why does increasing front tire pressure, reduce understeer?
12-27-2007, 02:39 AM
#31
Rennlist Member
So a spring set up that allows both side wheels to leave the ground in a hard turn, and a change in spring set up where this is fixed (or a situation where one corner is airborn, but with spring changes, this is fixed) doesnt change weight transfer?
I also remember some equations regarding G loading potential, that also didnt incorporate spring or other factors. Maybe those formulas are true for ideal compliance set ups, so they are true to compare relative setup us.
Intuitively, it seems like spring rates should effect weight transfer.
Thoughts?
MK
is allowed to
I also remember some equations regarding G loading potential, that also didnt incorporate spring or other factors. Maybe those formulas are true for ideal compliance set ups, so they are true to compare relative setup us.
Intuitively, it seems like spring rates should effect weight transfer.
Thoughts?
MK
is allowed to
Absolutely incorrect. Spring rates and tires pressures do NOT affect weight transfer.
Load (weight) transfer = acceleration x (mass x height)/wheelbase (or track depending upon whether you're calculating longitudinal or lateral load xfer). No where in that calculation does spring rate come into play.
Load (weight) transfer = acceleration x (mass x height)/wheelbase (or track depending upon whether you're calculating longitudinal or lateral load xfer). No where in that calculation does spring rate come into play.
12-27-2007, 07:29 AM
#32
Race Director
Correct, although if the height of the car were to change significantly (as wheels came off the ground), that would affect the weight xfer.
12-28-2007, 05:24 PM
#33
Rennlist Member
I think what is meant in the equations, is for a constant G loading. if a spring change allows for greater g loading, it will change the weight transfer values.
mk
mk
12-28-2007, 05:40 PM
#34
Race Director
That is a good and valid point. All other things being equal, if a spring change causes a change in the acceleration (g loading), that would indeed change the load xfer. However, review the formula. If a spring change increases the g loading, the load xfer will actually increase, not decrease.
12-28-2007, 07:40 PM
#35
I saw a lot of mis-information in this post. There are actually 2 things being discussed here, not one: contact patch and response. Sidewall stiffness is very difference from contact patch. You can only gain grip by adding friction between the vehicle and the road. This can be done several ways:
1. Reduce tire pressure to increase static contact patch
2. Increase camber. Most cars when cornering don't have the entire contact width across the tire on the road as the suspension loads and the tire shifts on the wheel. Adjusting camber can add contact patch while corner. You can also have too much camber, which will adversly remove contact patch.
3. Increase Caster. Caster is additional camber gain from the angle of the pivot point of the front suspension only while the wheels are turned. Increasing caster increases camber only while turning.
4. Using sway bars or spring rates to change weight transfer between the front and the back. The more weight that transfers to one end, the greater the grip on that end. For example, a larger rear sway bar or stiffer springs keeps more weight on the front wheels. Hence it will decrease understeer or increase oversteer as it actually reduces rear grip.
Please note, all these are exxplained in terms of contact patch; not response.
The disadvantage to lowering pressure is more sidewall flex. By physics, when you turn the wheels the car doesn't actually turn until the tires are fully side-loaded and friction forces them to turn. Remember and object put into motion stays into motion until a force is applied to it. That force in this case is friction.
The more air pressure in a tire, the greater the stiffness of the sidewall. When you first start to turn, the tire by momentum is still trying to go straight. As a result the tread is following the straight path and shifts to the outside. Once it has shifted to the maximum allowed by sidewall and suspension flex, friction takes over. The greater the sidewall flex the longer it takes to completely load a tire.
So can increasing tire pressure reduce understeer? It depends on how you are perceiving understeer. Is understeer the car doesn't want to turn in or in the middle of a corner it just pushes to the outside of the turn? The first is response, not understeer/oversteer. The simply solution is to adjust your driving style. Start turning in earlier and slow down you steering input. This allows the front end to load more naturally and turn in crisper. For the second, changing tire pressures can be very effective in changing belance.
1. Reduce tire pressure to increase static contact patch
2. Increase camber. Most cars when cornering don't have the entire contact width across the tire on the road as the suspension loads and the tire shifts on the wheel. Adjusting camber can add contact patch while corner. You can also have too much camber, which will adversly remove contact patch.
3. Increase Caster. Caster is additional camber gain from the angle of the pivot point of the front suspension only while the wheels are turned. Increasing caster increases camber only while turning.
4. Using sway bars or spring rates to change weight transfer between the front and the back. The more weight that transfers to one end, the greater the grip on that end. For example, a larger rear sway bar or stiffer springs keeps more weight on the front wheels. Hence it will decrease understeer or increase oversteer as it actually reduces rear grip.
Please note, all these are exxplained in terms of contact patch; not response.
The disadvantage to lowering pressure is more sidewall flex. By physics, when you turn the wheels the car doesn't actually turn until the tires are fully side-loaded and friction forces them to turn. Remember and object put into motion stays into motion until a force is applied to it. That force in this case is friction.
The more air pressure in a tire, the greater the stiffness of the sidewall. When you first start to turn, the tire by momentum is still trying to go straight. As a result the tread is following the straight path and shifts to the outside. Once it has shifted to the maximum allowed by sidewall and suspension flex, friction takes over. The greater the sidewall flex the longer it takes to completely load a tire.
So can increasing tire pressure reduce understeer? It depends on how you are perceiving understeer. Is understeer the car doesn't want to turn in or in the middle of a corner it just pushes to the outside of the turn? The first is response, not understeer/oversteer. The simply solution is to adjust your driving style. Start turning in earlier and slow down you steering input. This allows the front end to load more naturally and turn in crisper. For the second, changing tire pressures can be very effective in changing belance.
12-28-2007, 09:05 PM
#36
Race Director
I love threads like these because I'm something of a geek and love to research this stuff in an effort to understand it better. As a result, I've pulled out my highlighted copy of Carroll Smith's "Tune to Win" and have uncovered a comment Mark (and others) may find interesting, and also rediscovered something that I (and others as well probably) found interesting.
Regarding roll and load transfer, Smith wrote: "One of the most widespread misconceptions in racing is that the amount of load transfer taking place is directly related to chassis roll." (p36 of my copy)
"The greater the resistance of the springs, the less roll will result -- but there will be no significant effect on the amount of lateral load transfer because the roll couple has not been changed and there is no physical connection between the springs on the opposite sides of the car." (p37 of my copy)
Continuing the same paragraph, and the part I rediscovered and find quite interesting: "The same cannot be said of the resistance of the anti-roll bars. In this case, because the bar is a direct physical connection between the outside wheel and the inside wheel, increasing the stiffness of the anti-roll bar will both decrease roll angle and increase lateral load transfer."
Good stuff to chew on. I love reading Carroll Smith.
Regarding roll and load transfer, Smith wrote: "One of the most widespread misconceptions in racing is that the amount of load transfer taking place is directly related to chassis roll." (p36 of my copy)
"The greater the resistance of the springs, the less roll will result -- but there will be no significant effect on the amount of lateral load transfer because the roll couple has not been changed and there is no physical connection between the springs on the opposite sides of the car." (p37 of my copy)
Continuing the same paragraph, and the part I rediscovered and find quite interesting: "The same cannot be said of the resistance of the anti-roll bars. In this case, because the bar is a direct physical connection between the outside wheel and the inside wheel, increasing the stiffness of the anti-roll bar will both decrease roll angle and increase lateral load transfer."
Good stuff to chew on. I love reading Carroll Smith.
