Kartster.
The lads are correct but they could explain further.
You already know that the front OUTSIDE wheel lifts when you turn the wheel and you are not in the chassis. Now, imagine a chassis was as stiff-as-a-board and you can probably picture that, on entering a corner, the kart actually slows down and that will 'pitch' the chassis over and onto that outside wheel. If the chassis is stiff enough, then there's absolutely no choice, if the front OUTSIDE 'goes down' the rear INSIDE has no choice... it simply HAS to lift!
Ok.... the Caster angle (the angle that your front stub axle 'leans backward') controls how much your front OUTSIDE wheel lifts when stationary. The more the caster, the more the lift!
So..... WHY do we (supposedly) want the kart to 'lift'? Ok.... go into your garage and try an experiment... no really.... go and DO it! Disconnect your chain (unless you have a clutch, when you needn't bother) and simply PUSH the kart along the floor with one finger! So long as the floor is flat and the steering is STRAIGHT, you'll find that you (should be able to)/can manage that easily. Now try it with the steering TURNED........ you'll need a VERY strong 'finger', In truth.... if everything is set correctly, I'll be surprised if you CAN move it with a finger!
What we have shown is that the 'resistance to rolling' (shorthand: 'friction') increases with steering lock! Ok.... why?.....
When you were at school, you ran sports day races round a track. The outside lane was MUCH longer than the inside lane. That same effect comes into play on any vehicle. When turning, the inside tyre has LESS far to travel than the outside tyre. You won't notice this on the FRONT of a kart simply because the two wheels are NOT joined together.... however..... at the REAR of the kart...... you have SOLID AXLE. What on EARTH do the rear-tyres do when you corner? The INSIDE wheels 'wants' to rotate MUCH slower than the OUTSIDE wheel 'wants' to! They both 'want' to remain in FULL contact with the ground (i.e., not skidding/slipping)..... if they both 'had their way', then they'd simply PREVENT you from turning a corner at ALL!....... and that's the KEY to it!....
Given a choice, the rear tyres on a kart would NEVER let you turn a corner.... they will ALWAYS want to go 'straight on' simply because they are UNABLE to rotate at different speeds and a corner FORCES them to! That's WHY the 'friction' rises HUGELY when you turn the steering while pushing; you are feeling the 'fight' between the two rear wheels!
Ok..... we seem to have moved away from lifting rear wheels..... but we haven't!
If your rear wheels remain FIRMLY planted on the track, each with 'FULL grip' you can NEVER turn a corner...... how can we get over this...... you've worked it out!..... we simply LIFT the INNER rear wheel in a CORNER! That lets the OUTSIDE rear wheel to rotate at EXACTLY the speed it ‘chooses’ with ‘no argument’ from the inner rear wheel!
For precision, we do NOT need to LIFT it OFF THE GROUND.... all we need to do is to take SOME of the weight off it and that allows it to 'skid' just enough to get let you turn INTO the corner!
So..... ANY amount of 'caster-induced-front-lift' will make it possible to turn-in. The amount of 'lift' controls how much 'weight' is released from the INNER rear..... and you now know that the amount of caster controls the amount of 'lift'...
However..... beware!
Why NOT lift the rear a foot off the ground and have no problems with 'turn in'???? Ask yourself if you REALLY want a 'three wheel kart' because that is EXACTLY what a large-lift kart becomes! 4 wheels on the ground gives you better 'grip' on the track than 3!
Think about it.... almost as soon as you turn into the corner, you return the steering to 'straight ahead' to 'catch' the 'slide/spin' that will happen if you KEEP the lock applied! so.... when the steering is 'straight ahead' the caster has NO effect on the kart.....
Crudely speaking, the caster effect is ONLY USED on 'turn-in'. Watch all the GREAT karters.... you will see that they take most of the corner on ZERO lock with the steering 'dead ahead' for the WHOLE corner. They ONLY turn the steering wheel on 'TURN-IN'! Had you seen Terry Fullerton, you would have seen the PERFECT demonstration of this.... it used to look as though he didn't NEED a steering wheel as it seemed to ALWAYS be 'dead-ahead'!
Ok..... finally, the 'stiffness-thing'. If the chassis was made of wet spaghetti, it wouldn't matter if you lifted the front-outside by TEN feet, the back would still stay FIRMLY 'planted'. However, if your chassis was 'stiff-as-a-brick', then 0.01mms of ‘lift’ would lift the rear by exactly 0.01mms, too! So... you need MORE caster with a 'floppy' chassis than you do with a 'stiff one'.....
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Having said all that..... my suspicion is that you don't think your rear wheel is lifting (it actually IS or else you would NEVER turn into the corner AT ALL!) and THE most common reason for that is that you are not cornering HARD enough! If you generate ENOUGH force, most karts WILL lift a rear wheel off the ground.... however, you don't actually NEED to!
Therefore, lifting a rear wheel clear of the ground is NOT essential in the dry (but it can have other advantages in the wet). Put crudely, if you find that your chassis 'turns-in' to the corner well, then you have ENOUGH caster. If it feels OVER sensitive on 'turn-in' you MAY have TOO MUCH!
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Before any experts LEAP on me.... I KNOW this is an over simplification..... but it will DO for Kartster's current level of knowledge so far .... in my opinion!
Ian
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Inside/rear wheel lift by 'kartster' (23 Mar 2010 @ 17:03)
