Prop pitch vs gear ratio...

[SuperNova]

Re: Prop pitch vs gear ratio...

This is a common trap that's not readily apparent. Actually the components on the 16's and 3.73's are higher stressed, assuming equal friction losses, equal diameter axles, bearings, etc. In all three cases the engine is turning at the same rpm and delivering the same power to the rear end. The pinion gears are all turning at the same speed. But with the 16's the engine torque is multiplied by 3.73 (the rear end ratio) and transmitted to the wheel through the axle. The 3.73 axles are seeing 14% more stress (3.73/3.27 = 1.14) than the 3.27 axles on the 14" tires. All components (bearings, housings, etc) should be sized to take this greater load.

As I said earlier, as far as the motor is concerned all these combinations are equal (ignoring minor variables like moment of inertia, sidewall stiffness (assume equal aspect ratio), etc). Since the motor is turning an equal RPM and outputting an equal amount of power in each case, it is readily apparent that the bigger components are more highly loaded by simply looking at the definition of power. Since Power = Torque x RPM / 5252, and the bigger tires are turning at a slower RPM, it takes more torque to transmit the same amount of power through them as is transmitted through the smaller tires at the higher RPM. This is why you can't keep putting in smaller and smaller rear end components as tires get bigger and bigger! (Maybe I could put some 44's on the old F150 and twist'em with toothpicks .......NOPE!)



No argument that heavier components will reduce suspension life. I was ignoring minor differences. I doubt that the difference in weight between a 14" and 16" wheel would even be noticable over a typical component's lifetime. No suspension component (shocks, springs, etc) suppliers that I'm aware of even bother to ask for a wheel weight when specifying parts, or reduce the warranty if you're using a heavier wheel!

A higher moment of inertia will definitly reduce your acceleration. I seriously doubt if it would be enough to worry about in a street vehicle. If it was significant I bet all the drag racers would be running 6" diameter go-kart tires!

Please go on - always looking to learn more!

I sincerely hope this entire post was meant as a joke, cause if it wasn't, you have some serious misconceptions. I don't think I have ever seen the horsepower formula used quite like that before!! And that is only one of many errors! roflmao!!
--
Stan

 

[SuperNova]

Re: Prop pitch vs gear ratio...

Nova...Nova.... First you get the boat out of the hole, then you get it holding a acceptable plane speed and finally you get your final top rpm
and speed.......then ya try a bigger prop........then ya cant get out of the ....hole........it's a vicious cycle

As to gearing it's all what your looking for 2/1 1.8 for lower powerd 3.0.......1.8-1.60 for 200hp v6 and 1.5 for high hp and i believe it's all about how much tourqe the system will handle.

Now when ya get out of the hole and holding plane and hit wot at a good rpm.....well ok let's talk bow lift stern lift and slip ratio's aka that's when slip play's a big role........fine tuning..

I'm sorry, but I have no idea what the H*** you are talking about here, exactly. Can you clarify?

 

[45Auto]

Re: Prop pitch vs gear ratio...

No joke Stan!

Quote from Stan (Supernove):

I don't think I have ever seen the horsepower formula used quite like that before!! And that is only one of many errors!

I would be highly interested in seeing the HP formula you use! Any other mistakes you can point out would also be appreciated!

Your erroneous HP formula is probably whats causing you so many misconceptions. No wonder you're so confused. I thought everybody with any interest in an internal combustion engine knew the basic horsepower formula of:

Horsepower = Torque x RPM / 5252

I'll try to break it down for you, although all it would take is a simple Google search on your part to learn a little about the relationships between power, torque, and RPM.

The term "horsepower" was coined by the engineer James Watt (1736 to 1819) in 1782 while working on improving the performance of steam engines. This occurred while using a mine pony to lift coal out of a coal mine. He conceived the idea of defining the power exerted by these animals to accomplish this work. He found that, on the average, a mine horse could pull (lift by means of a pulley) 22,000 foot-pounds per minute. Rather than call this "pony" power, he increased these test results by 50 percent, and called it horsepower i.e. 33,000 foot-pounds of work per minute.

Under this system, then, one horsepower is:

1 hp ≡ 33,000 ft?lbf/min by definition, or

1 hp = 550 ft?lbf/s since 1 min = 60 seconds

This means it takes 1 hp to lift 33,000 lbs 1 foot in 1 minute. Or you could lift 66,000 lbs 6" in 1 minute. Or you could lift 550 lbs 1 foot in 1 second. Same thing in all 3 cases, takes 1 hp. There's an infinite number of combinations.

Okay, so the unit for hp is ft*lb/min. If you put a motor on a dynamometer you get 2 numbers: Torque and RPM. Notice that there is no RPM term in the HP units. The dyno units for torque are ft*lb. The dyno units for RPM are Revolutions Per Minute. To get rid of the revolutions you need to convert to radians. There are 2*PI radians in a circle (PI=3.14.....). To make it much easier on everyone to calculate HP, you want to get rid of all the big numbers (these numbers are ALWAYS the same, they're called "constants"): the 33,000 and the 6.28 (2*PI term). If you divide 33,000/2PI you will magically get 5252!!!

This is where the basic equation HP = T * RPM / 5252 comes from.

Maybe your confusion on tire sizes is caused by not understanding stress. Please publish the stress equation you use as well as the HP one.

You really think bigger tires (which require more torque multiplication to turn, hence the higher ratio rear ends) are easier to turn than smaller ones and put less stress on your drivetrain??? Put some 48" dia tires on your bicycle one day and try to pedal it. If your bicycle currently has 24" dia tires it will be exactly twice as hard to pedal. To accelerate at the same speed as with the 24"s you will have to put twice as much torque (therefore twice as much stress) into your drive train. You will be putting the same amount of horsepower into it in both case. Your RPM with the big tires is half as much as with the small ones, but you have to put out twice as much torque to develop the same HP since you are only turning half the RPM (Dang! That blasted HP equation you don't understand again!!). Why do you think Bravo drives are beefier than Alfa's? Using your theory the bigger props and higher torques put out by motors using Bravo drives should allow them to use lighter components than the Alpha's!

Works the same way with your car. Maybe that's why 454 Chevelles came with the heavier 12 bolt rear ends instead of the smaller 10 bolts?

 

[SuperNova]

Re: Prop pitch vs gear ratio...

I'm sorry 45. I really did think the post was meant as a joke. I do understand the HP formula and I didn't mean the formula itself was incorrect. I just had never seen it used to figure out how much horsepower it takes to turn something. Please excuse my ignorance.
My post, on the other hand, about rear driveline stresses WAS meant as a joke--thus the symbol. Sorry if you misunderstood my intention, I guess I wasn't clear enough.

Somebody else may be more able than I to clear up any other misconceptions.
--
Stan

 

[45Auto]

Re: Prop pitch vs gear ratio...

Hi Stan,

Sorry that I misunderstood your posts. Lots of my comments were pretty obnoxious since I didn't realize what you were saying. I guess that comes from my background where I became a strong believer that "the best defense is a good offense!". Please excuse my stupidity, I entirely misjudged the tone of your posts. I hate trying to communicate by this typing back and forth. It's even worse than telephones! I much prefer face to face communication.

Hopefully that little smiley thing means I'm happy!

 

[SuperNova]

Re: Prop pitch vs gear ratio...

Cool, we're buds again! The only other comment I'd like to propose, and I had this thought the other day, is when you talk about prop slip--I guess it would be the automotive equivalent of putting 10-speed bicycle tires on the back of that 454 powered 4.11 geared Nova. If you can't get bite, you're not going anywhere in a big hurry. Same with a prop in the water, although it takes a lot less change to make a big error with a prop and it's harder to tell exactly what is going on. With the car, the smoke lets you know.
--
Stan

 

[simmons527]

Re: Prop pitch vs gear ratio...

This is somewhat the same topic.

I have a 20 ft Century that had a alpha 1 stern drive with a 1.81 gear ratio. on a 170hp 4 cyl. mercruiser. What will happen to my boat if I put a pre alpha with a 1.64 gear ratio on it? The prop will be the same. Will my top ended be less or will it plane quicker?

 

[Bondo]

Re: Prop pitch vs gear ratio...

This is somewhat the same topic.

I have a 20 ft Century that had a alpha 1 stern drive with a 1.81 gear ratio. on a 170hp 4 cyl. mercruiser. What will happen to my boat if I put a pre alpha with a 1.64 gear ratio on it? The prop will be the same. Will my top ended be less or will it plane quicker?

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