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.

Originally Posted by BonzoHansen

Guide To High Performance Handling

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]

Originally Posted by BarneyMobile

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

I had the same thought.

[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.