Notes on top speed.

woot · Jan 13, 2006

I'm not particularily interested in top speed, however, I know it was one of the most popular questions people ask when purchasing a new motorcycle. As a result I'm posting this here - as a cross post - for the occasion someone wants to know here.

FROM: Will england :: motorcycle notes and tips
http://will.mylanders.com/outdoors/moto ... pl?file=97

Top Speed of Yamaha XJ

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We can do a few simple calculations to calculate what the top speed of the
bike should be in various gears and see if the claim is reasonable.

The final drive is 5.67 to one overall in 5th gear, so the rear wheel is
spinning 5.67 times slower than the motor.

Thus if an engine speed of 10,500RPM is observed, the rear wheel is spinning
at

10,500/5.67 = 1851.85RPM.

But we want revs per hour, so multiply by 60, giving 111,111.11RPH.

The wheel is 130/90 factory, and accounting for 5% squish height (pretty
common for higher profile tires with the weight of bike and rider) the
overall height of the wheel tire combo is the wheel diameter plus the twice
the profile of the tire:

19" + 2*130mm*.90/(25.4mm/") = 28.21" tall
Minus 5% is 26.80 inches.
Circumference is then 26.80*3.14 = 84.20".
In feet this is: 7.0167'.
In miles this is: .0013289 miles.

Thus for every revolution the bike travels roughly .001328 miles.

So multiply the wheel revolutions per hour by the revolutions per mile and
you get:

111,111*.001328 = 147.5MPH.

If he were observing the redline top speed would correspond to period tests
at about 141MPH.

John Vitamvas
85 Maxim X - Haven't yet determined if it will pull redline in top gear;
don't want to.

==== also ====

Actually I have much more than John does, but only have to check a chart I
did up. This came about last July when Peter asked about the gearing for
the various models.

It did make me conclude the gearing is designed for around town use. In
2nd through 5th gear you can go from 20.4 mph to 50.3 mph and keep the revs
between 3000 and 4000. Only in first would you shift at 4500 in order be
at around 3000 in second.

The main reason for my chart to figure out the gear set needed to decrease
the final drive ratio while in 5th gear and reduce the revs from 5566 @70
to 4502 or roughly a 20% reduction. Overdrive anyone?

An interesting point is the top speed at redline goes up to 160 mph. And
at 11,500, the most I've pushed it, that would be 184 mph. Not likely it
could pull that, but am sure 160 mph is obtainable.

Now were there a reasonably priced method to modify the gear, I'd go for a
25% reduction in 5th and a 15% or so reduction to 4th gear. Or just
shoehorn in a 6 spd gear set.

Oh yeah, a more simple and accurate method to translate would be:

RPM / final ratio * radius of tire (ft)* 2 * pi / 5280 *60 = mph

You measure the radius from the center of the axle to the ground while
sitting on the bike. This does not factor in this measurement increases as
the angular momentum of the tire increases, but found the result multiplied
by 1.03 does work.

With what I plugged in redline in 5th is 129.49 mph and my stint at 11,500
would 148.91 and puts Travis at 135.96 mph.

Might conclude that Travis's speedo reads high and mine low. Still, he did
come close to burying it. Mine was at least another 5 mph tick's width
past. Best be a real quick glance when doing around 150 mph or how does
220 feet per second sound.

Can have some fun with that figure. With 20/20 vision what is the closest
you can be and make out the plate, then figure how long it takes to read
the plate, then realize why worry about someone making the plate out.

Which brings us to Kenny's question...

Why stay around to find out.

Oh, and the top speed of the '88 FJ1200 was found to be 152 mph.

Jeff "The Mountin Man" - '86 MaximX

==== also ===

gear ratios for XJ650

Primary drive: 1.62
1st gear: 2.187
2nd 1.500
3rd 1.152
4th .933
5th .812
output gear 1.361
middle gear 1.055
final gear 2.909

OR

1st = 15.291
2nd = 10.485
3rd = 8.065
4th = 6.524
5th = 5.679
Click to expand...

mtnbikecrazy55 · May 28, 2012

whoa. interesting

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Notes on top speed.

woot Active Member

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