Engine power transferred to axle - Car vs Go Karts...

[jackw72]

Hey all,

Didn't really know how to name this question but I've been doing a little pondering on the difference between go karts and cars in terms of how they get their power onto the wheels.

From what I can see from a go kart the sprocket is mounted directly on to the rear axle and I am assuming that the engine power directly turns the rear axle? But surely it must need some time of gearing in order to not turn the wheels with the same RPM so there must be a reduction in there somewhere.

Then from a car perspective the engine's power goes to a differential that changes the power from a longitudinal torque to a lateral torque that then drives the rear axle?

I then looked at this random Model A chassis building YouTube video and the rear axle had some kind of connection to mount the power producing parts to it, which I assume contains the differential or some other magic?

So, TLDR why do go karts have like a direct drive sprocket on the axley thing and cars have differentials to reduce the RPMs at the axle? Or just explain why I am wrong on both accounts!

Thanks in advance,

Jack
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Suzuki GSXR 1000 K3
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[Monkeypony]

The size of the sprocket on the axle affects the rpm. The bigger it is, the slower it will turn. It's a cheap easy way to get the power to the wheels.

A car uses its gearbox to reduce rpm.
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[stevo as b4]

A kart is light, with a very low c of g and has grippy slick tyres. Straight line grip is improved by not having a differential, and when karts are cornering they kind of act in a four wheel drift and scrub sideways at the same time, a bit like a car would if set up for drifting. They because of the above don't need a diff, and are unlikely to go faster with one.

Cars are the complete opposite, with much higher centres of gravity, softly sprung long travel suspension, and much lower geared steering lock. They are generally set up to have a fair bit of body roll, and understeer going into corners. They are also too heavy to act like a kart does in a sideways drift, so they need the diff to alow smooth cornering without upsetting the suspension and traction.

All this is very generalised, as a car that's set up for say drifting motorsports, will have a very different chassis design and set up. Drift cars are also like karts in that they have a pretty wide steering lock compared to a road car.

[G]

A sprocket makes it very easy to change the ratio as you just change the sprocket to reduce the RPM as desired?

Cars setup for drifting with welded rear axles effectively work the same as a go kart. Both wheels turn at the same speed, which can cause problems in slow corners. On a go kart you either slide or can intentionally lift the inside wheel so the two wheels turning at the same speed but with different roads speeds.
In a car with a welded diff, you tend to get the back wheels 'hopping' when trying to park etc.

There's loads of guides out there on how differentials work and why they exist.

[B5234FT]

Both have a reduction gear (in a car this is gear ratio, then final drive ratio, then tyre circumference to give linear distance per revolution and in a kart it's the sprocket ratios and the tyre circumference)

The key difference is that one has a differential to allow different wheel speeds and the other doesnt. This is because karts have no suspension and massive castor so they corner on three wheels mainly.

[Teflon-Mike]

The differential lets the outside wheel turn faster than the inside wheel, when cornering, to account for the different lengths of the circumference they each have to travel.

The conventional differential 'gear' is a rather peculiar form of planetary gear; The entire pack rotates at driven speed, idler gear between the out-puts is geared 1:-1.

If you jack up the axle of a car, an 'lock' the axle by putting it in gear (engine off!) you can rotate one wheel, and the other rotates 'backwards' as much as the one you turn forwards.

In operation, then, you drive the diff, and the differential gearing lets near side wheel go as much slower as the diff makes the outside one go faster, the 'average' the same speed as you turn the diff.

Cars have a track width of about 2m.. go karts have a track width of about 1m....... so the difference in radius between inside and outside wheels and hence the amount of 'differential velocity' between them is an awful lot less, for any given arc of bend.

All tyres 'scrub' in cornerng; for the same reason, inside edge of the tyre doesn't have to travel as far as the outside; take a 2CV with skinny little 80mm wide tyres, and the scrub is pretty egligible... find a 6.6L transam with 18" wide Sticky Mickey drag tyres, and they inside & outside edges are almost as damn far apart as the wheels of a go-kart!

So Go-Karts and most Quad-Bikes with a rigid axle, are simply 'ignoring' the 'problem', and letting the tyres scrub at the eds of the axle... and mostly because its not 'such' a big problem..

On a Go-Kart, or Quad, you dot really 'care' about scrub. Main issue is tyres wear out fast... but then you aren't exactly cranking up transcontinental mileages round a kart track, and if tyre life was an issue, you wouldn't be using chewing gum soft compound 'slicks' really. On a road car, you rpobably do care a little more, and teh diff is more useful.

On a road car, a 'loose dff' also offer advantages to handlng, a solid axle, would tend to try and 'push' the vehicle in a straight line, whether power on or power off it would try ad resist turning, that would make stering heavier, and tighter turns more difficult, so it lets the car handle more neutrally and accomplish tighter turns.

A go kart has wheel base barely any longer than ts track width! They would tend NOT to have an awful lot of straight line stability, hence NOT hang a diff, can actually help a bit to find them some; whilst the loss on tight turns isn't so 'bad' when that tiny wheelbase needs to little effort to make it turn anyway, and they are so light, steering is never going to need power assistance!

In fact, most Karts have direct like steering, with maybe only 1/2 turn of steering wheel movement from full lock to lock, they don't even have the mechanical 'advantage' of a steering box, gving a gearing advantage of perhaps 3 full turns of steering wheel travel between locks!

So how useful would a 'diff' actually be?

Meanwhile, disadvantages are weight; diffs tend to be pretty heavy; you might get away with a smaller lighter one on a kart but on such a lightweight vehicle, is t really worth it?

Alternative's to a diff exist, and a rolaflex coupling, which is basically a big rubber donuts that can twist between shafts, can offer 'some' differential travel... problem on a Kart though is that they tend to only ever turn 'right'.. cos they go round and round in circles.... so left tyre goes further than right and winds up the rubber coupling, which ever gets 'unwound' by an equal and opposite about of left hand bend! And eventually they either unwind and unexpectedly drive the left hand wheel backwards... OR they break.

{Any-one familiar with Hilman Impswill know all about this; and any-one who has ever had to live with a stage rally prepped example wth a Limited Slip Differental, will probably be tearing thier hair out at the memory! These like some Alpha's ISTR had roleflex couplings instead of universal jonts to allow suspension movement, but could still twist themselves in knots under differential action!)

Another alternative would be a split shaft and one way roller clutch, direct drive sent to the insde wheel, the outer, allowed to turn faster under cornering 'free-wheeling' on the roller clutch.

Reasonably simple, and potentially ot too heavy, arrangement does cause some curiuse side effects; tr one way and the kart will effectvely accelerate a bit like a bike & side car fro the asymetric 'drive'!

Another is the Torsen Couplng or fluid coupling; bit like the torque converter i an automatic transmission, ts a drum full of oil, wth a shaft going into it wth a paddle on the end. Drum turns, and oil drags the paddle wth it. Stck two paddle shafts into teh drum, you have a Torsen Diff, drum drags the paddles, that can rotate 'differential' But with q few occasionally useful side effects. Can be very useful, but tends to be rather heavy and cumbersome.

So, there's ways and means of providing differetial drive between wheels... but, pro's and con's dont stack up so well to make it that useful on a Kart... and ultimately 'redundant' in Kart racing reg's where I believe that regs actually prohibit them!

Ariel may be the man here, but ISTR that the essential definition of a Kart, is no suspension, no diff; there was a vogue a few decades ago, ISTR to put out-regulated 'super-karts' with full down force body work, into Hill-Climb, where they did rather well, but had to be gven rubber block suspension and some fort of diff to be classed as a 'car'!

So, that's the 'differential action' part sorted; other issue is the conventional cars 'Differential Unit'.

If you look at one from something like an SD1 Rover, or an old RWD Granada; the differential is not just the differential, but incorporated the crown and pinion 'final drive' that turns the shaft rotation that's all in line with the chassis fro the engine, through 90 degrees to turn the wheels on the end of the axle.

Conventionally these usually offer around 3:1 reduction in the gear; but in the olden days, often needed an awful lot more, and reduction ratio's in the order of 10: weren't uncommon. This was because gear-boxes usually offered 'direct drive' 1:1 from the crank, as their highest ratio; 'for efficiency' as engines didn't make a lot of power; so they needed to get all thier reduction in the final drive; whch was usually 'unsprung' in the back axle, so yu dont want to make it too heavy, and yu tended to need a lot of reduction, both to get useful motive force from limited power, and because wheel sizes tended to be large to cope with rough roads.

10:1 or more reduction between two gears, would beg making them very large, and as they would have to be bevel gears, even larger; so some used reduction idlers, to make the assembly more compact, or they used worm drives, that provide a huge amount of reduction from relatively small gears, but have other issues, noteably back-lash and 'one way' drive. Whch lead to the inovation of the "Hypoid" Gear which is, sort of a spiral tooth form, that is as compact and strog as a bevel, but the teeth 'slide' tangetially over each other rather like a work-screw, to get a lot more reduction than the simple diameter ratio.. and takes topic off o another tangent.

BUT the 'compound diff' is dong two jobs; final reduction AND differential gearing twixt shafts.

Karts can get the final reduction from simple chain & sprocket, and they don't really 'need' and aren't I don't think allowed differential gearing.
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