v7 sub


Max. RPM Discussion....

I often get remarks from GC users that I have entered the wrong Maximum RPM in the GC database for their bike and therefore the top speed is calculated wrong. They mostly want me to put in their redline RPM or the RPM the rev-limiter kicks in. But I use the M.P.RPM (Maximum Power RPM), the RPM  at which the bike produces the maximum amount of horsepower in top gear. I will try to explain why:

When calculating motorcycle speeds, one of the key items is of course the RPM the engine makes.
The more RPM the more speed. So far so good.....

But, the more speed the more wind (and other) resistance will occur. In fact, the air resistance force is approximately proportional to the square of the speed. So doubling the speed will increase the wind resistance 4 times.
The more wind resistance the more back wheel torque your bike needs to increase the speed even more. The amount of  torque the bike can deliver depends (amongst other things) on available horsepower and the selected gear.

That is why a motorcycle can easily reach the redline RPM on the tachometer in the first 3 or 4 gears.
This redline RPM or the RPM that rev-limiter kicks in is the RPM the bike can make (in lower gears) without damaging the bike.

But when driving high speeds in higher gears (5 and 6), reaching the redline (or rev limiter) RPM depends on the available amount of horsepower. That is why some motorcycles (and cars) reach their top speed in 5th gear instead of in 6th.

To calculate the top speed of a bike, you need the RPM the bike can make at that speed in highest gear.
This is not necessarily the redline RPM on the tachometer ! Practically no street-bike can reach this redline RPM in top gear. This makes it very difficult to calculate top speed as the maximum RPM the bike can actually make is not known. But since you need more power to increase speed, it is safe to say that the maximum speed will be reached when producing the maximum power !

Since most manufacturers specify the maximum amount of horsepower at a certain RPM, it is also save to use this RPM (at which the bike produces the most HP) to calculate the top speed of the bike !

An even more accurate M.P.RPM is the same RPM but now taken from an actual dyna-run.
Have a look at the following dyna-run output for a Ducati 999s:


In this case the maximum horsepower (136) is delivered at 9750 RPM. Now this is the RPM that is entered in the Gearing Commander database as the Max Power RPM.  So when sending in data for your bike, please try to find out this RPM at which the bike delivers the most horsepower, either specified by the manufacturer or mentioned in a published dyna-run. If available on the Internet, please send me the link to this data.

When looking at this graph, you can see that at a higher RPM, lets say 10500, the available horsepower is less, it is about 122 HP. This means that at this RPM there might not be enough horsepower to accelerate the bike to this speed and RPM and the bike will become slower ! (There is less power available while more drag is generated at higher speed)

Using this M.P.RPM as the RPM in the GC database also means that your actual top speed in lower gears will probably be higher than indicated because in those gears you can actually reach the redline RPM which normally is at a higher RPM as the Max Power RPM.

So if you want to know your top speed in lower gears, just change the M.P.RPM field to reflect the RPM your bike can actually make in the gear you are interested in.


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