The impact of suspension changes on vehicle dynamics

[BonzoHansen]

Based on a few recent posts, I thought this list might be useful to some folks who are thinking about chassis changes.



Guide To High Performance Handling ( http://www.tirerack.com/tires/tiretech/techpage.jsp?techid=58)

Adjustments Decrease Understeer Decrease Oversteer
Front Tire Pressure Higher Lower
Rear Tire Pressure Lower Higher
Front Tire Section Larger Smaller
Rear Tire Section Smaller Larger
Front Wheel Camber More Negative More Positive
Rear Wheel Camber More Positive More Negative
Front Wheel Toe Toward Toe-Out Toward Toe-In
Rear Wheel Toe Toward Toe-In Toward Toe-Out
Front Wheel Caster More Positive More Negative
Front Springs Soften Stiffen
Rear Springs Stiffen Soften
Front Anti-sway Bar Soften (Thinner) Stiffen (Thicken)
Rear Anti-sway Bar Stiffen (Thicker) Soften (Thinner)
Weight Distribution More Rearward More Forward
Front Track Decrease track Increase track **
Rear Track Increase track Decrease track **
Front Toe-in Reduce Increase
Front tire downforce Increase Reduce
Rear tire downforce Reduce Increase


understeer = the nose pushes
oversteer = the tail gets loose

[thunder]

good call on this post

[BarneyMobile]

Nice post, just one concern. Its seems to me that the first 2 rows are backwards?

[CHRIS67]

Nice post, just one concern. Its seems to me that the first 2 rows are backwards?

I had the same thought. :scratch:

[BonzoHansen]

I'll do some more research. But I know with my civic it would oversteer when pushed hard and bumping the back tire PSI up absolutely helped. In any event I believe the concept is based on starting with PSI on being within an ideal range. So we’re talking maybe +/- a few psi, just fine tuning. IMO it’s tweaking after you’ve already set ‘ideal’ PSI via reading temps across the face of the tire. It’s not saying drop the tire to 10 psi or raise it to 60 psi.

I have read where if a car understeers entering and oversteers exiting a corner, raise all pressures. But I’ve never done that.

[BonzoHansen]

Taken from How To Make Your Car Handle by Fred Puhn

To Reduce Understeer:
Increase weight transfer at rear by increasing rear roll stiffness
Reduce weight transfer on front by reducing front roll stiffness
Reduce front toe-in
Increase aero downforce on front tires
Reduce aero downforce on rear tires
Wider front tires

To Reduce Oversteer:
Reduce weight transfer at rear by reducing rear roll stiffness
Increase weight transfer on front by increasing front roll stiffness
Increase front toe-in
Reduce aero downforce on front tires
Increase aero downforce on rear tires
Wider rear tires

He states the old rule of thumb was to increase front PSI to reduce understeer and increase rear PSI to reduce oversteer. He goes on to say this really applies to road cars where PSI is often too low to begin with. Best bet is to get PSI correct via temp readings via skidpad testing and changing psi only limits grip.

In Herb Adam's Chassis Engineering, he also does not appear to get into PSI other than working off temperatures to get to ideal and is generally in the same vein as Puhn.

So that jives with my gut that PSI is just fine tuning and Puhn talking about only going up on road tires matches my experiences.