please suggest

[crazykart]

As fabro said earlier, it is hard enough to get below a 6:1 ratio. With a clutch you will burn the clutch up before the kart moves. Without a clutch there will not be enough torque to move, and you risk doing damage.

I cant give you math, but give it some time and someone will. We're just saying, from experience...you're possibility for failure when trying a 3:1 ratio is near 99%.

Dont ever think you ask a stupid question though because there is no such thing. Believe me I ask what should be the stupidest questions as I get crazy ideas, but they always get answered factually and without (too much) fun being poked at them.

 

[Shraddha]

Yeah my team-mates wants 80km/h speed only. they are not listening :/
as per the rules the following specifications will suggest the maximum speeds that will be encountered on the course. Average speeds should be 40 km/hr (25 mph) to 48 km/hr (30 mph). Straights:No longer than 60m with hairpins at both ends (or) no longer than 45m with wide turns on the ends. Hairpin Turns: Minimum of 7m outside diameter (of the turn). Slaloms: Cones in a straight line with 7.62 m to 12.19 m Spacing. Miscellaneous: Chicanes, multiple turns, decreasing radius turns, etc. The minimum track width will be 3.5 m (11.5 feet). The length of each run will be approximately 0.805 km (1/2 mile) and the driver will complete a specified number of runs.

 

[crazykart]

30 mph is very attainable! For a homemade kart that is a great speed to shoot for. With hairpin turns though iiwm I would shoot for faster acceleration over a higher top speed. You want to get through those corners and build your speed back up as quickly as possible. Usually for a higher top speed you sacrafice acceleration, and in a track imo acceleration is more valuable than a high top speed.

 

[OzFab]

Given the course specs you provided, you'll never reach even 60km/h; using a lower gear ratio will actually move you around the course faster because you will have the benifit of acceleration...

Forget velocity ratio, what about power:weight ratio? 150kg weight (80kg driver, 70kg kart) powered by a 4hp engine, combined with 3:1 gearing (& you haven't mentioned what size tyres you intend to use) & this kart will run dead last because it will never move...

As a comparison, below are the specs for my kart:
Kart weight: approx 50kg
Driver weight: approx 80kg
Engine: mildly worked Honda GX160 producing ~7hp
Transmission: centrifugal clutch
Rear wheel height: 10" or 25cm
Gear ratio: 5:1
Max speed: 50km/h (30mph)

See the difference? more power, less weight but, if I use a higher gear ratio, it would struggle & I would damage the clutch...