes the speed not from the tire, but the driveshaft, like it is at our CB's, ...

No. 7 is the sensor:



So in case the rpm is higher, as usual, the clutch has to be the culprit.
Maybe because of a additive in the oil.
Maybe because of wear-slipping.

Maybe you confused yourself with the ABS sensors?

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Last edited by CBR-250-R; 05-25-2016 at 02:21 AM.
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post #19 of 36 Old 05-24-2016, 04:23 PM
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1 Tyres are not significantly elastic and a slightly soft tyre will have the same rolling diameter as a fully inflated one. The only difference will be in the size of the contact patch.
2 If clutch slip were the culprit then the disparity would be seen to vary with the load on the engine, ie gradient, throttle opening etc

The tacho measures the crank speed and I guess draws it's input from the ignition unit as there's no mechanical cable like older bikes used to have. The speed is measured by the sensor on the gearbox output shaft, hence the fault has to be confined to either components between those to sensors, ie within the crank cases, or the instruments themselves. The sensors I would expect to be 'one revolution - one pulse' and as such would be all or nothing, so unlikely to give the symptom reported.

My favourite would be the tacho. If the needle relies on a return spring then that spring could have been weakened, perhaps by exposure to excessive heat, corrosion or just faulty manufacture.


As an aside. Many years ago I owned and crashed a Suzuki GT200. After the crash the tacho only ever read about two thirds of the true reading. That was easy to work out once I got it back on the road. Unfortunately I forgot to explain that to a friend I lent it to for an afternoon and he spent his time with it still thinking that sending the tacho to 9000 was fine. That mistake cost me a new crank and big ends.

Last edited by Keith; 05-24-2016 at 04:31 PM. Reason: To add the aside
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post #20 of 36 Old 05-24-2016, 05:15 PM
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Quote:
Originally Posted by CBR-250-R View Post
Do you read what you write?
Yes. Everything. Academia makes you anal and very thorough.

Quote:
Originally Posted by CBR-250-R View Post
No, that is absolutely not changing the ratio from the rpm of the camschaft to the rpm of the driveshaft and "assumed" (programmed) speed, Honda want the speedo to show.
In gear one, gear 2, gear 3, ...., gear 6, at a specific rpm!
As long you don't change the gear ratio itself, 6000 rpm in 6. is always the same rpm at the driveshaft and about 100km/h on the speedo. In Reality it is maybe 94, 100, 106, whatever. But as long the clutch is not slipping, even a flat tire will show ~100km/h on 6000 rpm in 6.
Not true. Do you read what you write? (eg: "camshaft"? Nothing to do with gearing!) Or what I write? I never said the internal gear ratios are changed. Regardless, I can personally attest to a flat tire affecting the speedo. I had a complete flat on the freeway at 65MPH. When my speedo indicated 65, I was doing slightly less than that (and my RPM's were lower as well). GPS said actual speed was 6 MPH less than indicated. Ergo, my actual speed was under reported due to the decreased distance my wheel traveled with each revolution due to being flat, unbeknownst to the bike's ECU, which has always been accurate.

Bigger wheel = speedo reports lower than actual speed, therefore RPM's appear being higher at the given speed than usual. Smaller wheel = speedo reports higher than actual speed, and RPM's will be lower that usually appears at that reported speed.

Quote:
Originally Posted by CBR-250-R View Post
Maybe you confused yourself with the ABS sensors?
Absolutely not. I'm not talking camshafts here. The confusion is not on my end friend; there's obviously a misunderstanding about how wheels can change "true" speed here, of which I'm apparently failing to make clear.

Engine speed is measured against the crank and is reported as a true value, regardless of gearing. "Indicated" speed will assume stock tire/wheel sizes are present. Now actual speed is another story. Yes, the gears and drivetrain are locked together, and reported engine RPM will always correspond to the front sprocket RPM and rear sprocket RPM as the same. BUT, distance traveled will change depending on the circumference of the wheel/tire combination used, and so "actual" speed will change.

Say I turn 6k RPM at 60MPH with the stock 17" wheel. If I add a rear wheel with twice the circumference as the stock wheel, my "true" speed at the same RPM will be twice that as it was. The engine is turning the same speed, the sprockets are turning the same speed, but the rear wheel is covering a much greater distance with one revolution. So, if a wheel/tire is smaller in diameter than stock, the opposite is true. Not sure what's so hard to understand about that.

Don't believe me? Put a 20 foot diameter tire on your bike and see if you're still turning 6000RPM at a TRUE 60MPH. I'm not sure how much more plain I can make this, so I won't try. Main thing is what Shiso said: memory of what RPM corresponded to what speed before. Some bikes are slightly different; my tach reads 6900 at 65 while most read lower at that indicated speed (likely due to speedo calibration differences in regions).

This likely isn't the OP's issue, as stated before, he would witness his RPM's drop for the same indicated speed, because it would be underrepresenting the ACTUAL speed.

Tis better to be thought a fool and remain silent, than to open one's mouth and remove all doubt.

Last edited by cbrlocal; 05-24-2016 at 05:27 PM.
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