Calculating Maximum Speed

[oilburner]

I found a simple formula which is surprisingly accurate in estimating the top speed based only on horsepower.

top speed = [cube root of horsepower] * 30

For example, the Track diesel specs indicate 54 hp and a top speed of 185 km. Using the formula:
Calculated top speed = cube root of 54 * 30
= 3.78 * 30
= 181.44 mph

Using the formula, the 8.5 hp Robin DY41 is capable of 61.2 mph. Based on comments I've observed on the diesel forums, the Robin powered can apparently reach 60 mph with some coaxing.

Check the values for these British classics:

1970 Triumph T140 Advertised hp/mph = 50hp/111mph
Calculated=110.5 mph
1971 norton commando Advertised hp/mph=58hp/110-120mph
Calculated=116 mph

I ran all BMW motorcycles from the years 1923 to 1974 - 29 different models in all, through the formula and only 9 of the 29 were out by more than 5 mph compared to the advertised top speed (source BMW Motorrader Typen und Technik by Heinz Hartel)..

FYI
Avery

[Diesel Dave]

Thats a really interesting calculation, mapping this out I get:

MPH HP
30.0 1
45.0 3
47.6 4
51.3 5
54.5 6
57.4 7
60.0 8
62.4 9
64.6 10
66.7 11
68.7 12
70.5 13
72.3 14
74.0 15
75.6 16
77.1 17
78.6 18
80.1 19
81.4 20
84.1 22


Now this seems a little high for me so perhaps this is wheel measured HP?

[Rick]

54.5 out of SIX horses?
Man... If I can get 50 out of my 6 hp engine, I''l be one happy camper! I'd be thrilled if I could cruise at 45.

[oilburner]

The following quote is from the 1971 BMW owner's manual:

"The actually achieved maximum speed of a broken-in motorcycle depends to a large extent on the air resistance offered by the rider due to his size, posture, and clothing, and upon road and weather conditions".

Translation: To achieve a maximum speed of 60 mph from 8hp, pick a day with no wind, level straight road, lie on the tank and ride um, naked

Avery

[Diesel Dave]

I reakon that we can subtract 2hp for the Enfield chain drive/gearbox option and 3hp for the Comet friction drive - that seems to work out more accurately.

[balboa_71]

Best we could get out of 5 hp Bonanza minibikes with 2 speed auto clutch tranny's was 45 mph. Rupp minibike with 4 hp engine, and Comet tranny was about 40 ....nothing scientific, but some of our rigs (mine included) are built very simular. I think top end on my 10hp conversion is 50 mph, and sporting close to 650 lbs max combined weight, maybe a tad less. I think 45 mph is closer to max crusing speed for long periods of time....

Cris

[oilburner]

I think one reason that engines, especially older ones fall short of the cube root times 30 rule is that the *advertised* horsepower is not the *available* horsepower. In other words, the number may be either a *corrected* horsepower e.g. SAE standard, a number from an old dyno on a cool day, or an optimistic number from the sales people.

Take the Daihatsu 3 cylinder industrial engine. Specs shown on brochures and websites etc. indicate 3 ratings: Gross, Continuous, and Net.

At 3600 RPM, the figures are Gross=26.5hp, Net=24hp, and Continuous=15hp.

A paragraph in the Daihatsu installation guidelines explains the difference and I suggest this says a lot as why a particular engine falls short of expectations.

From Daihatsu:

When evaluating a performance curve for available horsepower use the following curves for the required duty cycle:

* Net Intermittent = Output for applications with varying speed and/or intermittent load. Examples: standby generator,set, vehicle, chipper shredder, etc.
* Net Continuous = Output for appplications with continuous load over long periods of time. Examples: base load generator, irrigation pump, air fan, etc.
Gross: = Nominal horsepower rating as defined by SAE J1349 (use net int. and net cont. curves for available horsepower)
[Note: Emphasis mine - Avery]

So, in the real world on a long trip, 15hp continuous @ 3600 rpm will allow me to cruise all day long at 67.4 mph and the 24 hp intermittent will come in handy for maintaining speed on hills and in headwinds. I will never see 26.5hp.

+++

It would be interesting to put one of the 10hp diesel clones on a dyno to verify the advertised horsepower figures

FWIW

Avery

[balboa_71]

Avery,
I'd love to get mine on a dyno just to adjust the injection pump rack setting.
There is a dyno shop near by, maybe I'll stop in someday and see what the testing rates are.....

Cris

[andrewaust]

Spot on balboa_71


I found my engine was overcharging the engine with fuel making lots of SOOT - not even smoke - smelt like a Steam Train.

So some of you will probably be surprised that to obtain maximum power you could be backing the fuel rack off instead of increasing fuel delivery.

It is normal for a diesel with mechanical injection to blow some smoke while under load, and the best way to adjust it is on a dyno - or if you have a hill nearby, get a run up to govern the engine out and feel what happens when you hit the hill with full throttle, if you back off a little and feel more power there - you could have a over fueling problem.

Those running the larger engines with Turbo charging "ya lucky buggers" you guys can have more of a play with the turbo inlet and exhaust etc etc to gain more power, but could find your engines are also over charging with fuel.

Electronic injection is totally different, most having sensors etc to have good control of fuel delivery - although with mechanical injection one can increase the revs of a 3600rpm engine to anything up to 4500rpm, where's most electronic systems have speed sensors "damn"

Word of warning Before increasing the engine rpm - be careful, especially with expensive engines I've got a spare engine for my bike sitting there for the day I might push the one lunger tooooo far, so take into consideration if you can afford another engine if something should decide to fail.



Cheers

Andrew

[balboa_71]

Andrew,
Most people think of broken rods when going to fast....I think in terms of exploding flywheels. Should a flywheel go, there would not be enough of ones right leg to ever ride again

I think the formula is not to let the O.D. of the flywheel exceed 60mph, someone please correct me if I'm wrong, thanks...

Cris

[andrewaust]

Yeah a flywheel is one of those things you don't want to fail. Actually a rod would be more prone to fail if the engine was run up to 5000rpm, and like you have hinted the flywheel will probably go beforehand, and is a very good point raised What is more likely to fail at higher than stated rpm

For myself 4500rpm is the absolute limit, I don't have a tacho on the bike but more than happy with 100 km/hr top governed speed. I'm using the standard 500 bullet gearing 17/38, so I guess the engine is actually running at max rpm around 4300 rpm. I also have a 19 tooth sprocket, but think the 17 works better all round, as the 19 was to tall for the bike.

If I get a chance to measure the engine speed I'll post it up!

The engine was run up to 4500rpm on a bench test after break in. The Yanmar clone engines do make for a good power plant.

There is now a V-Twin variety posted on this site that uses some of the same parts etc as the clone I'm running. With 21HP there is no way you would want to run the engine at high speed, rather stick to 2200rpm.

It's just a case of sqeeeeeeeeezing the power out of the singles and try to keep the road raging motorists happy.

The next Diesel bike project will be a multi cylinder one, more likely be Turboed if the silly regulations allow here "Polution requirements"



Cheers

Andrew

[John]

Hi Andrew. My advice, if you have a dual seat, is to shift back as far over the pillion bit as you can when exceeding 100 km/hr, wear some kevlar trousers, & pack a spare pair of pants. You'll need 'em if it finally lets go!

[Sphere]

What formula are you using to calculate theoretical speed (say, at 3600rpm, disregarding engine power) and how do you factor in the actual engine output? Your estimates seem a bit pessimistic.