Gear ratio for my build

[Allracing]

Hi,

I could need some advise on gearing for my wheelbarrow hotrod :

http://www.diygokarts.com/vb/showthread.php?t=38258

Data:
Engine: B&S Vanguard 6.5HP
Rear wheel diameter: 19"
Governed rpm: 3600
Torque converter TAV2
Estimated cart weight with driver: 300lbs
Rear axle ratio: 3.8:1

These numbers equate to a theoretical top speed of 54 mph, way too high for my application (kids kart)

I see that comet sells a device that can limit the travel of the torque converter and thereby reduce top speed. However I would never need this cart to go above say 30-35 mph so I am thinking I should reduce gearing further. I have the opportunity to introduce gear reduction between the torque converter sprocket on the driven pulley and what will be the drive shaft going to the rear axle. The sprocket on the TAV2 has 10 teeth and I could have a 15-16 tooth sprocket on the drive shaft. Top speed would then be 34-36 mph. I could also live with lower top speeds.

I guess what it comes down to what kind of gearing this kart would need to have decent acceleration with the 6.5hp engine (top speed being less important)?

 

[Tpdingo]

54mph out of a 6.5hp governed motor is going to get you no where. That engine will never put out enough power to manage that. Get a 5:1 - 6:1 ratio to get to 35mph and you should be good. I have 5:1 on a 6.5hp predator and 18" tires.

 

[mckutzy]

Way too high to maybe even get off the start... deff need to be around 6:1..

 

[itsid]

ratio needs to be dialed in properly..
depends mainly on weight and wheelsize..
with 19" wheels and an assumed weight of 300 lbs
you need a 5.3:1 ratio at the very least (43 mph'ish)

the more the better in terms of torque

6.3:1 would put the top speed to around 35.89 mph
so on the upper end of your target speed

7:1 would result in 32.3 mph

and 6.5 as somewhere in between.. somewhere in between of course:
KartInfo by kartcalc.net:

[FONT="Verdana"]
[B][U]KART[/U][/B]
[INDENT][I]RW diameter[/I]: [B] 48.26 cm | 19 in[/B]
[I]Total weight[/I]: [B] 136.08 kg | 300 lbs[/B]
[/INDENT]
[B][U]ENGINE[/U][/B]
[INDENT][I]Original HP[/I]: [B] 6.5[/B]
[I]Asummed HP[/I]: [B] ~6.5[/B]
[I]Torque[/I]: [B] 12.86 Nm | 9.48 ft lbf[/B]
[I]RPM[/I]: [B] 3600[/B]
[/INDENT]
[B][U]TRANSMISSION[/U][/B]
[INDENT][I]Final ratio[/I]: [B] 6.5:1[/B]
[I]TC type[/I]: [B]  series 30 6" driven[/B]
[I]Lowest ratio[/I]: [B] 17.55:1[/B]
[I]Highest ratio[/I]: [B] 5.85:1[/B]
[/INDENT]
[B][U]PERFORMANCE[/U][/B]
[INDENT][I]Max Wheel Torque[/I]: [B] 225.69 Nm | 166.46 ft lbf[/B]
[I]Min Wheel Torque[/I]: [B] 75.23 Nm | 55.49 ft lbf[/B]
[I]Acceleration[/I]: [B] 3.9 m/s² | 12.8 ft/s²[/B]
[I]Top Speed[/I]: [B] 55.98 km/h | 34.78 mph[/B]
[/INDENT]
[/FONT]

And 6.5 is a nice number since it adds quite a bit of weight allowance...
450 lbs with just 6:1 ratio would throw this Warning:
KartInfo by kartcalc.net:

[FONT="Verdana"]
[B][U]KART[/U][/B]
[INDENT][I]RW diameter[/I]: [B] 48.26 cm | 19 in[/B]
[I]Total weight[/I]: [B] 204.12 kg | 450 lbs[/B]
[/INDENT]
[B][U]ENGINE[/U][/B]
[INDENT][I]Original HP[/I]: [B] 6.5[/B]
[I]Asummed HP[/I]: [B] ~6.5[/B]
[I]Torque[/I]: [B] 12.86 Nm | 9.48 ft lbf[/B]
[I]RPM[/I]: [B] 3600[/B]
[/INDENT]
[B][U]TRANSMISSION[/U][/B]
[INDENT][I]Final ratio[/I]: [B] 6:1[/B]
[I]TC type[/I]: [B]  series 30 6" driven[/B]
[I]Lowest ratio[/I]: [B] 16.2:1[/B]
[I]Highest ratio[/I]: [B] 5.4:1[/B]
[/INDENT]
[B][U]PERFORMANCE[/U][/B]
[INDENT][I]Max Wheel Torque[/I]: [B] 208.33 Nm | 153.66 ft lbf[/B]
[I]Min Wheel Torque[/I]: [B] 69.44 Nm | 51.22 ft lbf[/B]
[I]Acceleration[/I]: [B] 2.4 m/s² | 7.87 ft/s²[/B]
[I]Top Speed[/I]: [B] 60.65 km/h | 37.68 mph[/B]
[/INDENT]
[B][U]PROBLEMS[/U][/B]
[INDENT][B]You might burn your clutch/belt![/B]
[B]Gear ratio too high![/B]
[I]Suggested final ratio[/I]: [B] at least 6.5:1[/B]
[/INDENT]
[/FONT]

see?

'sid

 

[Allracing]

Amen! Thank you guys

I shall aim for around 6.5 total gearing. With 3.8 in the diff that gives me a target of 1.71 for the chain drive.

I'll overshoot the target a little and try for a 10/18 combo and go for the TAV2 kit with a 10 tooth 4x(x) sprocket on the driven and try and find an 18 tooth sprocket for the driveshaft. Will give 1.8 x 3.8 = 6.84 final, should be ok

 

[itsid]

sounds like a plan to me

'sid