why do karts pull so many lateral g's?

[tuj]

I have heard of some of the shifter kart guys wearing rib protectors less they break a rib (which I hear has happened). Seems like these guys are consistently pulling over 2g's. Yet a road car even on autocross r-compound tires struggles to pull much more than 1.1 or 1.2g's. Yet the road car has suspension and the kart does not.

Is this a function simply of contact patch per unit weight of the vehicle? Let's assume the tire compounds are the same.

I am always surprised to see how fast the karter's are on an autocross course, often posting one of the top ten times of the day against vehicles with MUCH more hp. Likewise I am always surprised when I hop in a kart at how well it handles.

[Lycoming]

Is this a function simply of contact patch per unit weight of the vehicle? Let's assume the tire compounds are the same.

They're not the same though; an autocross R compound has quite a ways to go to match the performance of a proper racing slick.

It's mostly just down to weight I would think. A tire's peak friction coefficient tends to decrease, to varying degrees, as the vertical load on the tire increases. Karts have very little load on their tires. And karts do have suspension. The chassis is not infinitely rigid, nor is it free of hysteresis. Thus, you have springing, damping and articulation.

Power by itself doesn't mean anything. Power to weight is what actually determines your straight line acceleration. Since a kart doesn't weigh anything, it doesn't take much power to match road cars in a drag race. Plus, I don't think power matters too much on typical autocross courses anyways.

[tuj]

Ok, that helps.

Just one thing: you seem to think a racing slick is better than an autox tire? I have to disagree with this. Autox is where you can use the softest compound possible; the life-time of the tire is quite short in terms of miles. I say this because all the proper race cars I know of use Hoosiers slicks and they are designed for more heat cycles and longer duration of racing and higher speeds. Just what I thought... I mean, why else would Hoosier make the A6 tire if not specifically for autocross, and if it doesn't have the highest coef. of friction in the lineup, then why not?

[Shrieker]

Go karts can corner much tighter, no ? If you keep your speed relatively high in a much tighter turn, it will result in a net higher g.

[Jersey Tom]

Why can karts pull high G's? Pretty much because they're light. Helps tires in general, and as a further effect you can use fairly soft tread cap compounds on kart tires.

Autox is where you can use the softest compound possible

Not necessarily true. There's some balance. For example, autocrossing a sedan.. they're not particularly light (compared to an open wheeler) so you need to have some guts to the rubber to withstand that. And, being that auto-x is mostly a casual and recreational thing the customer is probably going to want to make more than one run on a set of tires before having to replace them, so you need to have some durability. Something else to think about.. I think I've heard of people preferring the R6 to A6 at times. There are reasons to that which fall into what I'd call Hoosier's general design strategy, but that's a separate discussion.

In any event it's far too much a blanket statement to say "racing slicks" are softer or harder than "autocross tires." Say we use the A6 for example.. I know of slick tires with way softer tread compounds, and others with way harder compounds.

And if you're saying that all proper race cars you know of use Hoosier slicks, then you must not be looking at many proper race cars

[Nuvolari_the_legend]

Firstly I just want to say that the automatic go karts like Rotax Max pull more g's than the shifters.

The thing about the tires is that you don't want any more grip from a karting tire. If you would give a karting tire much more grip you will get (more) understeer. You also said that autocross r-compound tires don't live really long. That is probably because cars, if you drive them fast round a racetrack, understeer a lot. You can here that when you are on a racetrack were road cars are racing or on car shows like top gear. On a karting circuit you almost won't hear that noise or just a little bit (if there are rental karts driving you can hear that noise but I think we are talking about real racing karts).

You also said that cars have suspension but the suspension on cars is not good at all for racing. Suspension on cars is soft for a better ride and not stiff for racing. A go kart hasn't got suspension for a reason: it doesn't need it and would only slow a go kart down.

And hire comes the actual answer to you question: A go kart is from the beginning designed for racing and a car just isn't. From the very beginning in 1956 go karts are designed to be as fast as possible. Every year people try to make the design better. Because of this search for perfection, go karts haven't changed very much in the last couple of years because we know the most of the ideas now. This applies for almost all kinds of racing.

[tuj]

Firstly I just want to say that the automatic go karts like Rotax Max pull more g's than the shifters.

Can you tell me why this is?

[Nuvolari_the_legend]

Firstly I just want to say that the automatic go karts like Rotax Max pull more g's than the shifters.

Can you tell me why this is?

The engine from a shifter go kart is a lot heavier than the engine from a automatic go kart. This is because of all the extra weight the gears bring with them. A shifter has profit on circuits with slow corners and long straights where you have to accelerate a lot but on circuits where you have a lot of corners and not a lot of straights, a automatic can be just as fast or even faster!

Still, on the most circuits shifters are faster but I wouldn't bother with a shifter if you budget is not very big. It's not nicer to drive, it's more expensive to buy and to run and there are more people with automatics then with shifters, especially Rotax Max is very popular.

[Ogami musashi]

As said previously, kart do pull lots of G's because they're light...and their tires have small radius (because they're light).
(that's also why shifter karts put less g's...they are heavier than non gearbox karts)

However i have a big pinch of salt concerning kart's cornering ability. For a whole set of reasons both kart tires and kart chassis themselves are not the best mechanical components in the world and to have a kart that actually sustains 2G's for a turn if very rare. Most of the time you have spike g's that averaged do produce between 1,6-2g's but compared to an openwheeler a kart is like an unpolished product.

If you could put a chassis with suspensions, differential, radial slikcs and thus stiff chassis on a kart you would end up with a machine much faster that the actual race karts.

You also said that cars have suspension but the suspension on cars is not good at all for racing. Suspension on cars is soft for a better ride and not stiff for racing. A go kart hasn't got suspension for a reason: it doesn't need it and would only slow a go kart down.

This is not correct. A kart needs suspension work of course, if not you would slide even more that it's the case (and one of the major reason why cornering performance of karts can't be steady) and each bump would smash down the inner rear wheel back onto the ground when cornering which is the main point of a kart's cornering performance.
I think it is also wrong to say it would slow down a kart...A suspension would enable a stiffer chassis leading to much more linear load transfer behavior and thus better cornering performance.

From the very beginning in 1956 go karts are designed to be as fast as possible. Every year people try to make the design better. Because of this search for perfection, go karts haven't changed very much in the last couple of years because we know the most of the ideas now. This applies for almost all kinds of racing.

I think you're view of kart is pretty wrong. First kart are not the fastest possible machine on a kart track...many innovations in the late 70's/80's got banned (for a starter: radial tires) and the state of research in karting today is none.
It's all about a stalemate. A 2013 race kart is not faster than a 2003 kart and if something has changed, it is heavier (a rotax is now 86kg dry...used to be 75kg...not even talking about ICA/FA at 66kg). There's no real research in kart, only empirical tests.
I just started to explore aerodynamics in karting by prospecting for kart project with downforce and reduced drag, got in conctact with two teams that also work for F1.....the end message was "nobody's (company) interested"...I also got to know how karting works at very high (international) level..you are miles away from cars that are miles away from aerospace.

AFAIK, the historic definition behind kart was "the purest and simpliest form of racing", certainly not the fastest.

[autogyro]

Kart chassis design has evolved over many years.
The chassis has to be flexible but that flexibility has to be balanced to the drivers ability to shift his weight.
All this has also to be matched to the ability of the flexible chassis to lift the rear inside wheel in a controlled manner.
A rigid chassis with suspension at this scale and with the high center of gravity cannot match this.
I do not agree that a kart with suspension and a diff would corner at higher g.

[Lycoming]

If you could put a chassis with suspensions, differential, radial slikcs and thus stiff chassis on a kart you would end up with a machine much faster that the actual race karts.

so like... an FSAE car?

[Tim.Wright]

If you could put a chassis with suspensions, differential, radial slikcs and thus stiff chassis on a kart you would end up with a machine much faster that the actual race karts.

so like... an FSAE car?

FSAE cars only have suspension because the rules mandate it. A kart would be faster around most FSAE tracks.

Back to the original question though, the latacc quoted certainly seem very high. Unless we are speaking of aero karts, I don't belive that a sustained 2g is possible. 2G transient is believable.

I don't know much about kart tyres but a racing slick will generally have a coefficient of friction of somewhere between 1.2-1.5. That means, without aero, you can acheive a maximum of 1.2-1.5G of sustained cornering.

For what its worth the rib protectors are not required because of the high Gs but because:
1. Long periods of driving without suspension will otherwise bruise your back and ribs
2. Karts don't have crumple zones, so if you hit anything solid, the driver's body reacts all the external forces through the seat which often means broken ribs if you arn't protected.

[Tim.Wright]

You also said that cars have suspension but the suspension on cars is not good at all for racing. Suspension on cars is soft for a better ride and not stiff for racing. A go kart hasn't got suspension for a reason: it doesn't need it and would only slow a go kart down.

This is also a common misconception. Racing car's have stiff suspension purely to control the body movements either for reasons of driver feedback or for to keep the aero platform at the correct attitude.

The stiffer you make a suspension, the more variation you get in contact patch load which in turn kills grip. Race cars are setup with the softest suspension you can get away with.

[Ogami musashi]

FSAE cars only have suspension because the rules mandate it. A kart would be faster around most FSAE tracks.

Please note that i said "stiff chassis" not "stiff suspension". This is the problem with karts. Since they need the chassis to twist the torsional rigidity of a kart's chassis is severly lower than a race car chassis. This has two side effects on tires working. First it doesn't transfert the load on the tires completely. This has its limitations even considering basic kart set up. too soft a chassis won't be able to maintain the inner wheel lifted when mid corner, but more importantly this doesn't transfer the load completely on the tire giving uneven load transfer on the three wheels making them work uneffficiently.
Also, a kart chassis has several members bolted on them like the engine, or seat struts and thogether they give the chassis a very non linear torsional response to an applied load.

Since the inner wheel must be lifted but not to much this makes working the tires consistently very hard and leads to unsteady cornering performance.



As to why such a kart would be faster is obvious: A differential would remove the need for the inner wheel to lift. a suspension would allow for stiffer chassis so better load transfer (this was tested in lab even without a suspension. Different chassis torsional rigidities were evaluated; the higher offered the better cornering performance). As to radial tyres, they would offer better combined longitudinal/lateral grip.

at some kart tracks FSAE are faster than Karts. And you have to account for the extra weight, but the sole fact that the best FSAE aero cars produce around their own weight in downforce at 90km/h shows you how little developped karts are.

And it worked in karts too! in the late 70's, wheel hub having differential rotation speeds were introduced in some races (mainly oval racing) and were instantly faster...and banned directly.

In the late 80's there was an experimental kart category in which radials were tested; Here in france, a driver put them..smashed the pole position by almost one second (in a high level kart event that's very rare)....this got banned.

A recent aerodynamic study managed while keeping the bodywork inside CIK rules to create about 4,5kg of downforce at 90km/h (that is without wings) instead of they quasi lifting situation now...The study was fueled by one manufacturer but it got canned despite the fact the study did no optmisation and that the author thinks much more than 4,5Kg was possible.

Back to the original question though, the latacc quoted certainly seem very high. Unless we are speaking of aero karts, I don't belive that a sustained 2g is possible. 2G transient is believable.

I don't know much about kart tyres but a racing slick will generally have a coefficient of friction of somewhere between 1.2-1.5. That means, without aero, you can acheive a maximum of 1.2-1.5G of sustained cornering.
.

Indeed, sustained 2g's is rare..but possible (on very soft compounds) and seen. Actual transient sometimes goes up to 2,5G's and i'm speaking of well calibrated telemetry.

According to vega manufacturer the size of the tyres give them very good working performance and the very light load on them allows a good friction coefficient.

As for racing tires in cars, i used to exchange with an engineer in F1 from michelin, he was tellling me a mu of 2+ was possible in F1....

[Ciro Pabón]

The main reason why karts have such good times around tracks and such a high grip is very simple and nobody has mentioned it: karts have a very low CG. This means that the transfer of weight is very low compared with a car. A kart runs at centimeters from the ground.

This is what I call a low CG!


Having such a balanced car gives you very good times: all the three (in turning) or four (in braking) wheels are working almost at their maximum grip. In a car, you brake mainly with the front tyres (and that's true even in cars with such a low CG as F1). When you turn you also get a large lateral weight transfer in a car than in a kart, of course.

Martin Lyell shows how it's done


I won't call nobody's post wrong but if softness of tyres or good suspensions were the key, well, why do not other kind of cars get better times when they have better compounds and... ummm... "sophisticated-er" suspensions?

To be frank, karts actually lack decent suspensions. You have to ponder this: a kart can flex like any spring, sure, but where do you get the damping, guys? A suspension is not merely a thing that moves up and down, it's a thing that actually keeps a constant height when you move over uneven terrain, ehem.

In a kart you do NOT want to have too much grip, in fact. It makes the car to bounce when you accelerate or brake (and I don't know why).

Another weird kart effect (but a fact) is that a wider rear track will give you more grip on certain tracks but most of the time a wider track will give you less grip. In some cases, narrowing the rear track will give you less understeer. Some times you will get more bite when you have a narrower rear track.

The wider track explanation is false. That's so true that kart setup manuals recommend to AVOID narrowing the rear track as the only way to get more grip


I believe suspension is not that important because karts have low vertical inertia. Simple: the thing weighs less than you (specially when you are as old as I am), so you can actually control with your body a lot of the kart. You wouldn't believe how much you can control the movements with your a$$ and your spine.

A low CG also means that understeer and oversteer are very low, when you compare them with the ones you get in a racing car. Frankly, that was the hardest thing for me to handle when I grew up and started to race in other classes: the oversteer is larger in the exit when you get serious power and a much larger CG (actually, is way less noticeable in lower kart classes, where you don't have the power).

On a side note (but relevant, when you ponder how low is a kart) you actually raise the height of the kart to be able to get a better grip in the wet.

The reason is that you have to raise the inner rear wheel (the darn thing, of course, do not have a differential) because you need to "bite", not to "slalom". On the other hand, while you can control the vertical position of the front axle, most kart rear axles only have two positions... but I digress.

It's not well known, but kart wet weather tyres are larger (slightly larger diameter) to compensate a little for the lack of lateral grip to raise the fourth wheel.

You want more grip in the wet? Get larger tyres (right of the image)


As for the cracked ribs, well, I've rarely crashed against anything in a kart (I've flown over other competitors) and I have them (sore ribs). To achieve this it's perhaps important to avoid tracks with grass (and I recommend that: I hate them and the few kart tracks I've designed have no grass, unlike car tracks I've contributed to) or to be careful and give a wide berth to grass, as I'm used to.

Do not walk on grass


It's not the collisions but the lateral load repetitions what make you sore (and the lack of padding in older seats, btw): that causes sore intercostal (between ribs) muscles, BUT the main cause of injuries, specially in children, is drivers crushing against the steering wheel.

A good rib protector protects you from the steering wheel lacerating your chest AND give you lateral support for your sore intercostal muscles


This thing is useless in a frontal collision


Finally, digressing even more, the grip of a kart varies during the race, in part because of the rubber, in part because of tyre temperatures, in part I don't know. The ideal race is run with a kart that slides a little at the beginning and starts to get grip towards the end. This means the chassis is a little stiffer than you should (stiffer than you use for the best qualifying laps) so you can get a little more grip at the end, just in case someone is interested.