Can someone explain this to me?

[rdhdfmn]

I reworked a friend of mines little 50cc go kart. It had a 50 cc motor on it, some type of transmission on it which reversed the rotation of the sprocket to run toward due to the way the motor was mounted. I didn’t tear the transmission down because he wanted to put it on another kart to see exactly what it did. He bought a 79cc motor from harbor freight, which I mounted on the kart. I went with an 11 tooth clutch and a 72 tooth sprocket. The original motor had a 10 tooth sprocket coming from the motor to a 49 tooth sprocket on the axle. The problem now is if they are on flat ground it runs fine, but when they go up an incline the kart slows way down. The original setup ran fine with no problems other than the motor wasn’t that reliable. My understanding (which may be wrong) is that the higher the ratio the more torque. Why is the kart slowing down with the new setup running a 6.54 to 1, but when the ratio on the original setup was 4.9 to 1 there wasn’t any hesitation going up hills. The pictures are of the original setup.
Thanks for reading

 

[JTSpeedDemon]

The previous 50cc engine must have had an internal gear reduction. Thus, you may actually have a higher gearing and not realize it.

Lower gearing means more torque, but less top speed.
Higher gearing means more top speed, but less torque and more wear on the clutch.


So you most likely have 2 issues here:

Gear ratio too high
Low horsepower(3 HP)


What's the diameter of the rear tires and the average weight of the driver? Those 2 factors make a big difference.

You may want to consider putting a 6.5 HP Predator 212 on instead if it will fit.

 

[rdhdfmn]

Hey speed demon. The rear tires are 13x5, and it’s a small child, around 65 pounds.

 

[JTSpeedDemon]

Try gearing lower or upgrading to a larger engine, of maybe modifying the old one to make more power.

Best option is to upgrade to the above mention Predator 212.

 

[J.S.@SMS]

I may be wrong, but the "transmission" is most likely a reduction gear box. And you're correct, the higher (numerically) the ratio, the more torque gets to the wheels. The trouble is that, while the final reduction on the 50cc (gearbox output to axle/wheel) is numerically lower than the setup with the 3hp, the 50cc actually has a higher reduction when the gearbox is factored in. It seems like those gear boxes are geared at about 3.5 ish to 1, so overall it had somewhere in the neighborhood of a 17.1:1, which makes sense since those little engines will turn at 8500 rpm with no load. So about the only thing to do is to gear it down some. What diameter tires does it have? Those will tell you what kind of ratio you can run.

---------- Post added at 08:51 PM ---------- Previous post was at 08:48 PM ----------

Ok, so there were some posts while I was typing and looking around at those gear boxes. But a 212 should move that kart with even a 6:1 reduction well enough with 13" tires. I havent had much experience with 3hp predators, but they might be a bit underpowered with that ratio..

 

[itsid]

^^ that

those tiny engines have an internal gear reduction on karts..
skipping that raises your ratio by a factor of threee at the very least.
and that had to be brought back down again of course.

'sid

 

[rdhdfmn]

Thanks for the help guys.

---------- Post added at 07:38 PM ---------- Previous post was at 07:37 PM ----------

I’m running 13 inch tires on it

 

[Smerft85]

I'd just like to chime in with, why buy the 79cc for $99 shelf price when he could have gotten the 212 for the same price with a coupon to begin with? Or am I thinking of a different size predator with that shelf price?

 

[J.S.@SMS]

Nope, The 79cc predator is $109.99 and the predator 212 is $124.99 without a coupon, $100 with a coupon.

 

[Smerft85]

That's what I was thinking. Last time I was in HF I thought that was crazy to get the smaller one at the same or higher price. Too bad they probably wouldn't exchange it for you.

 

[JTSpeedDemon]

I think if they sold the 79cc at like $50, that would be a steal, but it's just ridiculous to sell it for the same price as a 212!

 

[BigWes]

well to be honest it would be stupid for them NOT to sell those engines at almost the same price.

reason being as there is very little material difference between the actual parts and materials invested in the engine series up to the 212 size engine. It seems unfair to us as a consumer because we are comparing numbers but if you are honest with yourself you know I am speaking the truth.

 

[anickode]

I think if they sold the 79cc at like $50, that would be a steal, but it's just ridiculous to sell it for the same price as a 212!

Materials and manufacturing cost are almost the same, therefore the price is almost the same. The small savings comes from the small difference in materials, but the actual process of casting, stamping, machining, assembling, etc is virtually identical in time and complexity.

 

[JTSpeedDemon]

That's what I figured. I guess I should have watched my wording more carefully.
What I meant is that it's silly for a lot of people to get a 79 instead of a 212, unless a 212 is too much engine for their purposes.

 

[rdhdfmn]

Hey guys. With my limited knowledge on the transmission that came off of it I thought the 79cc would have been plenty, being that the one that came off of it was a 50cc. My friend little girl is 6 and I didn’t want that much power on a little kart like that. I have since learned the reason the 50cc worked so well. Live and learn.

 

[Smerft85]

Does that transmission/reduction box fit the 79cc? All we were saying is 99% of the time it makes much more sense to get the bigger engine for less money. Either engine would be just fine with proper gearing and leaving governors intact, you could get low 20mph ranges either way, or pull a governor and, at least with the 212cc, possibly into the 40mph range. If you already have the 79cc then I'd use it, but it may not fit the original transmission you have.

 

[itsid]

...
What I meant is that it's silly for a lot of people to get a 79 instead of a 212, unless a 212 is too much engine for their purposes.

if by people you mean "karters" then yeah you are about correct.

if you actually mean "people" however, you are not I'm afraid.

It does make sense to get the smallest viable engine for the job at hand,
even if the bigger engine is as cheap as the small one,
since it runs more efficiently and over the lifetime saves you enough fuel to still be the much better overall decision.

people do buy small (2kW) generators if they don't need a 5kW generator for example mostly because of size and fuel consumption.

Nevermind..

Now rdhdfmn.. you solved that already?
(btw that nick too much reminds me of "rtfm" to not say so
especially since if you had, you could have avoided this hiccup )

and if you didn't yet, what's your plan?
jackshaft and compound gearing, new engine, or trying to make the ol gearbox work again?

'sid

 

[rdhdfmn]

Hey sid. My plan is to put a 212 on it and a clutch. The reason I bought the smaller motor is because I didn’t understand how high the gear ratio was on the gearbox. If I had know what it was going to do I wouldn’t have bought that small of an engine. Thanks for the reply

 

[itsid]

Ah okay.. keep in mind that size does matter in this situation..
an engine bay to suit a 50cc may be too small to fit a 212..
so measure first

You'd always have the option to retrofit a jackshaft instead to match the gear ratio up

'#sid