Help with spring rate.

[V4641]

Hello,

I'm having a little trouble with a car I'm trying to setup for autocross. The issue is, I'm getting into the bump stops under braking which is hurting my braking performance. I think I've even hit the stops on hard cornering as well.

Vehicle Specs:
Total weight 3970 pounds (all fluids & driver)
Wheelbase is 112 inches
CG is 13 inches
Weight distribution is 54/46
Front unsprung mass is 117 pounds
Rear unsprung mass is 124 pounds
Maximum braking is ~1.1g
Front coilover travel is 5.5 inches and has a 1 inch bump stop

The coilover allows the ride height to be adjusted independent of spring preload.


Can someone help me out by going through the math and showing what the minimum spring rate/preload is needed to keep the car from getting into the stops under maximum braking?

I've been setting the car up so that I'm ~50% into the stoke at static ride height. I do this by setting the preload to get me in the middle of the stroke, then adjust the height of the coilover to set the ride height. Is this the correct approach with a MacPherson Strut?


Thanks,

James

 

[BUGGAR]

How about a rising rate spring geometry?

 

[BrianPetersen]

Many (most?) bikes have a rising-rate linkage on the rear shock, and changing the ride height does have a secondary influence on the effective spring rate because when you change the ride height you also change where the suspension is operating with the linkage.

A strongly progressive linkage causes other problems, so in reality, this effect is not large within the normal range of suspension travel.

 

[BUGGAR]

"A strongly progressive linkage causes other problems, so in reality, this effect is not large within the normal range of suspension travel."

Could you please elaborate on this? I think I disagree but need more information to understand. Thanks.

 

[BrianPetersen]

Roadracing application. When cornering, the suspension is more loaded. The suspension is more critical when leaned over to keep the tires in contact with the pavement, so you need compliance when leaned way over. On the other hand, you don't really care what happens when straight up and down. You don't want strong rising-rate in that application.

Most modern forks use long, soft top-out springs which have the effect of raising the spring rate towards topping out so that the suspension has at least some compliance during acceleration instead of simply being hard-topped-out against the stop. But when cornering, it is operating within the softer main spring rate.

 

[BUGGAR]

Got it. Thanks.

 

[Panther140]

You have 1.75" of effective travel right now. Thats probably not far from how much travel you should have left over while braking, unless you only hit the brakes on silky smooth pavement and never do it while setting up for a turn. This is they type of thing you might as well just go out and tune at the track. There are so many different forces acting on it that all you can do at your computer is get it kinda close. You're dealing with driving style, track conditions, weight transfers, lateral grip, Center of gravity moment arm, compression dampening, rebound dampening, suspension geometry, chasis rigidity, camber, castor, anti roll bar, wind resistance, driving style, course layout, and a bunch of other things that nobody on here can get enough info about. By the time you analyze all of that stuff and start calculating the finer points of it, your competitors already have dust on their trophies

"Formal education is a weapon, whose effect depends on who holds it in his hands and at whom it is aimed." ~ Joseph Stalin

 

[NormPeterson]

I'm going to question that 13" CG height. Everything else in the vehicle description and its autocross intent hints at the car in question being a 5th generation Camaro, the CG of which is going to start out somewhere much closer to 20". All of the load transfer numbers and suspension displacements will change accordingly.

Lowering also causes more of the lateral load transfer to be resisted by the springs and sta-bars, which results in greater suspension compression/extension than you'd get with the same springs and bars with the ride height set higher.


Norm

 

[Overrun]

Norm, wouldn’t lowering the CoG decrease the roll couple?

 

[NormPeterson]

It will reduce the lateral load transfer. But for a MacStrut suspension the geo roll center (strictly speaking, a geo RC defines the height at which a lateral force produces no roll) drops even faster, and the roll couple increases.


Norm