Help gearing big wheels

[Bigdogashwood]

Hey guys!! So I built a go kart and it has 20" car wheels and tires. The overall diameter is 25". I had a 212 in it for a while but couple days ago I bought a 2020 250cc Falcon bike engine. Looking it up, it's actually a 250cc gy6 water cooled engine so it has a cvt built into it. It's rated at 25hp, 13lbs of torque and 7500rpm. on the bike the engine shaft was splined directly to the wheel, so I can't get any gearing info from the bike(I don't think). And when I put all the info into the gearing calculator it shows 130+mph at 6:1 ratio.. I know this is probably unchartered territory and there will be some trial and error to get the gearing right. I'm just looking for some info on what ratio would be the best starting point. I would like for it to be able to do 50mph easy. Thanks for any info/tips

 

[itsid]

yeah gy6-style engines are terribly documented (if at all by the mfg)
and there is close to none reliable information to be found on the internet.

but it's actually quite easy if you open up the beltcover and take some observations and measurements.

first,
primary reduction
(the crankshaft has a cogwheel driving a spurgear that in turn drives the variator pulley)
either count the teeth or just mark one tooth
remove the spark plug and belt, then turn the crank over by hand counting until the larger wheel is back at it's starting orientation.

then: secondary reduction..
that's a bit harder, since the gears are on the other side of the crankcase,
easiest here is indeed revolution counting..
so rotate the clutch pulley (the rear one) and observe the output shaft

And then it's getting a bit difficult.
Take a fine point permanent marker and mark the top of the belt on the variator pulley
on it's sheave, use that marking to measure the diameter the topside of the belt runs at.
active diameter is a bit less (usually that diameter minus 1/4-1/3 the belt thickness)

measure the outmost diameter of the variator belt (substract the same amount of belt thickness again)

And measure the outer diameter of the belt on the clutch pulley (again substract the fractional belt thickness)

large active diameter clutch pulley divided by small active diameter variator pulley
gives you the starting ratio of the CVT.

that is the most mportant one since IT dictates what final ratio you can install without burning the belt or stalling the engine.

if you want the top speed, it's a bit nastier..
since you cannot just guess the smallest active diameter on the clutch pulley,
you can only calculate it..
it's dictated by the center to center distance of the two pulleys,
the largest active diameter on the variator and the belt length
since the conacting angle changes slightly it's a bit complicated math, but you can do it
if you put some effort in.

That then will tell how fast you will be able to go with that engine.

A good estimate can be taken off the bike's documentation however..
knowing it's top speed, the max rpm and the wheel diameter you can calculate
the overall ratio for the bike.
and derive the missing piece of info from that.

car wheels? that is not a good idea.. way too heavy to make sense.
especially on a low power engine like that.
You will vertainly NOT enjoy that outcome.

'sid

 

[anickode]

Depending on how much the whole thing weighs, 50 might be a good top speed to shoot for. The problem is those huge wheels will weigh a lot compared to go kart wheels, which means a lot of rotating mass. Big rotating mass = huge rotational inertia, which takes a lot of torque to overcome. I get the feeling that even geared for a max of 25mph, your acceleration will be abysmal with only 13 ft/lb peak torque.

Aerodynamic drag also becomes a huge factor above 50mph, and there's nothing aerodynamic about 4 massive wheels sticking out in the air, so 50-60 might be a realistic top speed even with 25hp.

The other big consideration here is brakes. Once you get those huge wheels spinning, it will take a lot more than a normal 4" band brake or small mechanical disc on the axle to bring it all to a stop. I hope you've taken this into consideration.

Steering can also be tricky. Again, large rotating mass develops a lot of gyroscopic force, which will be harder to overcome, and the large flat contact patch will make dialing in any sort of "ideal" steering geometry even more difficult, as a typical caster setting actually tilts the wheels slightly when cornering. That's not an issue with a small tire and a roundish tread cross-section.

I'm not trying to tell you it won't work at all, I'm just noting that huge wheels like that can open up a whole next level of design considerations that I hope you've taken into account.

Would you mind sharing some pics?

 

[Bigdogashwood]

yeah gy6-style engines are terribly documented (if at all by the mfg)
and there is close to none reliable information to be found on the internet.

but it's actually quite easy if you open up the beltcover and take some observations and measurements.

first,
primary reduction
(the crankshaft has a cogwheel driving a spurgear that in turn drives the variator pulley)
either count the teeth or just mark one tooth
remove the spark plug and belt, then turn the crank over by hand counting until the larger wheel is back at it's starting orientation.

then: secondary reduction..
that's a bit harder, since the gears are on the other side of the crankcase,
easiest here is indeed revolution counting..
so rotate the clutch pulley (the rear one) and observe the output shaft

And then it's getting a bit difficult.
Take a fine point permanent marker and mark the top of the belt on the variator pulley
on it's sheave, use that marking to measure the diameter the topside of the belt runs at.
active diameter is a bit less (usually that diameter minus 1/4-1/3 the belt thickness)

measure the outmost diameter of the variator belt (substract the same amount of belt thickness again)

And measure the outer diameter of the belt on the clutch pulley (again substract the fractional belt thickness)

large active diameter clutch pulley divided by small active diameter variator pulley
gives you the starting ratio of the CVT.

that is the most mportant one since IT dictates what final ratio you can install without burning the belt or stalling the engine.

if you want the top speed, it's a bit nastier..
since you cannot just guess the smallest active diameter on the clutch pulley,
you can only calculate it..
it's dictated by the center to center distance of the two pulleys,
the largest active diameter on the variator and the belt length
since the conacting angle changes slightly it's a bit complicated math, but you can do it
if you put some effort in.

That then will tell how fast you will be able to go with that engine.

A good estimate can be taken off the bike's documentation however..
knowing it's top speed, the max rpm and the wheel diameter you can calculate
the overall ratio for the bike.
and derive the missing piece of info from that.

car wheels? that is not a good idea.. way too heavy to make sense.
especially on a low power engine like that.
You will vertainly NOT enjoy that outcome.

'sid

That is alot of great info! I really like the idea of estimating it from bike documentation. The rear wheel on bike is 22" overall diameter with manufacturing specs rating the motor at 7500rpm and max speed of 90mph.(owner said he got up to 85mph on it) and So that means the ratio is 5.4:1, correct?

So with that being the case, should I gear for the same ratio? Or since the motor already has a gear reduction on it, should I go lower or higher.. thanks in advance

 

[mckutzy]

Lets see some pic of what you have???....

 

[itsid]

That is alot of great info! I really like the idea of estimating it from bike documentation. The rear wheel on bike is 22" overall diameter with manufacturing specs rating the motor at 7500rpm and max speed of 90mph.(owner said he got up to 85mph on it) and So that means the ratio is 5.4:1, correct?

So with that being the case, should I gear for the same ratio? Or since the motor already has a gear reduction on it, should I go lower or higher.. thanks in advance

90mph is a lie.. that motorpower is barely enough to go 84.51 mph..
so a tiny bit of inaccuracy of the PO's tach,
his 85mph is a decent assumption... the 90mph is clearly a lie
(well one could reach that speed downhill with enough bravery willing overrev the engine )

Anywhoo the overall high ratio is likely closer to 6.6:1.
(your math is halfway correct, but you calculated for 22" wheel diameter [at least that's when it matches up], the assumption unlikely was )

then again.. maybe the engine cannot rev up to it's full potential
under that kind of load; sooo yeah

it gets much worse for you I'm afraid...
those four wheels are essentially as heavy as the rest of the bike
(minus the engine mind you) maybe even heavier.
(a 20" rim is ~15-17kg, a tyre another 10-13kg that's at worst 120kg of just wheels [265lbs])
then adding more weight for a frame and seats and whatnot..

it will simply not going to be very pretty,
that speed is certainly out of reach.

if we assume ONLY wheels and engine as moved mass,
you will already need an overall gear ratio of 11.5:1,
And even worse, that weight alone is enough to drop the top speed to below 84mph already..

Now if we just assume a total vehicle weight of just a professionally made gy150 kart (manco carbide max weight 875lbs) and add the additional weight of the wheels we easily crack the 1000 lbs..
say 480kg (1060 lbs)...

aaaand that means the top speed dropped to 79.6 mph at best.

the minimum low gear ratio would need to be in the realms of 21:1
overall.
(hence you need to know the CVT low speed ratio!)

and the rest is going to be simple math..

Now since your goal was to hit a solid 50mph
you can certainly try to take that to calculate the ratio
and just hope for the best..
the torque you need to accelerate to 50mph however is likely going to bite you.

so for 53mph you'd need an overall high ratio of 10.5:1
with the engine's high ratio of 6.6:1 that means
you need a 1.6:1 chain ratio to get there

BUT keep in mind the low ratio needs to be in the realms of 21:1
and that means the engine's low ratio would need to be 13.1:1 or lower..
and that is very very!! unlikely...
at best I would assume something like 10:1 max..
means you need a 2.1:1 chain ratio and only be able to reach maybe 40 mph

Assumptions assumptions..

go measure and count teeth.. nothing else will be able to give you
a reliable enough answer to go and buy sprockets/chain.

'sid

 

[Budget GoKart]

There was a four wheeler with 25" wheels from a car it looked like the suspension wasnt happy neither was the motor and top heavy aswell but if you do it right it might be cool WE NEED PICS (or want them )

 

[Bigdogashwood]

So as I mentioned I previously had a stage1 212 predator on it and it actually did a decent job for what it was. Gps said 34mph.

I had a TC on it with a 10t going to 20t on jackshaft which had a 10t on other end going to a 60t on the sprocket. Would that make an 8:1 great ratio? If not l, what would it be? if I can get that same gear ratio then I will be golden. If you could, would you confirm that it was an 8:1 and if not what was it Thanks again itsid

There was a four wheeler with 25" wheels from a car it looked like the suspension wasnt happy neither was the motor and top heavy aswell but if you do it right it might be cool WE NEED PICS (or want them )

Here u go

 

[itsid]

So as I mentioned I previously had a stage1 212 predator on it and it actually did a decent job for what it was. Gps said 34mph.

I had a TC on it with a 10t going to 20t on jackshaft which had a 10t on other end going to a 60t on the sprocket. Would that make an 8:1 great ratio? If not l, what would it be? if I can get that same gear ratio then I will be golden. If you could, would you confirm that it was an 8:1 and if not what was it Thanks again itsid

2:1 JS and 6:1 axle makes a 12:1 ratio (multiply, don't add )
And that's WITHOUT the TC..
with a series 30 TC (I assume you had a TAvKit right?) you had a 32.4:1 low ratio

5000 rpms.. then.. governor removed, no insane mods..
that's good, really good actually!

But if we assume it was a good gear ratio, and you haven't burned through belts alot,
then the weight might still be in the 1000lbs realms..
KartInfo by kartcalc.net:

[FONT="Verdana"]
[B][U]KART[/U][/B]
[INDENT][I]RW diameter[/I]: [B] 63.5 cm | 25 in[/B]
[I]Total weight[/I]: [B] 435 kg | 959.01 lbs[/B]
[/INDENT]
[B][U]ENGINE[/U][/B]
[INDENT][I]Original HP[/I]: [B] 7[/B]
[I]Asummed HP[/I]: [B] ~7.78[/B]
[I]Torque[/I]: [B] 13.85 Nm | 10.21 ft lbf[/B]
[I]RPM[/I]: [B] 5000[/B]
[/INDENT]
[B][U]TRANSMISSION[/U][/B]
[INDENT][I]Final ratio[/I]: [B] 12:1[/B]
[I]TC type[/I]: [B]  series 30 6" driven[/B]
[I]Lowest ratio[/I]: [B] 32.4:1[/B]
[I]Highest ratio[/I]: [B] 10.8:1[/B]
[/INDENT]
[B][U]PERFORMANCE[/U][/B]
[INDENT][I]Max Wheel Torque[/I]: [B] 448.74 Nm | 330.97 ft lbf[/B]
[I]Min Wheel Torque[/I]: [B] 149.58 Nm | 110.32 ft lbf[/B]
[I]Acceleration[/I]: [B] 1.85 m/s² | 6.07 ft/s²[/B]
[I]Top Speed[/I]: [B] 55.41 km/h | 34.43 mph[/B]
[/INDENT]
[/FONT]

See? that's about the highest weight that doesn't trigger a warning,
since that calculator is rather conservative with belt/clutch life and such
maybe a few more pounds even...

Now the CVT in your new gy6 is not the same as the TC you had;
IDK what ratios it can provide you with, and IDK how low down it can gear..
but seeing some of those chinese 125cc scooters struggling uphill with a single person seated,
it cannot be too good I'm afraid considering they're using about the same CVT you have
(with similar but at least comparable internal gearing)

IDK, I honestly don't,
if you have the manual of the bike the engine was taken from..
could you attach it please?
maybe I can find some more usefull info, maybe I can dig up the partslist for your engine ...

[EDIT]

Okay.. I can only find the CFMoto 250 (172MM) but it only has 17horses max
eight less than your claimed 25hp
but it's got 17.6Nm of torque (12.98 ft lbf) so one of your numbers is incorrect.. I have a good idea which one tbh
anyways...
the bike had a 150/80-15 rear tyre which kinda comes close to the 22" OD (nominal 24" I think)

AND IT'S internal gearing I know
(primary is 2.64:1, secondary is 2.85:1 and the CVT ranges from 2.1:1 in low to 0.88:1 in high gear)
makes it's overall high gearing 6.62112:1 (HA! I guessed 6.6...not baaad)
and it's overall low gearing 15.8:1 (Shirt.. I guessed only 10:1 that was bad ... *giggle*)
Anywhoo
if that is your engine, a final 1.6:1 chain ratio should actually work
even with just 17horses.

Oh there are MANY different manufacturers for that generic chinese scooter engine,
some have different power, some have different internal gearing some both..
so be aware that the above info might NOT fit your actual engine just one of it's siblings

'sid