mj

For my RC car:

engine revs to approx 35,000,
primary gear ( cluth bell ) has 15 teeth.
secondary gear has 51 teeth

the above combination gives approx 50 - 55 mph using one engine. If the car has two engines what size primary gear do i need to up the top end speed to about 75 - 80 mph. there is a choice of 15,16,17,18 teeth ( primary)cluctch bells available.

Apologies for such a boring topic on a friday afternoon, but I need to order some parts.

Any calculations appreciated.

cheers,

mike.

midget1500

assuming there is no differential we'll need the diameter of the size of the wheels.

midget1500

oh wait, maybe we can work backwards using the speed given...too tired to think

mj

wake up, here, try this:

midget1500

being silly, the 51/15 is the "diff" ratio of 3.4

after some calculations can u confirm that the wheel diameter is 4.2cm?

if so i can work out the rest

midget1500

worked out using...

35000/3600 = revs per sec. then divide by gear reduction ratio of 3.4 (i.e. 51/15) = 171.5 revs per sec.

now, 171.5 revs per sec times the wheel circumerference will give the distance travelled per sec. hence, 171.5 * Pi * Diameter.

distance per hour = [(171.5 * Pi * D) * 3600] = u said 50mph(=80467 metres per hour).

so 171.5 * 3.14 * D * 3600 = 80467

therefore D (wheel diameter) = 4.2cm.

that's how i got the number...not holding my breath though!

so, if D is correct, working backwards u can see what ratio u will need for 70mph, etc. then just get the gears that give that ratio.

steven

midget1500

i don't know how this works with 2 engines, i.e. these calculations just show the max speed u can get as long as the engine can reach the provided revs (i.e. it has the power to overcome all the aero drag etc).

the only way u can change the top speed is to increase the revs, decrease the ratio, increase the wheel size.

mj

does this help :

this is the layout in plan view, the tyres are about 90-100mm diameter i think, the arrows show direction of rotation. there are also front , rear and centre diffs but i havnt got a clue about ratios for these.




this is an app i found for working out speed, the only way i found to get the speed to eqaute to the known speed of 50-55 is to put a figure of 2.2 as the gearbox ratio, as below :





I tried it again with 18 teeth and got this :




does any of this make any sense?

thanks for the help by the way.

cheers,

mike.

midget1500

oh my head hurts. need to go to the pub

was basing everything on a real car. my figure is out coz i haven't accounted for diff ratio/gearbox. i thought the 51/15 was the gearbox ratio (1 gear) as such.

that program should give u the new "51/15" to get your desired top speed as long as the engines can get to the 35000 rpm (i.e. they have the power).

i would imagine that the front/rear diff ratios would be the same and the centre diff is just for providing grip (i.e. no ratio change), i.e. 1/1 ratio. but not sure.