RPMs running higher than usual. - Page 3 - Honda CBR250R Forum : Honda CBR 250 Forums
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post #21 of 36 Old 05-24-2016, 05:25 PM
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Quote:
Originally Posted by Keith View Post
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...
The contact patch will indeed widen, but with weight pressing down the tire's sidewall, you will lose rotational radius of the tire due to flattening too, which will reduce the overall circumference.

In my case, loaded on a COMPLETELY flat tire I lost ~2.5 inches of total radius, which is significant in a tire/rim combo that is only 23 inches total in diameter (~11% reduction in diameter and circumference).

My fully inflated tire and rim is 23 inches in diameter.
1 Rotation Inflated = 3.14*23 = 72.22 inches traveled.
1 Rotation Flat = 3.14*20.5 = 64.37 inches traveled.

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 #22 of 36 Old 05-24-2016, 09:07 PM
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lots of interesting observations here..
but sequence 'memory/tyre pressure/clutch'
is an example of sequential problem solving,,
simplest/easiest thru less easy etc..

on tyres, it is self evident that reducing radius
will reduce actual speed of the vehicle..
[all else being equal]

still, interesting interchange of opinion etc..
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post #23 of 36 Old 05-25-2016, 01:34 AM
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Quote:
Originally Posted by cbrlocal View Post
... 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.
Ah, here you point it out yourself:
You're fighting a lost fight, mate!

I even believe you this "and my RPM's were lower as well"
Because it is a analog tach, there is "a bit" difference to expect.
Every time, every day, every now and than.
But not 500 rpm on ~65mls/100km/h!
That is a 12% difference!
That would be a very flat tire, dont you think?
Even flater as your "complete flat" tire, you "did" 65 mls/h with


And your "speed was 6mph slower", on a flat tire, because of a flat tire?
Here goes the myth of "mls speedos are spot on", huu?
Or are you saying, with your flat tire it was 10% less real speed?
And, of course, you where driving on the rim, 65 mls/h, right?
And had nothing else to do, as to check GPS, Tach, Speedo. And all together.
I get it now!


Anyway:
It's not about real speed, this topic, it never was!
So forget about gps and such.

This topic is about shown speed on the speedo, compared with the rpm on the tach.

"I've noticed over this last winter that my tachometer indicates about a 500 RPM increase than when I was riding last summer."

The real speed, and the speed, that honda lets show the speedo, has nothing to do with each other. They (Honda) "tell" the speedo, to show this "exact" speed/numbers at that amount of rpm of the driveshaft!
And as long you don't change the gearing between the crankshaft (thanks to CBRLOCAL to tell me about that mistake) and the driveshaft, even in case you change the CBR 250 rearwheel to one from a groom, the bloody speedo will show 100km/h at about 6000 rpm on the tach.
Because the connection/ratio from the crankshaft to the driveshaft is fix, set by the chosen gear, not negotiable, as long the clutch is working and closing 100%.

Or maybe simple:

Put the bike on a pitstand
Run the engine in 6.
Go to 6000 rpm
read the speedo
Now dismantle the rear wheel and the chain
try again
Changes?

With this setting of reading the speed at the driveshaft, RPM on the tach and km/h (mls) are "hard" connected. With the ratio of every gear in the gearbox.
Nothing behind the driveshaft has influence on the shown speed to tach rpm's.
NEVER, with this setting!

But even reading the speed at the wheel will not change that!
Because the speedo is not knowing, if the tire is flat or overpressurized
Or a 17", it's made for, or a15", a12" rim
It reads rpm's of the wheel. And is showing, what is the default setting (in meter) for (one rpm) x rpms.
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Last edited by CBR-250-R; 05-25-2016 at 02:01 AM.
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post #24 of 36 Old 05-25-2016, 01:40 AM
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Cool

Quote:
Originally Posted by shisoshin View Post
...

on tyres, it is self evident that reducing radius
will reduce actual speed of the vehicle..
[all else being equal]

still, interesting interchange of opinion etc..
Saying actual speed, shisoshin, real speed you mean, right?
What has the actual/real speed to do with the shown speed, which shows given numbers on given/calculated rpm's of the driveshaft?

Or asking the other way round:
A 17" wheel will spin as quick, as a 15", 12" wheel would.
Or none, with only the front sprocket but no chain
Shown speed still identical, real speed not that much.

Right?

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Last edited by CBR-250-R; 05-25-2016 at 01:46 AM.
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post #25 of 36 Old 05-25-2016, 01:59 PM
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Quote:
Originally Posted by CBR-250-R View Post
Ah, here you point it out yourself:
You're fighting a lost fight, mate!...
I'm not fighting any battle mate; don't have the energy or desire for it. If you actually read all my posts, you would realize I just pointed out speedo differences will result from REAL, ACTUAL speed due to tire diameter changes. Take it how you will.
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post #26 of 36 Old 05-26-2016, 05:03 AM
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Talking

Quote:
Originally Posted by cbrlocal View Post
I'm not fighting any battle mate; don't have the energy or desire for it. If you actually read all my posts, you would realize I just pointed out speedo differences will result from REAL, ACTUAL speed due to tire diameter changes. Take it how you will.
Right, it's not a fight, it's a lost case!
And without arguments you have to say that, huu?
Lets see how it started, dude:

cbrlocal: 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.

https://www.cbr250.net/forum/627890-post11.html

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?
Quote:
Originally Posted by cbrlocal View Post
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.
And now tell someone that you didn't get it wrong in the first place.
For whichever reasons, dude, but "as engine RPM is measured in the motor and speed is measured from the back wheel" is wrong, here.
And in case you would have read my posts, you would understand, that I pointed on that one.

Quote:
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.
Yep, you ridin a dead horse, here!
Have fun with it.

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post #27 of 36 Old 05-26-2016, 08:00 AM
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Guys please keep it civil and don't start a personal feud over that.
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post #28 of 36 Old 05-26-2016, 09:10 AM
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Quote:
Originally Posted by CBR-250-R View Post
Right, it's not a fight, it's a lost case!
And without arguments you have to say that, huu?
Lets see how it started, dude:

cbrlocal: 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.

https://www.cbr250.net/forum/627890-post11.html





And now tell someone that you didn't get it wrong in the first place.
For whichever reasons, dude, but "as engine RPM is measured in the motor and speed is measured from the back wheel" is wrong, here.
And in case you would have read my posts, you would understand, that I pointed on that one.



Yep, you ridin a dead horse, here!
Have fun with it.


The output shaft gives speed readings; the output shaft drives the chain and rear wheel. Hence why I said speed is measured from the rear wheel; many motorcycles are measured at the FRONT WHEEL.

You're still missing the point (actual vs indicated speed), particularly the part where I never said indicated speed would change. But that's ok; do what makes you feel special. I'm not going to get worked up about it.

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 #29 of 36 Old 05-26-2016, 11:43 AM
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Wink

,,0..0,,t: I just have some time, sorry!


Did I read right?

"I never said indicated speed would change"

I'm sure it's my fault then

So yes, the back wheel does influence indicated speed, but not RPM. the back wheel rotational speed dictates the gear speed inside the crankcase transaxle



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.



I'm confused, right now!


cbrlocal, you probably know the case
But it went the wrong way at the keyboard, methinks.

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post #30 of 36 Old 05-26-2016, 02:16 PM
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Ah, those were at the end of an indeedly long day; my bad. Point was really indicated vs true, and the RPM perceived change therein. Problem with forums is, you cannot speak. Things get written poorly/super late/early in the morning (my fault; lots of long days), or read out of context; it happens.

I do believe at the core we're saying the same things, and just full of sound and fury signifying nothing.

Regardless, no relation to the OP's problem. Hope the clutch isn't cooked.

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