Springrates - understeer and oversteer


_JT

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I've been looking at springrates etc. What I read here is that higher springrates on the front makes for understeer. What I don't get however is why that is so.

If you get understeer, you lose grip on the front and still have some left at the rear. So if you take a 8F/6R setup, you'd have less understeer than 12F/6R. But I don't get it. One of the reasons is to get higher spring rates is to prevent body roll and have more grip, isn't it? So a higher springrate should mean more grip on the front and more oversteer....what am I missing here?
 


konradinu

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that's why by time you'll get your perfect setup. you'll learn how to cope with your setup
 


_JT

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Yeah, but do you have an answer for my question? :p
 


Kozy

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Harder spring rates mean you load the outside tyre more as there is less give in the suspension to even the load about between both front wheels. This means you'll push wide as tyre slip angle increases with load.

Same applies for the rear, and visa versa.

Roll bars also have the same effect, a big rear bar will load the outside tyre quicker and create oversteer. In the case of FWD cars, it's less full blown oversteer rather than the required slip angle for the tyre to create optimal grip is reached quicker as the the bar generates a higher load on the outside wheel than would otherwise be seen with a stock bar. This can be felt as the rear end being more responsive.
 

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ijwhiteman

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For neutral handling regardless of what the rate is, keep it the same all round
For oversteer have it harder on the rear
For understeer have it harder on the front

Same applies to tyre pressures, ARB's etc etc
 


blinx9900

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Dont forget tire compound and camber angle play a huge role in all of this, understeer can be combated with tire pressure alone sometimes even.
 


_JT

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Harder spring rates mean you load the outside tyre more as there is less give in the suspension to even the load about between both front wheels. This means you'll push wide as tyre slip angle increases with load.
I don't get that? If you use harder springs, there will be less weight shifting to one side meaning better spreading of the weight over both sides? At least, that's what I'm inclined to think...

Roll bars also have the same effect, a big rear bar will load the outside tyre quicker and create oversteer. In the case of FWD cars, it's less full blown oversteer rather than the required slip angle for the tyre to create optimal grip is reached quicker as the the bar generates a higher load on the outside wheel than would otherwise be seen with a stock bar. This can be felt as the rear end being more responsive.
Roll bars I understand, they link the suspension so when the outside tire is pushed into the wheel arch, the inner tire will be taken into the wheel arch by the roll bar. But I don't see how that compares to harder springs, as suspension will still be independent.

And why does everyone take harder springs on a track, if softer springs mean more grip :p

If I'm wrong correct me, I'm just typing how I thought everything works ;)
 


Kozy

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It's easy to see why you think that way, let me try and explain a bit further.

The tyre generates cornering force by its slip angle, which must be optimum, push it over that angle and you loose grip. If you're not up to scratch on what a slip angle is I'd recommend looking it up, but basically it's the angle between which the tyre is pointing and the actual direction of travel.

The SA reacts to load, more load means more mass trying to push straight on in a corner, and as such, generates a higher slip angle. So more load = more tendancy to slide straight on. The tyre reacts better to gentle changes in steering angle and load as it takes time for the slip angle to be generated, load up the suspension too quick, you'll have no angle built up and the tyre will slide.

This is where the springs come in.

You still have the same amount of weight trying to go to the outside regardless of spring rate, however with a hard spring it will load up that corner quicker, where a soft spring will acts as a damper. (Imagine trying to hammer a nail into a bit of wood, then doing it again with a pillow over the nail.) That quick load up quickly pushes the tyre beyond it's optimal slip angle and thus creates understeer. With a softer spring, when you turn in some of the load gets stored in the suspension, meaning a slower load up and less likely to push the tyre over it's optimum slip angle.

The downside of soft springs, is that body control is reduced, and also a by product of body roll is wildly changing suspension geometery. Too much roll, and your outside wheel gains positive camber, and tyres really do not cope well with that at all. (Which is the reason we have negative camber dialled in to combat this situation)

The roll bar works in the same way, loading up the outside wheel quicker. Same effect as harder springs, but with a softer ride. :))

Hope that goes some way to explaining it!
 

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_JT

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Wow, it really does, thank you. But what you just told, is that independant of cornering speed? Eg, can you induce more oversteer but gain on cornering speed when the balance is right?
 


Kozy

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I don't follow you? You mean can you go faster by creating oversteer? Not really, the fastest way round a corner is to have all four tyres generating optimum grip, and as such, no slip.

This one of the reasons why the EK9 is so fast in corners, it has neutral handling balance. :nice:
 

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_JT

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No, I mean that if you use harder springs and thereby create more over- or understeer, you would expect a car to be slower in the corner, if your create over- or understeer. What about that? And why do you want a very hard suspension for track use? Track or not, harder spring creates more slip angle on a tire and thus less grip, right?
 


Kozy

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Not necessarily less grip, no. They impove the car in other ways, body control, suspension geometery and weight transfer for example. A soft sprung car will roll excessively, pushing the suspension geometry beyond it's optimum settings, creating positive camber for instance, which ruins grip. The weight transfer will not be well controlled causing unpredictable handling

Hard suspension on a track car only makes sense because the driving surface is completely smooth, and the steering inputs are conducive to ideal tyre loading. It is a set of conditions you seldom find on the road.

On the road, hard suspension is deeply flawed. Bumpy roads will throw the car around, and irregular steering and throttle inputs will cause the tyres to rapidly load and unload creating often unpredictable under/oversteer situations.

It's all about setting the car up for what you will use it for. If like me, your local roads are largely abysmal quality, then a rock hard setup simply won't work. You'll see all the downsides and none of the benefit.

Likewise, if you have soft suspension and your local roads are track-like, wide fast and perfectly smooth, you'll be seeing all the downsides of soft springs and none of the upsides.

Sorry if I make no sense, I am simply typing out as I think! Really, you want the hardest suspension you can get away with before it becomes a hinderance. Unfortunately, this is not something that can be calculated, it is very much a trial and error.
 

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_JT

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Ok...so let me summarize this:
- You need to adjust your geometry for maximum cornering grip/speed
- Then you want the softest springrates at which your geometry doesn't turn into positive camber under the hardest cornering situations. Because higher springrates creates more slip angle.

Also:
- Weight transfer doesn't play as big a role in cornering as slip angle and geometry; pushing geometry beyond it's optimum settings is a bigger problem than some more weight shifting. But it's still all about balance.
- And you actually wouldn't need arb's on the ideal road right? You could create the same effect with higher springrates but still have independent suspension and not have lifted wheels in the corners

Summary of summary:
- Biggest emphasis is on geometry and slip angle when setting up suspension, but don't forget other factors.

Are these all right?
 

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Kozy

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Ok...so let me summarize this:
- You need to adjust your geometry for maximum cornering grip/speed
- Then you want the softest springrates at which your geometry doesn't turn into positive camber under the hardest cornering situations. Because higher springrates creates more slip angle.

Yes that's one way to look at it, and you definately need to set up the geometery to get the most out of any suspension set up. Simply lowering and stiffening will not get the best out of it.

Also:
- Weight transfer doesn't play as big a role in cornering as slip angle and geometry; pushing geometry beyond it's optimum settings is a bigger problem than some more weight shifting. But it's still all about balance.
- And you actually wouldn't need arb's on the ideal road right? You could create the same effect with higher springrates but still have independent suspension and not have lifted wheels in the corners

Weight transfer is something I do not know a great deal about unfortunately, but the one fact I do know is about the the effect on geometery, and also having the weight bouncing from side to side on a soft sprung car is not conducive to a well balanced car.

And I think you have a point on the ARBs, as essentially they do link the wheels together, the stiffer the bar, the less independant the wheels on the axle are. They are a compromise design to allow the fitment of softer springs for ride comfort.


Summary of summary:
- Biggest emphasis is on geometry and slip angle when setting up suspension, but don't forget other factors.

Are these all right?

Yes, but don't get the impression I am saying increasing slip angle is a bad thing (as I think you are). Remember that the whole principal can work in your favour. A softly sprung car will build slip angle slowly, and so turn in and repsonsiveness will be slower as a result. So it might not understeer, but by the time the tyres have decided to respond and create cornering force, you could be beyond the corner! (Over exaggeration) This is another reason I forgot to mention in favour of fitting hard springs.

I know that in between driving my EK and my girlfriends Ford Ka, when I turn the wheel in mine, the car changes direction instantly. In hers, I turn the wheel, the car wallows, the nose dips down, and then it turns.

Hers is still good fun to drive with soft suspension, but it is never going be as responsive as mine.

The point is, you can safely fit harder springs without creating boat like understeer, you simply have to find the right balance between front and rear rates. Keep the balance the same as stock, and the over/under steer situation should be the same as stock, but the whole car will be more responsive to inputs.

Forgive me not making much sense on this, I know all the theory in my head but writing it out has never been my strong point!
 

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_JT

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I think I understand you :) Though in a given bend with given path and given lateral g-forces, don't you want as less slip angle as possible? Cause that would mean you would have more grip left, assuming geometry is still ok, right?
 


_JT

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And I was also thinking, less bodyroll means that the inside tire will be used more for corneringgrip, meaning less slip angle (further away from the grip limit) per tire right?
 


Kozy

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Wrong. Less bodyroll, or increased stiffness, will increase slip angles. Slip angle = cornering force, until the point you exceed the tyres limits. So yes, less bodyroll means you operate further away from the limits of grip my keeping the slip angle down.

You want the slip angle as close to optimum as possible. Soft isn't optimum on smooth roads, but hard will find the limits very easy, resulting in your understeer situation. Somewhere in the middle, depending on the roads you drive on is where you want to be. :)
 



_JT

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Wrong. Less bodyroll, or increased stiffness, will increase slip angles. Slip angle = cornering force, until the point you exceed the tyres limits. So yes, less bodyroll means you operate further away from the limits of grip my keeping the slip angle down.

You want the slip angle as close to optimum as possible. Soft isn't optimum on smooth roads, but hard will find the limits very easy, resulting in your understeer situation. Somewhere in the middle, depending on the roads you drive on is where you want to be. :)

Yeah, I understand that increasing stiffness wil increase slip angle absolutely, but you are saying that in a given situation with given cornering speed and path, even though the inside tire will have much more contact with the road cause there's a bigger weight load, the outside slip angle will not decrease (again, in a given situation with all other variables controlled)? For example, 30% more slip angle because of increased stiffness, but 15% less because the inside tire is more into play? Cause some cars oversteer when the rear wheel lifts, so that must be because of a decrease in grip.
 


Kozy

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Sorry yes I get you, I misread the post and didn't realise you were talking about the inside tyre.

Yes I guess what you state is more or less true in this case, the inside tyre maintains grip more on a stffer car.

I'm not going to pretend I 'know all' about this though. I am simply making educated guesses now!
 


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