# How much HP is required to reach different speeds

Discussion in '2-Stroke Engines' started by markham26, Jan 3, 2012.

1. ### markham26New Member

Hey
I have a question that may have been spoken about already but mine is slightly different.
A bloke in my town thinks his motorized bike can go 90km/hour oop:, i don't believe a word that he has said and i would like to know how much horse power is need to hold such a speed, in consideration with the conditions such as wind, incline etc.
i would also like to know how much horse power is required to hold speeds on flat ground.
40km/hour
60km/hour
80km/hour
90km/hour

Any feedback would be greatly appreciated
kind regards Markham26

2. ### blue 48Member

on a good day im happy to get 60 out of mine 80k iv seen it done 90km i doubt it

84.7 km/h downhill with serious mods and lets say the thing didnt make it back up the hill ten minutes later...

1hp = 33000 foot pounds per minute.

learn the difference between power to accelerate to and power required to maintain velocity (or newtons three laws...)

get a fibreglass shell, do some serious and well planned port modifications, make a good pipe, and get the gearing right...yeah, youd get 90km/h... for a few hours.

40 to 50 is the standard average... while maintaing reliability

if this guys getting 90 then wheres the evidence?

i agree, he is talking oop:

Last edited: Jan 4, 2012
4. ### 5-7HEAVENMember

My friend's 11yo son ran 68.8kph downhill (43mph).....on a pedal bike.

LOL, let the guy talk. That's all it is, and it doesn't hurt anyone but his own credibility.

Now if you want to BUILD a similar bike and RACE him for \$\$\$ or bragging rights.....

that'd be exciting!

5. ### motorpsychoActive Member

there's a lot more to speed than just horsepower.
no one mentioned gear ratio and wheel size.
I can probably make a stock 66 c.c. 2 stroke go 70 mph given the right gear ratio and wheel size.
there's a difference between something being fast, and something being quick.
quickness is what you want on a m.b....get up to top speed as soon as possible.
anything can have a high top speed as long as you have the room for it to actually get there.
it's like drag cars and salt flat cars.
drag cars get from point a to point b (a 1/4 mile) in the quickest amount of time as possible all due to horsepower and gear ratio.(low gearing, tons of horsepower). but after that 1/4 mile run the engine is wound out to the max.
a salt flats car is more focused on top speed, (higher gearing) so it can take a mile or more for it to get up to that top speed.
you can make a 200 horsepower car go 100 mph in a 1/4 mile, or you can make that same 200 horsepower car go 200 mph on the salt flats, all with a simple gear ratio change.
tire size also has a big roll (no pun intended) in this as well.
a taller tire will give you more top speed, and a shorter tire will give you more low end torque.
so...lets say your bike is geared with a 44 tooth sprocket and it has a 24" tall tire on it, and you are getting 25 mph top speed for example.
throw a 26" tall tire on it and you will gain some top end speed.
now, take the same bike with a 24" tall tire and a 44 tooth sprocket, and drop the rear sprocket down to a 36 tooth. now you will gain top speed but loose low end torque....so it'll take longer for it to get up to speed. now, throw a 26" tall tire on it and you will gain even more top speed, but you will loose even more bottom end torque.
gear ratios and tire sizes are the easiest way to make a stock 2 stroke bike engine quick, or fast.
of corse, horsepower does come into play here as well because you can alter the low end torque or the top speed by making more horsepower....everything goes hand in hand when it comes to speed and torque (quickness).
so maybe this guy has his bike geared really high and it takes him 2-3 miles to get up to 90 kmH top speed....it is possible with the right gear ratio.
would i do 90 kph on a bicycle? no way.

Last edited: Jan 4, 2012
6. ### jaguarWell-Known Member

90kmh = 56mph
My engine is more highly modified than just about any Grubee engine out there and I think it might be able to go that fast. Might. But most guys that brag that can't back it up with proof. And it is not just a matter of gearing to go fast. These resistances build up with more speed:
gear, chain, wheel bearings, tire, wind, and last but not least the resistance caused by gravity pulling bike/rider towards the ground causing more rolling resistance.

7. ### jamz1378New Member

I have a standard 48cc motor, 26" wheels and 44 tooth rear cog and get 40 km/h out of her without pushing her hard. The way i see it if you want to do 90 km/h then go out and get a motorbike.

8. ### MikeJMember

Hi Everybody -

I have often thought the same questions as did markham26 did, way back on 3 Jan 2012. I crunched some numbers in a spreadsheet and used the basic forumula of:

HP = (Torque x RPM) / 5252

Torque supplied by the wheel between the rubber and road surface can be directly translated into how much rope pulling tension is needed for the same bike and rider. Imagine pulling a rider far behind a car with a rope. A certain amount of rope tension can be measured. On flat road with no engine input and dead still air, tension will be caused by only wind resistance. (Assume bike frictions will be zero. Assume a constant speed and a windless day.) Low speeds produce low tension; high speeds produce high tension. The weight of a rider is not a factor, only resistance as the bike moves.

With measurement by some mechanical device, the rider can calculate how much horsepower is put out by the engine. (Maybe a local hot rod shop will let you borrow their dynomometer. Anyone have a wind tunnel so the rider is stationary? Maybe set up a raised platform on back of a pickup truck with a broken Walmart bicycle and a full-size non-human dummy? The setup would have to be able to move forward and backward with wind resistance.)

Horsepower output by the engine can be calculated if it can be measured how much pulling tension is needed to keep the rider at a constant speed. Calculations can get really messy in a hurry. But let's keep it simple for now. The spreadsheet bike has a wheel radius of one foot, or 24 inches diameter.

At 30 mph, the wheel turns 420 rpm. But for now we have to assume wind resistance that can be measured from the rope with some kind of scales. Put this measurement in for Torque.

If the scales reads 20 lbs of tension (this is wind resistance) at 30 mph, the engine would have to do the same through whatever gearing is on the bike. That tension would be seen between the tire and the road surface. Anyway, given these conditions, HP needed would be:

HP = (20 lbs x 420 RPM) / 5252 = 1.6 HP (assume 20 lbs wind resistance at 30 mph)

Assume a strong headwind producing 40 lbs of rope tension at 30 mph.
HP = (40 lbs x 420 RPM) / 5252 = 3.3 HP (Need a bigger engine to hold that speed)

We all have experienced going uphill or riding into a strong wind; we will naturally slow down if the engine cannot put out the horsepower needed to overcome the additional resistance at a constant speed. Riding downhill or with the wind allows us to back off the throttle and produce less horsepower.

At 90 kph (56 mph), this 24 inch diameter wheel will be turning about 785 RPM. Assume rider and bike are really streamlined and measures only 20 lbs of wind resistance. His bike needs:

HP = (20 lbs x 785 RPM) / 5252 = 3 ; 3 HP coming out of his engine.

Without some resistance measurement, one can only guess how much HP his engine is putting out. If you need speed into strong headwinds or going up hills, just get a big engine right from the start.

MikeJ

Last edited: Jan 8, 2012

10. ### jaguarWell-Known Member

I just tried it and the result was 2.9 horsepower for 56mph.
1/4 mile is 1320 feet.
To go 90 miles/hour it takes 12.2 horsepower.
So obviously a speed to horsepower graph would not be linear but logorithmic.

Last edited: Jan 9, 2012
11. ### MikeJMember

Hi Jag -

You are correct! The speed to HP graph would be logarithmic. Aero drag at 40 units of speed is considerably more than twice that at 20 units of speed. I did some quick research and found this statement: "When the drag force becomes equal to the force the engine can exert, the car reaches its drag-limited top speed." The same applies to bike and rider. The non-motorized-bike person would see this as a decrease in fuel economy at high speeds on open stretches of highway.

This site makes the statement for automobile drag:
http://en.wikipedia.org/wiki/Drag_(physics)
"Note that the power needed to push an object through a fluid increases as the cube of the velocity."
(Edit: If you want to go twice as fast through air, you need 8 times the engine output than needed at half speed. - MikeJ)

Some more sites to see:
http://craig.backfire.ca/pages/autos/drag
http://www.rbracing-rsr.com/aerohpcalc.html

Last edited: Jan 9, 2012

drive a car at 80km, drive 100 km.

now drive the 100km at 100km, and compare fuel useage.

just that 20kmh can make a huge difference on making it to the next service station or hitching with can in hand...

13. ### gothicguy64Member

hi

the fastest ive been on my rse shaft 3spd bike with a rse hp1.5 48cc motor with a long street pipe was 64 kph with a 44 tooth rear .
that was going up to penrith on the gw hwy and i passed a cop booking a speeder in a car ..lol