Rev counter over-read

[speedy231278]

No, not a problem with the bike, but something I've been pondering for a while, and guess what... I got bored. Now, we all know that lots of people are obsessed with the speed their bike can or can't do. Being really bored, I did the sums on a stock NC35 by taking the all the gear ratios from one end of the bike to the other. I got a figure of a few tenths under 150mph @14500rpm. Now, we all know that a 150mph NC would far exceed the capability of even a very well tuned engine. So I checked everything, and did it several times more and got the same figure. I then found an online calculator for gearing. It told me the same.

Now, we all know our speedo reads fast. I then remembered an article from America, where Yamaha for some reason felt the need to issue some sort of statement about a new R6 model where the brochure had suggested the engine revved to 17K, but in reality it wasn't getting as high as the tach said. When I was young and stupid, I saw a whisker over 140 on my RVF, and enough revs left to reach what I reckoned would be an indicated 150. Now, that indicated 150 and the calculated figure match quite well. So the obvious conclusion to me is that the tach over-reads by almost exactly the same as the speedo. Some years ago at the Bulldog Bash, I was indicating 120 through the lights and getting slips for 104. That's about 13% high. Interestingly, people seem to reckon that at a push 125/130 is the absolute limit on an RVF, and lo and behold, take 13% off of 150 and you get 130.5mph.

If one concludes that both clocks are over by the same amount, this gives the red line of 14500 an actual figure of about 12600. And this is where things start to fall down. Honda suggest max power at 12500. Fair enough. However, most dyno charts for our V4s run up to 14000 or higher. Do dynos read high, or are they pretty accurate? Is Honda's claimed figure the true engine speed or the indicated one? My brain hurts!

And as we can barely do more than half the true top speed of a 400 on the road anyway, who cares?

[Yakama]

What about those racers using agtermarket rev counters and ignitechs or such. Does that mean that setting a rev limit of say 13.5k revs on the ignitech is too high??

[speedy231278]

Interesting.... looks like 13% would be way high....

http://www.superstreetbike.com/how-to/y ... ythbusters

So, maybe an issue on doing the sums with wheel size? Perhaps the fitted height of a 150/60-17 isn't 17 inches plus 60% (twice) of 150mm? Clearly when it's worn a bit it won't be, but is the 60% the height above the edge of the rim, or the height of the actual carcass including what goes inside?

[newtothis]

I'd have thought a dyno would be pretty accurate for RPM. The roller/tyre surface speed ought to be reasonably good too. Speedos are only required to read within +10% (i.e. optimistic) at 30 and 70(?) mph and tend to go way off as the speed gets higher.
How much tyre slip do you get as the speed builds? There could be a few percent there just to add into the mix. Or does the tyre carcass grow in diameter as the rotation speed increases and counteract it ??

There are GPS speed logging apps for smartphones, you could set that going then ride for a few seconds at a steady indicated 40, 50, 60, 70 ... ahem and check against the GPS values.

[Neosophist]

you only need to GPS the speed against a stock tacho.

i think the nc30 one was pretty bob on.

and just because you could reach 140mph and theoertically the extra rpm will allow it go faster doesnt mean the engine has the power to rev that far.

http://www.gearingcommander.com/

this is maybe the site you used? it has a lot of stock gears already in it.

in theory if did a +3 at the front and -3 at the back (dunno if youd get an 18 on the front in reality) but the bike should be able to reach 165mph.. but i doubt youd have power to get that far)

it's not just about the gearing oyu need the power to get there too.

stock gearing isnt far away from balacing the maximun power of the engine with accellration, you can get a bit more top end or a bit more acceleration by playing whith the gearing but one comes at the expense of the other

[madcusslerfan]

Just a thought....would we also need to take into account the weight factor (ie do these gearing charts and calcs allow for the added weight of moving even just the swingarm and rear wheel for example let alone the weight of a rider)?

[newtothis]

Weight doesn't affect top speed - other than the tiny difference in the amount that the tyre deflects under load. It's there but miniscule on comparison to drag. As in air resistance...not riding in a dress.

[RickOliver]

Random observations -

Using the tacho correction facility on the Ignitech units I generally have to dial in about -4% to get the tach accurately matching actual engine RPM.

The speedo drives off the front sprocket so any alteration to stock gearing will affect the speedo reading by an equivalent amount - speedo reads fast on lower gearing, slow on higher gearing.

Variations of tyre profile design and wear levels will make a difference but broadly speaking yes, the overall diameter of a tyre should be rim size plus 2x (width x aspect ratio/100).

As stated, power/weight ratio only affects acceleration, top speed is more about aerodynamics (power vs drag coefficient) - witness Moto Guzzi`s fully streamlined GP bikes of the late 1950s, 500cc single cylinder engine, 42bhp, 235lb in race trim, top speed 155mph...

I would say that as standard the NC30 is slightly overgeared, that`s a part of what gives them that relaxed `not really trying` feel and also allowed the road test models back in 1990 to make around 135 mph in favourable wind conditions with a good run up. If you add two teeth to the rear sprocket it will improve acceleration with no `real world` loss of top speed but the bike will feel a bit more `busy`.