Max RPMs, Gearing and Power

[Mato]

Hi guys,

Im new to the forum, Ive been reading a lot and found out that there are plenty of really knowledgable guys in here, which is great.

Right now I dont have any project in my hands, but im looking forward to start with my own karting in the next few months. The big issue is Im from Argentina, and nice tools (saw MIG welder, drop saw, etc) are really expensive, and I rather save for a nice one than buy a crappy one that wont last all that much.

The reason of this topic is Ive been refreshing my mind with gearing and jackshafts (Ive even made a nice excel sheet to make all those calculations, I can upload it and share it if youre interested) and run into a few "conceptual" doubts.

While calculating the max speed you can reach with your go kart, everything is asuming that the engine can make it to a certain RPMs, let say 3600. So, if you think than both, a 6hp and a 8hp engine could reach the same max 3600 RPMs, thats to say they both could reach the same final speed (asuming same gearing on both)? If so, clearly the 8hp will get there faster (due to the extra torque available) and that would be the only difference?

Following the idea, I'm not sure if, for example, the 6hp could actually get to the 3600 RPMs carryng a load of 180kg (say thats kart + driver weight), but I wouldnt doubt that the 8hp could get there driving the same weight. If that would be the case, the actual max RPMs of the 6hp could drop to a max of 3000 RPMs, wheather the 8hp could take advantage of the full 3600 RPMs. So that would add an advantage on the actual final speed the 8hp engine could reach (added to the fact that it "gains" speed faster). Values are just to put my ideas into numbers, Im just making them up more or less.

And a final thought, how is that the wheel diameter would affect the gearing calculations? I've been using wheel circunference for getting the km/h, but I havent involved its diameter for gearing purposes. If I think about it, it may be working as an extra sprocket, but because the gearing ratio is calculated with "teeth" and not using the actual radius of each srocket, Im having a hard time getting it. For getting the most accurate possible values, should the wheels be included in the calculations?

Ok, that was pretty long! haha, hope I find some guys interested in discussing this a little bit.
Thanks!
Mato

 

[r97]

Hi guys,

Im new to the forum, Ive been reading a lot and found out that there are plenty of really knowledgable guys in here, which is great.

Right now I dont have any project in my hands, but im looking forward to start with my own karting in the next few months. The big issue is Im from Argentina, and nice tools (saw MIG welder, drop saw, etc) are really expensive, and I rather save for a nice one than buy a crappy one that wont last all that much.

Sounds good, you wont regret spending the extra money now to get some proper tools!

The reason of this topic is Ive been refreshing my mind with gearing and jackshafts (Ive even made a nice excel sheet to make all those calculations, I can upload it and share it if youre interested) and run into a few "conceptual" doubts.

If it isn't much trouble you might as well post it. I'm sure someone will be able to use it, and I'm always curious to see what others are doing.

While calculating the max speed you can reach with your go kart, everything is asuming that the engine can make it to a certain RPMs, let say 3600. So, if you think than both, a 6hp and a 8hp engine could reach the same max 3600 RPMs, thats to say they both could reach the same final speed (asuming same gearing on both)? If so, clearly the 8hp will get there faster (due to the extra torque available) and that would be the only difference?

In general with industrial engines, yes. The two engines could reach the same top speed, but the 8hp engine would be able to accelerate to that speed in less time.

Following the idea, I'm not sure if, for example, the 6hp could actually get to the 3600 RPMs carryng a load of 180kg (say thats kart + driver weight), but I wouldnt doubt that the 8hp could get there driving the same weight. If that would be the case, the actual max RPMs of the 6hp could drop to a max of 3000 RPMs, wheather the 8hp could take advantage of the full 3600 RPMs. So that would add an advantage on the actual final speed the 8hp engine could reach (added to the fact that it "gains" speed faster). Values are just to put my ideas into numbers, Im just making them up more or less.

You are correct here as well. A 6hp engine could easily be bogged down to 3000rpm by the same load that an 8hp engine is able to pull all the way to 3600rpm

And a final thought, how is that the wheel diameter would affect the gearing calculations? I've been using wheel circunference for getting the km/h, but I havent involved its diameter for gearing purposes. If I think about it, it may be working as an extra sprocket, but because the gearing ratio is calculated with "teeth" and not using the actual radius of each srocket, Im having a hard time getting it. For getting the most accurate possible values, should the wheels be included in the calculations?

Diameter is used because it is easy to measure, and it allows someone to easily find other measurements. One use of the diameter is to enter it into a formula to find the circumference of the wheel so speed can be calculated. Another use is to utilize the diameter to find the radius of the wheel to calculate how much force is pushing the vehicle forward. If you are just trying to figure out a gear ratio, the axle rpm, or the amount of torque being put into the axle, wheel size is unimportant. Other calculations like vehicle speed, amount of force pushing the vehicle, etc. require the size of the wheel.

If you are doing something like pressing the engine PTO against the tire to drive the tire, then wheel size would matter in calculating the gear ratio, but it doesn't sound like that's what you're interested in.

Ok, that was pretty long! haha, hope I find some guys interested in discussing this a little bit.
Thanks!
Mato

Hope that helps, let me know if anything needs clarification. Please keep in mind that there are other variables that, although unlikely to affect things, they could. Like you said these numbers are just for example, once you start using more specific gear ratios, wheel sizes, engines, etc. you can look at the other variables, if you want to that is.

 

[Mato]

Hope that helps, let me know if anything needs clarification. Please keep in mind that there are other variables that, although unlikely to affect things, they could. Like you said these numbers are just for example, once you start using more specific gear ratios, wheel sizes, engines, etc. you can look at the other variables, if you want to that is.

Hi r97,

Thanks for your replies, you helped me to get some concepts straight!

I've uploaded the spreadsheet I talked about, its in this post:
http://www.diygokarts.com/vb/showthread.php?t=21633

BTW, what would be an average engine for a 90kg guy like me? Ive done some local research and for a new chinese engine (Just 5.5 hp and 2900 RPM) they are asking around 150 dollars.

I don't thinks that's enough power to have some fun, with a gear ratio of 6 (12/72) I only get under 15 miles/hr, and Im hoping there would be some torque, but Im not all that optimistic about it.

Hope it helps!
Mato

 

[r97]

With the setup that you described, in an average go kart (relatively flat street use, maybe 12" OD tires, one seat, centrifugal clutch, etc.), 15mph is probably about what you would expect to still have a decent ability to accelerate. If you could get an engine that revs closer to 4000rpm, or possibly remove the governor from the one you mentioned, then you might expect something like 25 to 30mph.

What engine you want depends on a lot of factors. Will you be using a centrifugal clutch, a CVT, or something else? What size tires do you want to use? How big will the kart be? One seat or two? Where will you be riding? Will there be hills? Mud or sand? How fast do you want to go? How important is the ability to accelerate quickly to you? Electric start? What kind of engine budget do you have? (150 USD really isn't a terrible price for a new Honda GX160 clone). Do you need an engine that can just bolt in, or are you willing to make special mounts, do some wiring, etc.?...

That list could go on even further, but in short we need more details about what you're planning to make if you want us to help you choose a good engine.