just dynoed the 1150

[gsBert]

yeah, i've had the 750 up to 240 km/h. what's so weird about that?

i dont get the land to sell thing? must be a cultural reference im missing...

is it surprising? should it not be able to reach these speeds? well, i never dynoed it but im guessing it makes at least 60 something hp, and i've had 100 hp cars that weigh 4 times the weight and have more wind resistance to those speeds and higher.

it does take a really long time to get there though and you need a long straight road and there is not much acceleration left, and youre scared to pieces.

but if you think a 750cc bike cant reach those speeds you need to stop thinking in terms of v-twins, heck, my friend's shadow 750 reaches 200km/h if you keep it open long enough... my 1976 250 cc thumper in the honda xl reached 160 km/h with a tailwind, and i'd be surprised if that made even 20 hp...

 

[RacingJake]

Back in the 70's Honda had a small twin, maybe 195. That thing could go over 100 mph. A guy at our High School had one with open pipes sounded almost like a 750 back then. Of course the 750 was the biggest bike around back then too.

I was riding a 350 back then and that thing could turn over 14 grand in the first 3 gears......time warp again :? and now they call a 1200cc bike a girls bike

 

[scotty]

Thats because THEY Never saw a reworked sportie run :lol:

 

[mark m]

yeah, i've had the 750 up to 240 km/h. what's so weird about that?

Because it's physically impossible for a stock GS750 to reach much over about 110mph. Your speedo is lying.

is it surprising? should it not be able to reach these speeds? well, i never dynoed it but im guessing it makes at least 60 something hp, and i've had 100 hp cars that weigh 4 times the weight and have more wind resistance to those speeds and higher.

No it WILL not reach those speeds, not "should not". My last bike before the GS was a 97 ZX-9. It made 125+rwhp and had a magazine tested top speed of around 160mph. Your 60hp 750 will not even come close to that. You've never had a 100hp car go 150mph, either, unless it was a Bonneville streamliner. Physics again (and speedos lie grossly). The cars likely have about the same amount of drag as the bike in the end. The bike is smaller, but has a horrible drag coefficient and the cars are larger, but have a much better drag coefficient. So it ends up being a push. The last fast car I had was a 5.0L Mustang LX and it had a top speed of 145mph or so (again, magazine tested). It also had 225hp, not 100.

but if you think a 750cc bike cant reach those speeds you need to stop thinking in terms of v-twins, heck, my friend's shadow 750 reaches 200km/h if you keep it open long enough...

Yes, and not much faster. He also has pretty close to the 60hp you mentioned for your GS. The power required to reach a speed veries as the cube of the speed, so to go 2 times as fast requires 8 times the power. This means to get your GS to 150mph it will need (240/200)^3 = 1.73 times the power of his Shadow. I can't imagine you have that much more power than he does.

I'm not trying to be a smartass here, Bert. Just pointing out that physics say many of the things we think our bikes can do are impossible if you think about it a bit. And speedos are often GROSSLY inaccurate at higher speeds.


Mark

 

[gsBert]

well, since the thread has all gone down the drain anyways...
let's talk about physics.
im leaving the mustang out of this, the last fast car i had was my modded v6 fiero with unknown hp (prolly around 220) and it reached 290 km/h on a 4 km 30 degree banked oval (timed on laps 6, 7 and 8, not magazine or speedo tested). it was however a racecar that had been lowered, rebodied and lightened.

the power required to reach a speed does NOT vary as the cube of speed.
you must've heard that somewhere that it requires 8x the power to accelerate twice as quickly and misinterpreted it.

if your statement were true, there'd be no way a 180 hp hayabusa could reach 300 km/h since it only has one fifth the power of a formula one. oh, by the way, the busa also accelerates faster to 100km/h...

here is the correct aerodynamic science of it:
(dont flame please, im keeping it VERY simple, i know its got more stuff involved)

the power required to MAINTAIN a speed only needs to overcome rolling resistance (which decreases with speed) and aerodynamic resistance (which increases linearly with speed up to Mach 0.1 and to the square thereof above those speeds; roughly 110 km/h at sea level)

i dont know the drag coefficient of a GS, so ill use a bike i know it of, the hayabusa (i know its not realistic, but im just going to calculate for you how much power the busa needs to maintain 240 km/h)

i also dont know the rolling resistance of the tires, so im going to leave it out all together for now (i'm guessing between 5% and 10% of the hp will be used to overcome this though, ill calculate it later from horsepower and top speed)

the busa has a drag coefficient (Cx) of 0.561 and a frontal area of 0.558 square meters.

to calculate the drag we use this formula since we're in the subsonic speed range:
-Rx (drag in newtons)= [Cx (Drag coefficient)] x [air density/2 (in kg/cube meter)] x [frontal area in (meters square)] x [Velocity Squared in (meters per second)]

a 240 km/h speed equals roughly 66 meters per second
air density (dry) at sea level and room temperature is 1.225 kg/m3

let's have some fun now (i know im a pervert):
0.561 x 0.6125 x 0.558 x 66 x 66 = 835 Newtons
that's the force the air exerts on our busa at 240 km/h

lets calculate the newton meters (work) per second required to produce this force (we're gonna get watts out of this)

work is force applied over distance (say our 835 newtons over 66 meters)
66*835=55110 newton-meters

since we're dealing with work over time (watts) and we're talking about a single second, that is 55110 watts (1 hp = 745.7 watts)

wow! the busa is using 73.9 horsepower to overcome wind resistance at 240 km/h.

lets assume it uses all 180 horses at 300km/h , because im curious how much hp is wasted and used on rolling resistance.
wind resistance on same bike at 300 km/h:
1320 newtons of drag *83 m=109560 newton meters
that's 147 horses...
last time i saw a stock busa on a dyno it made 155.1 at the rear wheel.
so that assumes 6% rolling resistance at that speed...

so a bike with the busas aerodynamics needs 79 hp to drive at 240 km/h

hmm... im pretty sure my 750 doesnt make 79 hp on top of not having the busas Cx
im guessing 60hp and an aerodynamic top speed of 192 km/h (120mph), you're right, something must be wrong with the bicycle speedo...

now im gonna have to see just how big the magnetic field on the wheel pickup is and at what velocity it becomes inaccurate... damn!

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...

 

[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

 

[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

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.