Roll Stiffness and Spring Rates, Trying to Understand 3

[sleighty]

Hi Everyone,

I've had a search and haven't found the answer to my exact question so here goes.

I will just inform you initially I'm trying to work from two conflicting views regarding differing spring rates, roll stiffness and tyre loads as a result.

I'm in my head trying to work out how a go kart works and how and when certain setup alterations should be made. However this isn't the point initially.

I've read in some articles that softer springs or softer roll stiffness accept weight transfer more easily. Meaning it creates more cornering 'G'
I've then read the opposite in another article, stiffer springs or stiffer roll stiffness transfer more weight to the tyres.

Before I started university i was also of the simple belief that more weight or normal force on a tyre creates more 'grip'. I now understand this isn't the case, the traction increases but this traction divided by the normal force equates to the lateral acceleration the tyre can withstand which of course is less.

So....What I'm asking is which of the above is true.

Assuming 50/50 lateral and longitudinal weight distribution, Does a rear roll soft vehicle in a corner transfer more load to the outside tyre than the front roll stiff end? Or vice versa? I currently understand it to be that the roll stiff end transfers more load to the outside tyre. This if my tyre loading belief is correct means the tyre loses lateral acceleration ability. (Currently reading Milliken brings me to this)

If this is the case, why would some literature indicate the opposite?

I've recently watched a video regarding go kart 'roll stiffness' in which the author talks about how a softer roll stiffness accepts weight transfer more easily and as a result loads the tyres harder and can cause a go kart to lose its unloaded inside rear tyre state earlier causing binding or slowing of the kart off of a corner. In a way...this makes some sense to me, however everything I'm reading states this isn't the case.

Perhaps I'm thinking about it too hard.

Any help would be appreciated.

 

[GregLocock]

Perhaps the source of your confusion is that karts don't have suspension, so their setup is heavily dependent on tire pressures and chassis stiffness. Using a kart as an analogy for a car is going to lead to tears before bedtime.

So, which textbook are you using?

Cheers

Greg Locock


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[gt6racer2]

"I've recently watched a video regarding go kart 'roll stiffness' in which the author talks about how a softer roll stiffness accepts weight transfer more easily and as a result loads the tyres harder and can cause a go kart to lose its unloaded inside rear tyre state earlier causing binding or slowing of the kart off of a corner. In a way...this makes some sense to me, however everything I'm reading states this isn't the case. ".
This makes sense to me, if you read it knowing the answer, but it is confusing. What he's trying to say is that if you drop the roll stiffness at the rear (and nothing else is changed) the softer roll stiffness will provide more load on the inside rear tire and less on the outside. The inside tire will be low in it's load range - so in this case more load = more grip. This is good for grip at the rear, but has a negative in the kart world that doesn't apply to the typical car. Karts have locked rear ends ( no differential). To get good corner speed you want to unload the inner tire (and even better -lift it) so that the two tires aren't fighting each other or the front wheels. The lower rear roll stiffness will lift the wheel at higher g ie-later, and put it down at higher g ie-sooner - hence the loss of speed off the corner.

 

[sleighty]

Gt6Racer2,

Thanks for that input, so its more a case of less load being applied to the outside rear in relation to the inside rear. I think I understand now, karts have always fascinated me in the sense that some manufacturers say a softer axle or chassis setup in the rear causes less grip in the rear and some state the reverse of the rule.

Realizing that it wasn't this simple I've worked to attempt to understand what is really going on.

Its more a case of balance, to load the outside rear enough to allow enough traction but not too much in order to lose cornering force. Likewise to load the inside rear enough so as to not transfer too much load to the outside but enough to stop the inside from returning to the surface too quickly or at too higher of a G which will bind the kart off the corner.

Greg Locock,
I guess I'm trying to use car dynamics in order to try understand karts better. There is a distinct lack of reliable literature which doesn't include A is better than B. I don't like reading rules which seem very general and with very little explanation. Sadly Kart chassis setup literature is typically written this way. I'm trying to understand each possible setup area of a kart in order to understand when certain changes should be made and why. As for car dynamics, I have read in certain literature and on the internet that softer springs accept more load which loads the tyre harder giving more grip. However upon reading certain chapters of Race Car Vehicle Dynamics by Milliken it seems this is totally incorrect? If anything stiffer springs transfer more load to the outside tyre which can eventually overload the tyre causing a lack of cornering G.

As perhaps seen from some of my previous posts, I eventually want to build a very accurate multibody model of a kart which can be adjusted in certain parameters to understand how chassis, axle, and other component flexure alter a karts performance in certain conditions.

 

[imcjoek]

softer springs accept more load

This is all kinds of confusing from both language and technical standpoints.

I would have to suggest you try going back to first principals. Draw yourself some free-body-diagrams of car cross sections with different spring rates, track widths, CG heights, etc and solve them for reaction forces. Once you have a handle (ugh) on that, look at how those forces influence tire grip. Then finally, move on to dynamic situations. IE: the RATE of load transfer can be rather important...

I think this approach will give you a more intuitive grasp of what is going on. Milliken can be pretty deep, so that can maybe obfuscate instead of clarifying.

 

[GregLocock]

I briefly dabbled in kart tuning and agree with your general comments about the state of knowledge there in the public domain.

OK, on a given axle on a sprung car, increasing the spring rate will increase the load transfer to the outside wheel on that axle, and lessen it on the other axle.

Generally that will cause that axle to understeer more.

But not always. As it reduces the roll gain of that axle, and if the axle was set up with a lot of roll steer, that will reduce, or even negate the understeer change caused by the weight transfer. Also, some tires are more resistant to load sensitivity than others.

The combination of those two provisoes leads to cars which do not respond in the way the rule of thumb suggests, although as Milliken says somewhere or other, if you design using rules of thumb, you'll get a thumb, not a suspension.

Cheers

Greg Locock


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[patprimmer]

My very limited understanding of Kart handling is it's all in chassis flex, tyre pressure AND C of G control with seat position and driver movement.

Due to the very short wheelbase vs track width, like typically almost square, the steering effect from applying power only through the outside back wheel is very important and is a bit similar in effect to scrub or steering brake steering as used in tractors where you apply a brake to the inside wheel as you apply power with an open differential.

This is controlled by the back having a lot more roll resistance than the front so it picks up the back inside wheel. This is tuned on the track by tyre pressure and axle stiffness itself pre race, and by the driver moving forward or back in the corners.

Regards
Pat
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[sleighty]

Greg, Thanks for that point. I understand that it is only part of the big picture in which the vehicle operates. However its easy to forget.

imcjoek, I'm going to go back and read something more basic and take the steps you suggested to try and understand exactly what is going on. I'm also going to go back to something such as Fundamentals of Vehicle Dynamics by Gillespie to aid me with this investigation.

Thanks for your help guys, I'm sure I will pop up from time to time in order to ask further queries.

 

[imcjoek]

Carol Smith (Tune to Win) & Herb Adams (Chassis Engineering) are both written for a target audience more in line with what you are trying to accomplish. IE: Casual enough to get the basics down.

 

[AeroFC]

Traditional go karts dont actually use springs for suspension because of size. Drivers use over rotation to turn the car go kart. You would have to fabricate your own chassis and utilize custom casing with dampers and springs. The spring rates would be quite low as the weight of the go kart is far less than a car. Roll stiffness is really more attributed the to anti roll (sway) bars. Spring rates have more to do with the rebound of the damper. The tire on the go kart acts as the damper merely because there is nothing else to absorb impact and deflect impact.

 

[sleighty]

AeroFC

Yeh Im aware, I've raced karts for nearly 15 years now, But with adjustments available in axle and chassis stiffness, which would alter the pitch and roll stiffness of the vehicle, I was trying to understand how certain adjustments would affect the overall balance and why.

When you read kart literature, they read along the lines of, 'a softer axle will reduce grip' Well I know from an engineering point of view that nothing is ever that simple and when you look at the dynamics of what is going on it becomes even more interesting and highlights how these books are a far cry from what is actually happening.

 

[AeroFC]

Well softer suspension set up properly will yield more mechanical grip. I personally like a car to oversteer a little bit for fsdter corner speed. But set ups are purely driver oriented.