just dynoed the 1150

[oldschoolGS]

My new, stock, freshly broken in 16v 1983 GS750ED could only manage 125mph back in 1984. This was with me laid out flat on the tank and the throttle pegged over a 10 mile streach of road. My friends new VF700 Interceptor Honda was also not capable of 130 mph under similar conditions.

Fully modified factory Suzuki Superbike 750's were just approaching 160mph in the early 90's at Daytona. These were factory tuned and built 390lb streamlined bikes geared for a high speed track like Daytona.

I don't see a stock GS750- any model- reaching 150mph. I think you are placing too much emphasis on inaccurate speedo info. Indicated and actual speed are two very different numbers.

[Hoomgar]

serves me right for trusting speedos instead of stopwatches...

Hey no worries Bert. They all are like that. Even the new ones! They do it on purpose now I am told? About 10 to 12% off. Reading faster than your going. My 78 GS1000 (both of them for that matter) has pegged the 145 MPH speedo on many occasions. From riding with GPS attached friends I know that when my speedo says I am pegging it I am doing around 131 to 132. I imagine with a downhill slop and a tailwind I could reach closer to 140 but would surely run out of gears somewhere around there.

So yeah those darn speedo's lie to us. Because I assure you my liter bikes can eat a stock GS750 and I can't do no buck fifty

It's all good man

I enjoyed that physics break :P

[mark m]

well, since the thread has all gone down the drain anyways...
let's talk about physics.

Hey Bert,

We should kick this around off-line, unless everybody else is interested in the math and stuff associated with aero drag.

Well laid out example, I can follow perfectly.

please understand i dont want to start a flame war mark, i just felt the need to explore the physics. and hey, turns out i was wrong, the 750 cant reach 240 km/h. serves me right for trusting speedos instead of stopwatches...

I'm with you, flames aren't terribly productive in the long run... The reason I said your 750 won't get there is that all the magazine testing showed top speeds of around 110mph, tested by either timing or radar. Hard to refute that, as with your Fiero example. Nothing beats real numbers from a track. Your example shows the Busa to be a pretty slippery machine for a bike, that drag coefficient is excellent for an unstreamlined bike. The reason it needs less than an F1 car is they also have horrible Cd's because of all the downforce they generate. The last number I saw for an open wheel race car was around 0.70-0.75, which is really poor for a car.

The power cubed relationship is because the aero drag goes as the square of speed, as you say, but also power = thrust x velocity. So, as you go faster it takes more power to generate the same thrust force. Naturally, thrust has to equal drag to maintain a speed. There is definitely no mention of acceleration in there anywhere. Your above example shows this, because you have a 25% increase in speed (from 240 to 300kmh) and the calculation shows a required power increase of 86%. 1.25 squared is only 1.56, which would show a required power increase of 56%. 1.25 cubed is 1.95, which isn't quite right either, but the difference is because of the mostly constant factor of rolling resistance being thrown in there in reality. The cube effect is coming in where you multiply the drag force (with the velocity squared term in it) by the velocity to get the power required. This gives you the velocity cubed if you combine the two equations.

PM me if you want to kick this around some more and talk about bike areo stuff.


Mark

[melray]

aero drag

It seems to me that a wind tunnel or some other method of measuring actual aero drag is needed to make a practical use of drag formulae. There has been a fair amount of press on this in recent years coming from the comparative RWHP and top speed figures of ZX12 vs 'Busa.

I can tell you from personal experiance that aero drag is a huge factor- won't bore anyone with details unless this turns to another thread.

The main point being, reducing aero drag by reducing frontal area was called out as a main reasons for going from the plan form of the GS engines to the GSXR form, and the development of ever smaller aero presentations as the GSXR line developed. I won;t give up the high wide and handsome face of my GS engines, but they sure present a great big shoebox to the wind. Actually, more like 2 milk crates.