Explanation wanted! Rear roll - front grip?
29 March 2006, 05:16 PM
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Explanation wanted! Rear roll - front grip?
Could someone boffin-esque explain exactly what's going on here? Another forum i use is daring to question how to imrove handling and i would appreciate some info on this before i wade in 
since i know there are a few people here who are very clued up. 
I know that reducing rear roll is said to reduce understeer and promote front end grip (at least on a scooby with the famous WL adj. ARB!) - is this a diagonal relationship?
Thanks for any replies
since i know there are a few people here who are very clued up. I know that reducing rear roll is said to reduce understeer and promote front end grip (at least on a scooby with the famous WL adj. ARB!) - is this a diagonal relationship?
Thanks for any replies
30 March 2006, 06:31 AM
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as far as i am aware the whiteline rear roll bar on the scooby actually removes rear end grip, which helps the rear slide around giving you more over steer
i was led to believe it does not give the front end anymore grip in real terms TBH
i was led to believe it does not give the front end anymore grip in real terms TBH
10 June 2006, 12:57 PM
#5
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Originally Posted by StickyMicky
as far as i am aware the whiteline rear roll bar on the scooby actually removes rear end grip, which helps the rear slide around giving you more over steer
i was led to believe it does not give the front end anymore grip in real terms TBH
i was led to believe it does not give the front end anymore grip in real terms TBH
Last edited by silent running; 10 June 2006 at 01:01 PM.
12 June 2006, 11:40 AM
#6
It's pretty much as posted in the above posts.
You don't increse grip at the front, it just reduces rear grip at the back to the point that it prefers to over steer moreso than under steer. As essentially your ties both rear wheel together more to act as a single wheel or live solid axle - where one wheel hits a bump, the opposing wheel is also affected, even though the opposing wheel is on a smooth suface.
Done correctly it will make the car quicker round a track, as grip levels between the front and rear axles are more balanced - A car which oversteers too much will be slower on high speed corners and more unstable on turning into corners, especially under braking as well as being more sensitive using excessive throttle movements. A car that understeers too much will be slower on sharp, tight bends, but is more stable under heavy throttle movements and on high speed corners (presuming RWD or 4wd
)
A few cautions need to taken, especially with Imprezas, which are predominently an understeering car, that does suffer sudden snap oversteer. Reducing the progressional stiffness and increasing the overall stiffness of the rear antiroll bar will make the back end snap out of line easier and also importantly, can make it snap faster with less provocation. So attention need to be taken with respect to overall ARB stiffness, and elasticity or compliance of the linkage (i.e droplinks) that still gives individual wheel independance on light bumps. dispite the use of a stiff ARB.
You don't increse grip at the front, it just reduces rear grip at the back to the point that it prefers to over steer moreso than under steer. As essentially your ties both rear wheel together more to act as a single wheel or live solid axle - where one wheel hits a bump, the opposing wheel is also affected, even though the opposing wheel is on a smooth suface.
Done correctly it will make the car quicker round a track, as grip levels between the front and rear axles are more balanced - A car which oversteers too much will be slower on high speed corners and more unstable on turning into corners, especially under braking as well as being more sensitive using excessive throttle movements. A car that understeers too much will be slower on sharp, tight bends, but is more stable under heavy throttle movements and on high speed corners (presuming RWD or 4wd
)A few cautions need to taken, especially with Imprezas, which are predominently an understeering car, that does suffer sudden snap oversteer. Reducing the progressional stiffness and increasing the overall stiffness of the rear antiroll bar will make the back end snap out of line easier and also importantly, can make it snap faster with less provocation. So attention need to be taken with respect to overall ARB stiffness, and elasticity or compliance of the linkage (i.e droplinks) that still gives individual wheel independance on light bumps. dispite the use of a stiff ARB.
Last edited by ALi-B; 12 June 2006 at 11:49 AM.
12 June 2006, 07:30 PM
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The following is a copy of the Whiteline website with a small summary at the end.
The fitting of larger swaybars (rear and in general) has two main effects, vehicle balance in terms of understeer and oversteer, and increased roll resistance. Both of these can provide increased overall grip levels that can be achieved by the vehicle.
As most factory vehicles are biased towards understeer, fitting of the larger rear swaybar will help in providing a more neutral characteristic in the handling at the limit. This is due to the increase in roll stiffness at the rear, which loads the rear wheels more unevenly and provides slightly less grip at the rear than previous.
At first this may sound sacrificial, however, as the rear end is resisting more of the roll, the front end resists less in proportion, leaving the front wheels more evenly loaded, therefore more available front end grip. In the end an increase in overall grip can be achieved by balancing the vehicle. A WRX or other front torque biased all wheel drive vehicle will benefit even more due to combined front end steering/traction demand.
Another effect of introducing larger rear bars is that the roll stiffness is increased, and chassis roll is reduced, this also reduces the effects of "roll camber". Roll camber is the variation in the wheel/tyre camber setting due to chassis roll, and during cornering usually results in the outside wheels gaining positive camber.
By increasing the roll stiffness and reducing roll camber effect, the wheel/tyre stays closer to its wheel alignment setting or optimal setting. This can increase the overall cornering grip available, as the wheel/tyre does not lose as much negative camber at the limit.
The balance (and grip increase) of the car could also be achieved by reducing the front swaybar stiffness, however its roll stiffness would be reduced and roll camber would suffer. This would lead to large amounts of positive camber being gained on the outside wheels/tyres when cornering. This would result in a wheel/tyre that would not be at its optimal camber setting at the limit of handling.
This could be remedied with large amounts of static camber to counter act the positive camber gain, however the resulting tyre where, and straight-line handling effects would suffer.
To maximise wet weather grip, a softer overall setting would be required from dry settings. The reason for this is that a wet track cannot give the same friction values as a dry track and therefore overall grip will never be as high as in the dry (hence the amount of chassis roll will be lower as well). The suspension can therefore be soften slightly until the camber setting start to be compromised due to camber roll.
So to conclude a larger rear bar in the wet should leave the car balanced however it would be slightly disadvantaged due to the high stiffness
How big a bar do I need and how will it work?
Lets start first by assuming that every vehicle has a certain optimum "anti-roll" value, typically expressed in pounds or kilos. Don't ask me the ideal for the WRX as there are many formulas that have to take into account the COG (centre of gravity), RC (roll centres), the resultant roll couple and roll axis front to rear. We at Whiteline establish this optimal amount thru experience and testing though we are starting to use more computer modelling software to fast track development. With this amount in hand, the next issue is to determine where these amounts are needed and under what circumstances.
Best next to jump to the issue of cornering and how much we need where and when but its important to first identify that we need to split any given corner into at least 3 segments. Corner entry is that first phase where you initially turn in, you are generally decelerating either under brakes or on a trailing throttle. Needless to say the weight shift is moved toward the front. Mid corner is when you start aiming for the apex proper with either a neutral pitch (for-aft weight or movement mode) stance or slight power application. Corner exit is once you seriously start applying power and can be either just before the apex or after. Either way, the main steering line has been established and corner exit power is being applied to maximise forward speed. Accepting the above, its equally important to then segment handling bias according to corner position. That is, a mid-engine car with factory setup will typically understeer on "corner entry" and "middle" stages but very easily oversteer on exit if too much power is applied.
Understanding that everything is a compromise, it is useful to have a disproportionately high rear roll rate on corner entry to deliver more front roll to improve initial turn-in "bite" through the front outer. A heavier front spring like a lot of front anti-roll, will act to oppose the momentary (as little as a 1/10 sec) weight transfer required to give that "bite" which can result in more turn-in understeer. On the other hand, corner exit is all about maximising grip in longitude and latitude with throttle steering used to optimise outcome. Here, disproportionately more rear roll control acts to load the inside front (particularly useful on a fwd and WRX) as the outside rear is held more upright.
The correct total amount of "anti-roll" is inclusive of spring rate, which complicates the issue but it is still safe to assume that it is better to start with less spring rate than too much. The object is to maximise grip through adequate suspension travel at each wheel, a heavier spring rate makes it difficult for an individual wheel to react to changes in road surface under roll so contact is lost. Having said that, there is still no absolute correct number or recipe with two different racecars in any 2x car leading race team using largely different setups depending on the driver's preference. However, in most cases it will still function within a certain optimal pie of anti-roll force.
Click here to see how fitting a larger Whiteline rear bar in this example significantly increases total grip resulting in higher cornering speeds. Larger bars DO NOT simply change the handling bias as some people think, they can actually improve handling and INCREASE grip. (Test vehicle is P-Rex III GD series MY03 WRX).
Multiple adjustment holes on Whiteline Blade swaybars
A swaybar is actually a torsion spring not unlike a coil spring. Imagine you could uncoil your coil springs, and then hold each end and twist. This is pretty much what happens with the coil when it does its work as the shape forces the material to twist through out its length as it's compressed.
Going back to our straightened piece of spring steel, with your arms holding each end, imagine that each of your arms represent the swaybars "arms" and the spring steel rod represents the "back" or centre portion of the bar between the arms. As you try to twist the ends, a certain amount of flex happens in your arms but most of the action happens across the back of the bar. Hence, the formula calculating swaybar rates requires a value for the length of the back of the bar as well as the length of the arms.
The formula does not allow for a left or right arm value, just a total that acknowledges that the total arm length vs. the back length is the key calculation affecting the rate. We know that shortening the effective length of the arm by choosing a hole closer to the back of the bar will reduce the leverage ratio hence increase the rate but this affects the total arm length so can be done on one side only. In this way, a 2 hole per arm Blade adjustable bar does actually have 3 different settings; a 3-hole bar has 5 different settings. Additional mounting holes on the chassis end multiply the options further.
There are some assumptions and exceptions to these examples, specifically the issue of ultra short arm lengths and its effect on suspension preload. That is, asymmetric adjustment can preload the suspension if the arms are very short. The other issue is that the bars arms also deflect so that is taken into account by the formula but the amount of deflection is governed by the size and shape. Friction and deflection in mounting bushes will also affect the bars outright behaviour.
You may also notice that some of our adjustable use a longer Blade (flattened area) than others. We use this to fine-tune the adjustment range as the Bladed portion deflects a lot less than the simple round bar. (Think of a structural steel I-beam). Even the height and width of the Blade is used to tune the final outcome when designing new bars.
As for different materials, our XRD swaybars use 27mm base material machined down to 24mm outside the chassis D mount points coupled with a specific Blade length to deliver a very broad range of adjustment through a more progressive working curve. This delivers a hybrid outcome tunned for a particular result and is another design tool we can use.
It's therefore important to understand that a 22mm physical diameter swaybar can be made to behave like something totally different just by changing the shape of the bar and its ends. Adjustable bars are a very useful suspension balance tuning tool and a better understanding of how they work should help you get a better result.
--------------------
So, the main thing to remember is that an anti roll bar has a significant effect on the diagonal axis of the car. Fitting an uprated rear bar on an Impreza does not so much reduce the rear grip as it increases the weight on the inside front wheel during cornering. The normal practice in (RWD/4WD)race car set-up is to increase the front roll resistance until turn-in is compromised. Then use the rear bar as a 'trimming tool' to increase the inside front wheel grip and induce a very small amount of inside rear wheel slip to stop the car pushing in the direction that ther rear wheels are pointing.
Cheers
The fitting of larger swaybars (rear and in general) has two main effects, vehicle balance in terms of understeer and oversteer, and increased roll resistance. Both of these can provide increased overall grip levels that can be achieved by the vehicle.
As most factory vehicles are biased towards understeer, fitting of the larger rear swaybar will help in providing a more neutral characteristic in the handling at the limit. This is due to the increase in roll stiffness at the rear, which loads the rear wheels more unevenly and provides slightly less grip at the rear than previous.
At first this may sound sacrificial, however, as the rear end is resisting more of the roll, the front end resists less in proportion, leaving the front wheels more evenly loaded, therefore more available front end grip. In the end an increase in overall grip can be achieved by balancing the vehicle. A WRX or other front torque biased all wheel drive vehicle will benefit even more due to combined front end steering/traction demand.
Another effect of introducing larger rear bars is that the roll stiffness is increased, and chassis roll is reduced, this also reduces the effects of "roll camber". Roll camber is the variation in the wheel/tyre camber setting due to chassis roll, and during cornering usually results in the outside wheels gaining positive camber.
By increasing the roll stiffness and reducing roll camber effect, the wheel/tyre stays closer to its wheel alignment setting or optimal setting. This can increase the overall cornering grip available, as the wheel/tyre does not lose as much negative camber at the limit.
The balance (and grip increase) of the car could also be achieved by reducing the front swaybar stiffness, however its roll stiffness would be reduced and roll camber would suffer. This would lead to large amounts of positive camber being gained on the outside wheels/tyres when cornering. This would result in a wheel/tyre that would not be at its optimal camber setting at the limit of handling.
This could be remedied with large amounts of static camber to counter act the positive camber gain, however the resulting tyre where, and straight-line handling effects would suffer.
To maximise wet weather grip, a softer overall setting would be required from dry settings. The reason for this is that a wet track cannot give the same friction values as a dry track and therefore overall grip will never be as high as in the dry (hence the amount of chassis roll will be lower as well). The suspension can therefore be soften slightly until the camber setting start to be compromised due to camber roll.
So to conclude a larger rear bar in the wet should leave the car balanced however it would be slightly disadvantaged due to the high stiffness
How big a bar do I need and how will it work?
Lets start first by assuming that every vehicle has a certain optimum "anti-roll" value, typically expressed in pounds or kilos. Don't ask me the ideal for the WRX as there are many formulas that have to take into account the COG (centre of gravity), RC (roll centres), the resultant roll couple and roll axis front to rear. We at Whiteline establish this optimal amount thru experience and testing though we are starting to use more computer modelling software to fast track development. With this amount in hand, the next issue is to determine where these amounts are needed and under what circumstances.
Best next to jump to the issue of cornering and how much we need where and when but its important to first identify that we need to split any given corner into at least 3 segments. Corner entry is that first phase where you initially turn in, you are generally decelerating either under brakes or on a trailing throttle. Needless to say the weight shift is moved toward the front. Mid corner is when you start aiming for the apex proper with either a neutral pitch (for-aft weight or movement mode) stance or slight power application. Corner exit is once you seriously start applying power and can be either just before the apex or after. Either way, the main steering line has been established and corner exit power is being applied to maximise forward speed. Accepting the above, its equally important to then segment handling bias according to corner position. That is, a mid-engine car with factory setup will typically understeer on "corner entry" and "middle" stages but very easily oversteer on exit if too much power is applied.
Understanding that everything is a compromise, it is useful to have a disproportionately high rear roll rate on corner entry to deliver more front roll to improve initial turn-in "bite" through the front outer. A heavier front spring like a lot of front anti-roll, will act to oppose the momentary (as little as a 1/10 sec) weight transfer required to give that "bite" which can result in more turn-in understeer. On the other hand, corner exit is all about maximising grip in longitude and latitude with throttle steering used to optimise outcome. Here, disproportionately more rear roll control acts to load the inside front (particularly useful on a fwd and WRX) as the outside rear is held more upright.
The correct total amount of "anti-roll" is inclusive of spring rate, which complicates the issue but it is still safe to assume that it is better to start with less spring rate than too much. The object is to maximise grip through adequate suspension travel at each wheel, a heavier spring rate makes it difficult for an individual wheel to react to changes in road surface under roll so contact is lost. Having said that, there is still no absolute correct number or recipe with two different racecars in any 2x car leading race team using largely different setups depending on the driver's preference. However, in most cases it will still function within a certain optimal pie of anti-roll force.
Click here to see how fitting a larger Whiteline rear bar in this example significantly increases total grip resulting in higher cornering speeds. Larger bars DO NOT simply change the handling bias as some people think, they can actually improve handling and INCREASE grip. (Test vehicle is P-Rex III GD series MY03 WRX).
Multiple adjustment holes on Whiteline Blade swaybars
A swaybar is actually a torsion spring not unlike a coil spring. Imagine you could uncoil your coil springs, and then hold each end and twist. This is pretty much what happens with the coil when it does its work as the shape forces the material to twist through out its length as it's compressed.
Going back to our straightened piece of spring steel, with your arms holding each end, imagine that each of your arms represent the swaybars "arms" and the spring steel rod represents the "back" or centre portion of the bar between the arms. As you try to twist the ends, a certain amount of flex happens in your arms but most of the action happens across the back of the bar. Hence, the formula calculating swaybar rates requires a value for the length of the back of the bar as well as the length of the arms.
The formula does not allow for a left or right arm value, just a total that acknowledges that the total arm length vs. the back length is the key calculation affecting the rate. We know that shortening the effective length of the arm by choosing a hole closer to the back of the bar will reduce the leverage ratio hence increase the rate but this affects the total arm length so can be done on one side only. In this way, a 2 hole per arm Blade adjustable bar does actually have 3 different settings; a 3-hole bar has 5 different settings. Additional mounting holes on the chassis end multiply the options further.
There are some assumptions and exceptions to these examples, specifically the issue of ultra short arm lengths and its effect on suspension preload. That is, asymmetric adjustment can preload the suspension if the arms are very short. The other issue is that the bars arms also deflect so that is taken into account by the formula but the amount of deflection is governed by the size and shape. Friction and deflection in mounting bushes will also affect the bars outright behaviour.
You may also notice that some of our adjustable use a longer Blade (flattened area) than others. We use this to fine-tune the adjustment range as the Bladed portion deflects a lot less than the simple round bar. (Think of a structural steel I-beam). Even the height and width of the Blade is used to tune the final outcome when designing new bars.
As for different materials, our XRD swaybars use 27mm base material machined down to 24mm outside the chassis D mount points coupled with a specific Blade length to deliver a very broad range of adjustment through a more progressive working curve. This delivers a hybrid outcome tunned for a particular result and is another design tool we can use.
It's therefore important to understand that a 22mm physical diameter swaybar can be made to behave like something totally different just by changing the shape of the bar and its ends. Adjustable bars are a very useful suspension balance tuning tool and a better understanding of how they work should help you get a better result.
--------------------
So, the main thing to remember is that an anti roll bar has a significant effect on the diagonal axis of the car. Fitting an uprated rear bar on an Impreza does not so much reduce the rear grip as it increases the weight on the inside front wheel during cornering. The normal practice in (RWD/4WD)race car set-up is to increase the front roll resistance until turn-in is compromised. Then use the rear bar as a 'trimming tool' to increase the inside front wheel grip and induce a very small amount of inside rear wheel slip to stop the car pushing in the direction that ther rear wheels are pointing.
Cheers
Last edited by jgevers; 12 June 2006 at 07:53 PM.
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