So confused... Ninja 250, same gear, same speed, different rev speeds???

[MJinZ]

Someone is going to have to explain how a motorcycle going downhill in 1st gear (near vertically) at 100mph is going to be requiring say 8000RPM. Since, you know, gravity tends to be doing most of the work.

Also, how you can go the same speeds at different weights with the same RPM. 1000lb man on bike should require more power to maintain the same 100mph vs 100lb man.

But this is just intuition, don't remember my AP Physics from HS.

 

[jlee]

Someone is going to have to explain how a motorcycle going downhill in 1st gear (near vertically) at 100mph is going to be requiring say 8000RPM. Since, you know, gravity tends to be doing most of the work.

Also, how you can go the same speeds at different weights with the same RPM. 1000lb man on bike should require more power to maintain the same 100mph vs 100lb man.

But this is just intuition, don't remember my AP Physics from HS.

You're confusing RPM with load. Read the above posts again.

 

[MJinZ]

You're confusing RPM with load. Read the above posts again.

But Power = Torque X RPM.

Since I believe power is rate at which work is performed, and more (or less) work is needed depending on the situation (extremely fat guy bike, extremely downhill)... so you can see where intuitively, I would even guess that an engine could be idling while the vehicle could be flying down a hill. I suppose if a vehicle can't do that, it would be extremely poor engineering.

 

[jlee]

But Power = Torque X RPM.

Since I believe power is rate at which work is performed, and more (or less) work is needed depending on the situation (extremely fat guy bike, extremely downhill)... so you can see where intuitively, I would even guess that an engine could be idling while the vehicle could be flying down a hill. I suppose if a vehicle can't do that, it would be extremely poor engineering.

Tell me how much power my car makes at 5000rpm with my foot off the gas pedal - and then you may realize what's wrong with your theory. 😛

The drive wheel is directly connected to the engine through the transmission. Unless the clutch is either disengaged or slipping (or the chain is broken), a certain RPM in a certain gear will always result in a certain speed.

For example, consider a child's tricycle. Same concept -- the rate the pedals are moving is your RPM. Are you going to be able to ride that tricycle downhill at 30mph without the pedals moving at a rate directly corresponding to the speed? The faster the tricycle goes, the faster the pedals spin around.

 

[skull]

Revolutions Per Minute its how fast the motor is going not how much gas it is using to stay at that speed. In a manuel transmission the the engine speed is locked straight to the wheels. With different ratios for different gears. The RPM at a constant speed in a given gear will not change. However the engine might require more gas say going up hill to maintain that speed.

 

[MJinZ]

Tell me how much power my car makes at 5000rpm with my foot off the gas pedal - and then you may realize what's wrong with your theory. 😛

The drive wheel is directly connected to the engine through the transmission. Unless the clutch is either disengaged or slipping (or the chain is broken), a certain RPM in a certain gear will always result in a certain speed.

For example, consider a child's tricycle. Same concept -- the rate the pedals are moving is your RPM. Are you going to be able to ride that tricycle downhill at 30mph without the pedals moving at a rate directly corresponding to the speed? The faster the tricycle goes, the faster the pedals spin around.

So manual transmissions are just inefficient then eh

 

[jlee]

So manual transmissions are just inefficient then eh

A vehicle with a manual transmission engine braking downhill is using less fuel than an engine at idle.

If you brake in my truck while going downhill, the automatic transmission downshifts so the RPM's go HIGHER. I'll let you chew on that one for a while. 😀

 

[skull]

So manual transmissions are just inefficient then eh

No. The engine is only using as much gas as you give it. Have you ever driven a stick? Going downhill in gear with your foot off the gas pedel isn't using any gas. Well except what it gets to stay idling. The engine still spins because the wheels are locked to it. So your RPMs going downhill at 35mph will be the same as going uphill at 35 in the same gear. Except uphill you have to give it gas.

 

[MJinZ]

A vehicle with a manual transmission engine braking downhill is using less fuel than an engine at idle.

If you brake in my truck while going downhill, the automatic transmission downshifts so the RPM's go HIGHER. I'll let you chew on that one for a while. 😀

So your automatic transmission sucks eh :sneaky:

 

[MJinZ]

No. The engine is only using as much gas as you give it. Have you ever driven a stick? Going downhill in gear with your foot off the gas pedel isn't using any gas. Well except what it gets to stay idling. The engine still spins because the wheels are locked to it. So your RPMs going downhill at 35mph will be the same as going uphill at 35 in the same gear. Except uphill you have to give it gas.

That makes sense, I question manuals because nowadays, the manual models are always less fuel efficient.

 

[LTC8K6]

A vehicle with a manual transmission engine braking downhill is using less fuel than an engine at idle.

True for the auto as well, even with a downshift.

 

[Zenmervolt]

But Power = Torque X RPM.

For a given amount of fuel injected.

If your car is rated at 200 hp at 5,000 RPM and you're cruising at 25 mph in 1st at 5,000 RPM without accelerating, you're not making anything near 200 hp. You're throttled back so far that you're only making 10-15 hp because the throttle plate is nearly closed and that's choking off the engine.

Open the throttle, then you get more air (and, consequently, more fuel) and the engine starts to make more power, but then you're accelerating because the engine is making, at the same RPM, far more power than with the throttle closed.

Think about it this way, if you pulled out all the spark plugs and cranked the starter, the engine would spin at 500-1,000 RPM, but it wouldn't make any power.

Just because an engine is spinning doesn't mean it's making any power at all (modern EFI systems shut off the injectors when the engine's above idle and the throttle is closed), much less that it's making rated power.

Now, if someone is used to driving an automatic transmission, this line gets a little blurred because a torque converter does not work well at all in reverse. When the wheels are attempting to drive the engine, the torque converter slips. A LOT. For this reason, when one lifts throttle in an automatic, the engine spins down to near idle because the torque converter unlocks and the fluid coupling slips massively and is unable to transfer very much force to the engine. In this way, an unlocked torque converter is similar to a one-way clutch.

ZV

 

[jlee]

True for the auto as well, even with a downshift.

Correct.., I was responding to the idea that manual transmissions are inefficient.

 

[LTC8K6]

Correct.., I was responding to the idea that manual transmissions are inefficient.

Have to look at the one in control... :biggrin:

 

[Zenmervolt]

So your automatic transmission sucks eh :sneaky:

His automatic is smart enough to engage the lockup clutch and allow engine braking. 😉

It's not that manuals are less efficient; they are still superior to automatics if all else is equal (same ratios, same number of gears, etc). The difference is that modern automatics are adding gears like crazy (6-speed automatics are common now, and there are even 7 and 8 speed units out there) while many manuals are still only 5 or 6 speeds. Also, the target market for manuals, at least in the US, has shifted away from cost-conscious persons and towards sport-oriented drivers.

It used to be that manual transmissions were geared for higher mileage (except on outright sportscars) because the manual was the "budget" option and people who bought manuals were trying to save money, so fuel mileage was very important to them. As feature content in cars crept upwards and automatics became standard (or at least very inexpensive options) the people buying manual transmissions shifted to mainly sport-conscious buyers. Because of that shift, most modern manuals have closer ratios and overall shorter gearing than their automatic counterpart.

Manuals getting less mileage today is due more to marketing concerns than engineering concerns.

ZV

 

[jlee]

That makes sense, I question manuals because nowadays, the manual models are always less fuel efficient.

Courtesy of a friend of mine:

An engine can apply both positive and negative torque in varying amplitudes depending on throttle position, so at say 8krpm there can be a lot of torque being applied to the drive wheel, or a lot of engine braking, depending on the throttle position.

Newer automatics are sometimes more efficient that stick-shifts because they have lots of gears and a shifting schedule to increase fuel economy, which requires more frequent shifting that a typically manual-transmission driver would do.

jch13 - EIT - Bachelor of engineering, mechanical engineer

 

[PlasmaBomb]

Someone is going to have to explain
Sure

1) how a motorcycle going downhill in 1st gear (near vertically) at 100mph is going to be requiring say 8000RPM. Since, you know, gravity tends to be doing most of the work.

Yup Gravity is doing all the work and is causing the back torque from rear tyre to rotate the engine. Since the clutch isn't engaged there is no slip and the number of wheel rotations to engine rotations is fixed by the gearing. No fuel is being used (fuel cut off).

2) Also, how you can go the same speeds at different weights with the same RPM. 1000lb man on bike should require more power to maintain the same 100mph vs 100lb man.

This is indeed correct - propelling a 1000lb man requires the engine to do more work than propelling a 100lb man. This is achieved by burning more fuel, which produces more torque and hence more power, enabling the bike to do more work in a given period. Granted in extreme circumstances (such as above) the engine won't be able to produce enough torque to keep accelerating the load up to 100 mph.

But this is just intuition, don't remember my AP Physics from HS.
Intuition isn't great...

six of the most common intuitive errors:

inattentional blindness (failing to see things that are in plain sight);
the belief that our memories are more reliable than they are;
the tendency to think someone is competent if they are confident;
the illusion of knowledge (we know much less than we think);
the assumption that things that occur together must be causally related (think MMR vaccine and autism);
and the increasingly popular notion that cognitive exercises make us smarter (in fact, physical exercise has a much stronger effect).

Hope that helps.

PB

 

[DivideBYZero]

Sometimes I miss Fleabag.

 

[GuitarDaddy]

So wind and resistance in general could seriously cause this much consistent clutch slippage?

Not clutch slippage, traction/wheel slippage!

When assuming direct drive with the only variable as clutch slippage you overlook the connection of tire to the road which has the most possibility to slip because of rough bumpy surface, loose impediments, moisture, incline, wind resistence, etc.. ect....

 

[jlee]

Not clutch slippage, traction/wheel slippage!

When assuming direct drive with the only variable as clutch slippage you overlook the connection of tire to the road which has the most possibility to slip because of rough bumpy surface, loose impediments, moisture, incline, etc.. ect....

The chances of a Ninja 250 having constant wheelspin at highway speed are just about zero.

 

[Toastedlightly]

Could be the speedo being a bit wonky.

 

[caspur]

I suppose I should clarify the initial post regarding headwind. Leave it to a tech forum to break this down into a physics discussion.

This never should have been about gear ratios, mechanical drivetrains, load, power, etc.

(This is actually a reverse of the airplane taking off from a treadmill, in which in a non-gear driven vehicle such as an airplane, can actually take off due to an effective airspeed which is independent of wheel speed.)

There has been some confusion then when I said headwind. In the airplane example, a strong enough headwind can in theory allow an airplane to fly with an effective ground speed of 0. However, unless there is clutch slip or wheel slip, this is not possible with a gear driven vehicle.

So, how does headwind play a part in the OP's concern. Simple...you don't have a fixed throttle hand. A 500rpm difference at speed in a bike with little torque is a variation of throttle control, a human condition. A datalogger would reveal the OP is opening the throttle more. The speedometer on motorcycles is at times wildly inaccurate (see references to top speeds in supersports vs. gps actuals). Therefore even minute changes in throttle position, which accounts for any rpm differences, are not accurately reflected on the speedometer. These small changes are a direct result of headwinds or tailwinds. Ride a motorcycle on a windy day and try it.

 

[angry hampster]

Your bike has a cheap tachometer or speedometer that is slightly inaccurate. Not hard to figure out.

 

[SithSolo1]

I was cruising the Kawi forums day in and day out a month or two ago and this popped up as a common issue. As far as I could tell wasn't a fix.

 

[DominionSeraph]

Holy cow, 3/4ths of the people in this thread don't know how power gets from an engine to the wheels.
And to all of you going on about torque converters, have you ever thought that maybe the term "slushbox" might have been coined because it contrasts with how a manual transmission works?

It's true. Manual = no slush.

There has been some confusion then when I said headwind. In the airplane example, a strong enough headwind can in theory allow an airplane to fly with an effective ground speed of 0. However, unless there is clutch slip or wheel slip, this is not possible with a gear driven vehicle.

So, how does headwind play a part in the OP's concern. Simple...you don't have a fixed throttle hand. A 500rpm difference at speed in a bike with little torque is a variation of throttle control, a human condition. A datalogger would reveal the OP is opening the throttle more. The speedometer on motorcycles is at times wildly inaccurate (see references to top speeds in supersports vs. gps actuals). Therefore even minute changes in throttle position, which accounts for any rpm differences, are not accurately reflected on the speedometer. These small changes are a direct result of headwinds or tailwinds. Ride a motorcycle on a windy day and try it.

An optimistic speedo != a varying one.
The speedo on my bike is off by 10%. This does not mean that when I'm doing 60, it reads between 66 and 54. It reads 66. At 59 it would read 64.9. At 58, 63.8. At 10, 11. At 150, 165. See how this works?

If you're slowing down or speeding up due to wind, you can hear it in the engine speed. And 500-750rpm is noticable.

Slowing down is like RRRRRRRRRRRRRRRrrrrrrrrrrrrrrrrrrr
Speeding up is like rrrrrrrrrrrrrrrrrrrRRRRRRRRRRRRRRR

See how that works?