Could my engine RPM some how be LIMITED?!

[bgm94]

I use the speed calculator on the site a lot, was pretty accurate for my minibike speed. I then tried to use it on my reverse trike gokart i built. on the GPS it said my speed was 38mph (engine sounded topped), but on the calc fully revved it said 46mph.
GoKart Setup:
Engine: 11hp 346cc HF greyhound
Tire Diameter: 16in
Max RPM: 3600
Axle Teeth: 45
Clutch teeth: 12
Comet 40 series Torque Converter

But when i plugged in 3000rpm, i got 38mph in the calc.
Is it possible my engine could be limited for some reason?

 

[Jake.B]

Well i dont think the calc adds in weight. The weight from the driver and cart will slow your top speed.

 

[Blazkowiez]

Are you sure you want to go 46 in that thing?

 

[landuse]

Jake.B is correct. The weight of the kart and the rider is not taken into account in the calculator. A minibike is smaller, therefor it doesn't matter too much, but a kart is a whole lot bigger. It is a theoretical answer that you get if all conditions are perfect. In reallity you have a heavy kart, more tyres on the road (more friction), wind resistance etc.

 

[robbie]

Jake.B is correct. The weight of the kart and the rider is not taken into account in the calculator. A minibike is smaller, therefor it doesn't matter too much, but a kart is a whole lot bigger. It is a theoretical answer that you get if all conditions are perfect. In reallity you have a heavy kart, more tyres on the road (more friction), wind resistance etc.

I don't want to be a nitpicker, but wind resistance is negligible at these speeds.

Wind resistance is a function of the square, meaning that when you double your speed, you get 4 times the wind resistance. It starts becoming a major factor at highway speeds, but under 50 mph you're not going to notice it much. You'll feel it on your face, of course, but you can ignore it in your calculations.

 

[landuse]

I don't want to be a nitpicker, but wind resistance is negligible at these speeds.

Wind resistance is a function of the square, meaning that when you double your speed, you get 4 times the wind resistance. It starts becoming a major factor at highway speeds, but under 50 mph you're not going to notice it much. You'll feel it on your face, of course, but you can ignore it in your calculations.

All I am really saying is that there are a lot of factors that the calculator doesn't take into account, no matter what they might be.

You are correct though, but wind can rob you of 1 or 2 mph

 

[jason4207]

Have you tried going downhill to take the load off the TC, so that it can fully open into top gear?

Maybe the TC needs a higher engine speed to fully engage the top gear?

 

[bgm94]

Have you tried going downhill to take the load off the TC, so that it can fully open into top gear?

Maybe the TC needs a higher engine speed to fully engage the top gear?

great idea! will try soon.

also, top speed is not affected by weight
Source: My Physics Teacher

 

[jamyers]

I suggest getting one of those $12 tachometers and confirming actual rpm.

Like this: http://www.ebay.com/itm/230760981942

 

[Jake.B]

Right but engine power is correct? If your engine is under load your not going to get FULL potential. For example, You can run 15MPH try doing that with 90 pounds on your back. I understand what your physics teacher is saying. He may be right but this just seems like common sense.

 

[bgm94]

Right but engine power is correct? If your engine is under load your not going to get FULL potential. For example, You can run 15MPH try doing that with 90 pounds on your back. I understand what your physics teacher is saying. He may be right but this just seems like common sense.

only acceleration is affected by top speed, and yeah i should invest in a tach.

 

[Grimace]

great idea! will try soon.

also, top speed is not affected by weight
Source: My Physics Teacher

Your teacher is correct, but in a limited amount of space, weight will affect top speed since the engine might not have enough time to achieve max power even though it sounds like it is wound out. Another suggestion is to get into the engine and remove the governor gear. That will allow you to make more power via higher rpms.
Think about 1/4 mile race cars. The lighter they are, and the more power they have let's the achieve top speed quicker. Of course gearing and aerodynamics come into play. And other crap.

 

[bgm94]

Your teacher is correct, but in a limited amount of space, weight will affect top speed since the engine might not have enough time to achieve max power even though it sounds like it is wound out. Another suggestion is to get into the engine and remove the governor gear. That will allow you to make more power via higher rpms.
Think about 1/4 mile race cars. The lighter they are, and the more power they have let's the achieve top speed quicker. Of course gearing and aerodynamics come into play. And other crap.

thanks for that but i already am aware of all those things, im just wondering if the engine might possibly be limited somehow

 

[Grimace]

It is limited by the governor. I guess I don't understand your question if that's not what you're talking about. I see right around 3550 rpm wide open throttle on my predator 420. I have been reading that by removing the governor it will spin to 4600 rpm. I may or may not do this since the clutches are limited to 3600 (I think).

EDIT: I just read that clutch engagement is min 1600 rpm max is 3100 rpm. Does that mean that I shouldn't spin over 3100 with this clutch? If so, that sucks. Series 40 driver from gokart supply.

 

[r97]

I think there are three possible scenarios:

1. Engine is reaching top speed, but something is slipping (belt?).
2. Engine cannot reach top speed because of a large load (need to change gearing).
3. Your throttle linkage, or governor linkage is not working correctly.

 

[jason4207]

great idea! will try soon.

also, top speed is not affected by weight
Source: My Physics Teacher

Yes, but the clutch is affected by load (weight).

Also, I'm not so sure if that physics statement applies to something under 1G of downward force (gravity). Take any car and add 1000lbs to it. I seriously doubt it would reach the same top speed on a flat. Going down hill it might get there, but then you are removing some of the effects (or adding them?) of gravity.

But out in space I completely agree with that statement.

 

[jason4207]

EDIT: I just read that clutch engagement is min 1600 rpm max is 3100 rpm. Does that mean that I shouldn't spin over 3100 with this clutch? If so, that sucks. Series 40 driver from gokart supply.

No, it means you set the beginning of the clutch engagement at 1600-3100rpm. Top rpm is of no concern here. Once it fully engages you can rev as high as your engine setup will allow. Just set it enough above your engine's idle speed so that it doesn't move the cart at idle.

 

[Doc Sprocket]

To attempt to condense all of this into basic english-

You're geared too high for the amount of power your engine can make. Your current gear ratio is 3.75:1. In a brief statement- The calculator might indicate "X"MPH@3600 RPM, but if you don't have enough power to push it that hard, you will not reach that RPM and therefore will not reach that speed. You need to lighten the engine's load and allow it to reach the upper RPM limit.

How? Gearing. Let me give you a real-world,"I-did-this" example to put it into perspective. My kart featured an 11hp GX340, Comet 40 CVT, 17.5" Tires. Final gear ratio was 4:1. I had the EXACT problem you did. I DID mess with the governor and determined that it was not the problem. I DID mess with the CVT engagement/shift points to see what it would do for me. Not a lot. What did I do?

I regeared from 4:1 to 4.8:1. Not only did my acceleration improve (totally predictable) but so did my top speed. The load on the engine was reduced, making it easier for my engine to spin up to full RPM. Overall, I picked up 3-4 MPH- This is one of those rare circumstances where gearing DOWN increases top speed.

This is assuming everything is more or less copacetic with your engine, and your throttle mechanism does allow full cable/throttle travel.

One final thought- I have absolutely no idea what the EXACT context is, in which your physics teacher indicates that weight does not affect speed, but in OUR context, it most certainly does. I will defend that statement to the death. Theory is one thing, but the proof is in the practise.

 

[fowler]

your teacher is correct
but as a mathamatican and pyshics teacher he is thinking in a perfect world
kinda like the speed calculator is

more weight adds more strain on the tyres thereby making more friction

 

[Doc Sprocket]

he is thinking in a perfect world

LOL. Probably my favourite saying to date, been using it for years- "Theory and practice are two very different things." And yes, you may quote me...