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Old 08-18-2019, 07:38 PM
zbillz zbillz is offline
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Default Changed tav2 jackshaft sprocket from 12 to 15 tooth

Hi,

I recently replaced the motor in my sons kart. It had a briggs 5hp and we replaced it with a go power sports tillotson 10hp.

Current setup, 11 inch tires, tav2, 15 tooth jackshaft sprocket and 55 tooth rear axle sprocket. Kart plus driver weight around 250lbs.

Ran with a 12 tooth torque converter sprocket first and the thing accelerates like crazy and got a top speed of 26mph around 3650 rpm. Governor still intact.
Put a 15 tooth converter sprocket to get a little more mph and it looks like it lost some top speed...could only get around 22 mph at 3650rpm.
Almost looks like the belt on the driven side of the converter does not pull all the way in when the kart is at its top speed.

Any ideas on why this would happen?

Ultimately will be removing the governor but want to get this straightened out first.

Thanks!
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Old 08-18-2019, 09:30 PM
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Originally Posted by zbillz View Post
Hi,

Put a 15 tooth converter sprocket to get a little more mph and it looks like it lost some top speed...could only get around 22 mph at 3650rpm.
Almost looks like the belt on the driven side of the converter does not pull all the way in when the kart is at its top speed.

Any ideas on why this would happen?
...
SURE, craptacular gear ratio!

with a chain ratio of 3.67:1 you just do not have enough torque for the TC to shift all the way up,
essentially to 3.3:1

luckily, the 9.9:1 at take off keeps the belt from shredding,
but still the ratio is just insanely bad, even for a lightweight pack and small wheels.

what you want is to hit the calculated speed with the gear ratio you set.
and you haven't even with the 12T TC sprocket (assuming true 11") .. 28.6 would have been your goal, not 26'ish

Ideally you measure from the center of the axle down with your kid seated,
then multiply by two to get the effective wheel diameter
and use that for calculations

say 10.5" of effective wheel diameter bring the sped down to 27.3 mph
a straight 10" to exactly 26 mph

And that's why effective wheel diameter and some extra effort can be your friend.

I highly doubt the 10 horses btw...
since if it really had 10 horses
it'd also had the torque to reach top speed with your terrible ratio btw.
(as long as the kid's not growing that is)

So IMHO: back to the 12T and get a proper measurement..
maybe loose a tooth or two on the axle (which is a totally different thing than adding two teeth to the TC!!!)

'sid
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Old 08-18-2019, 10:04 PM
zbillz zbillz is offline
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Thanks for chiming in Sid.

I just measured the slicks and your correct, even though they are marked 11inch they measure 10.5 without any weight.

Are you saying I should try a 52 or 53 tooth rear and back to the 12 tooth to get a better final drive ratio, 4.33 overdrive 3.9?

If this motor is not really 10hp the kart may never achieve calculated speeds?

Anybody have experience with this motor?
I figured since its rated 10hp out of the box, it's better than a new predator and purchasing mods. Liked the idea of not having to run a loud header pipe.

Thanks again!
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Old 08-19-2019, 07:22 AM
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I'm saying you should first verify you have enough power to actually reach the geared-for speed

Or, to put it in the correct order:
to gear such that you can actually reach the speed the motor is able to push the kart(and kid) at.

And at first that means 12->55 and a verification run or two.
12->53 is certainly ALOT better than 15->55

IDK what the motor actually is able to provide,
one would only know with a series of dyno runs really..

My best guess is: it's not better than any other 212 with light mods
so it's a ~12-13Nm max torque at around 2500 -2800 rpm (and getting less from there on)

And yes in theory that could still mean you reach 9-10'ish horses at 5.5-6k rpm
but it does NOT mean you have the power of two briggs flatheads
In fact the 5hp flathead has more torque than the 212 [not higher peak, but for a much wider band]

And then there's also the downside of torque converters ..
namely they're less than 80% efficient!
so more than 20% of the engine power is lost before it reaches the rear axle.


'sid
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Old 08-19-2019, 08:29 AM
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Thanks Sid!
Will go back to the 12 tooth TC sprocket, re-measure wheels with him in kart and see if we can match calculated speed. Will ultimately try a 53 tooth also.

Will removing the governor help for more MPH or will the Kart just be stuck at that speed since the motor can't shift the TC into OD due to power loss from belt or lack of power from the motor?
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Old 08-19-2019, 08:56 AM
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Will removing the governor help for more MPH or will the Kart just be stuck at that speed since the motor can't shift the TC into OD due to power loss from belt or lack of power from the motor?
That's what we're trying to find out..

My guess is: it will not have the power to reach top speed with 12->53
frankly I doubt it would with 12->55
I'd say roundabout 12->58 is needed to rev to ~5500 or so
(but that's nothing but a wild guess, since I'm too lazy to calculate a better guess)

IDK why you want to propell your kid to 50mph in a kart to be honest,
but it's your kid, you pay the hospital bills, just try not to kill it please

So let's see how much juice is left, and then decide how much can be converted to speed, and what should be left for torque.

'sid
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Old 08-19-2019, 10:29 AM
zbillz zbillz is offline
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Not looking to get to 50 but he is asking "can you make it a little faster" he is 14 and we have a big long straight stretch of street in front of the house...not looking to kill him, he will be fine.
Will switch back to 12 tooth tonight and see what happens .

Thanks for the help and concern.
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Old 08-19-2019, 11:57 AM
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15->55 on 10.2" is 50.57mph @5500 rpm (removed governor)
Sooo if removing the governor was the plan and the 15->55 would've worked.. 50 mph!

12->55 would still end up at roughly 40+ mph under the above conditions

(not sure it'd sweep up to 5.5k but chances are it will)
Sooo lower gearing is my recommendation (almost all the time)
12->58 [as I said above] at a 'very likely possible' 5k rpm is roundabout 35'ish mph
(38'ish mph @5500 rpm)

plus it'd be even quicker at take off

'sid

PS for all speeds above ~28 mph or such you want the governor to be gone or you'll risk burning the belt/clutch... no matter what hp engine, since it's the takeoff speed (road speed required for clutch engagement) killing it then...
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Old 08-19-2019, 04:26 PM
zbillz zbillz is offline
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Thanks again Sid,
I really never did the calcs at 5500rpm to see the 50 mph speeds......
Going back to 12>55 tonight.
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Old 08-19-2019, 08:01 PM
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I only did because you said
Quote:
Ultimately will be removing the governor but want to get this straightened out first.
so I was under the impression the governor removal is on your to do list already
Nevermind..

'sid
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Old 08-19-2019, 08:27 PM
zbillz zbillz is offline
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Originally, my bright idea was replace with 15 tooth TC sprocket,get 35mph governed and call it a day.
Didn't quite work out that way. Ha!
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