Vehicle dynamics question for the engineers

[Boobietrap]

I have a question regarding spring rates. Apart from track smoothness and aero, what is the purpose of having stiffer springs (or vice versa)? I'm not talking about F/R balance, my question is what is the reason of having higher (or lower) frequencies on all four corners, from the perspective of physics/engineering, preferably backed up with some math. Please don't respond with anecdotal or "common belief" type of answers.

Similarly, why have higher damping rates, say, 1.1 of critical vs 0.7? Again, not asking about transient balance here. And how should a damper velocity histograms look and be used to tune overall damper stiffness?

Much appreciated!

 

[PhilS13]

Two main things for higher ride frequencies, disregarding aero and track smoothness.

Lower CG. Higher frequencies reduce travel so usually the point of them is to allow a lower car and the gains that come with it.

Response. There is simply a point where too low ride frequencies become undriveable because of the time it takes for the car to fully change state.

The damping ratios are always given in terms of ride but sometimes you want to be looking at roll critical damping and if all you have is corner dampers then you might end up with pretty high damping in ride to get a car that rolls and sets nicely.

 

[Miguel Batista]

Two main things for higher ride frequencies, disregarding aero and track smoothness.

Lower CG. Higher frequencies reduce travel so usually the point of them is to allow a lower car and the gains that come with it.

Response. There is simply a point where too low ride frequencies become undriveable because of the time it takes for the car to fully change state.

The damping ratios are always given in terms of ride but sometimes you want to be looking at roll critical damping and if all you have is corner dampers then you might end up with pretty high damping in ride to get a car that rolls and sets nicely.

Interesting so basically you want to stiffen the springs to reduce low frequency oscillations and modify the dampers to modulate the return frequency? So in a car that does a chicane in 0.2 seconds the best way to do it would be to set the spring to have a stiffness that would take 0.05s to fully compress (first corner apex) then have the dampers bounce the car back in another 0.05s so that it is set for the next corner? How does the weight of the car come into play because the change in direction will cause the car to shift its weight around.

 

[Ghoults]

A race car typically has not much ground clearance. With stiffer springs you can run the car low while still having suspension in the car. The harder the springs the less they compress under the same load and if you have only an inch to work with you probably don't want the suspension to bottom out as soon as you steer or accelerate/brake.

As for dampers just quoting the 0.7 damping value is a bit misleading as dampers have different damping values at different speeds (speed of damper, not car). But even then you look at a damping value plot you can see the 0.7 has little overshoot. Above 1.0 is overdamped and below is underdamped. For race car you want higher numbers because it gives you more stable platform and for road car you want comfort.

Damper histograms tells you how your damper was working on a track. If drive on rough track your historgrams are flatter because there is more bumps and the damper is more evenly used on its full speed range. There are some pretty good guides and explanations for damper histograms online.

 

[Boobietrap]

Two main things for higher ride frequencies, disregarding aero and track smoothness.

Lower CG. Higher frequencies reduce travel so usually the point of them is to allow a lower car and the gains that come with it.

Response. There is simply a point where too low ride frequencies become undriveable because of the time it takes for the car to fully change state.

The damping ratios are always given in terms of ride but sometimes you want to be looking at roll critical damping and if all you have is corner dampers then you might end up with pretty high damping in ride to get a car that rolls and sets nicely.

So stiffen roll resistance to set the car faster and reduce suspension travel allowing lower CoG, and by extension, reduced alignment changes... By that logic, the ideal spring rates would be as stiff as the track surface permits?

I don't follow your comments regarding the dampers. Can you explain?

 

[mclarenf1papa]

By that logic, the ideal spring rates would be as stiff as the track surface permits?

Also worth keeping in mind that the tires might not like being loaded super quickly versus a bit more gradually (due to things like relaxation length), even on a smooth track.

 

[Emery]

If a car is production-based and is camber-challenged (has McPherson struts), then stiff springs are used to keep the camber in a range that works for cornering because you typically have no way to legally alter the camber curve.

Damping ratio depends on downforce, too. If the car weighs X at rest, you end up with one damping ratio based on the sprung weight, however at 90-120 mph, the car's effective sprung weight increases by 10%-30% which changes the damping ratio. For oval track cars with high downforce at 200+ mph, you're looking at much larger downforce.

 

[Ghoults]

Lowering the suspension for lower cog is not always ideal either. Suspensions have been designed to work at the stock ride height and if you just lower the car you may cause things like bump steer or just mess up your roll centers causing terminal understeer or just bad behaviour. Of course in sim you don't need to worry about tire rubbing against fenders or parts exceeding their available motion but at the same time in sim you can not change parts, add spacers or adjust the geometry to counter effect the negative changes. I don't remember driving any car in ac which was improved with just making it lower. While ride height is important, especially in race cars, I'd still focus on keeping it where it is unless you know better.

 

[Boobietrap]

Ok so if we take ride height out of consideration as well, and maintain the stock instantaneous centers and roll centers (or assume that we can lower CoG and keep RC above ground without messing up elastic/kinematic proportions), stiffer springs will reduce unwanted (and wanted) alignment changes. In this scenario, what would the rational be to select softer springs? There was mention of tire characteristics... Anything else?

 

[RasmusP]

Ok so if we take ride height out of consideration as well, and maintain the stock instantaneous centers and roll centers (or assume that we can lower CoG and keep RC above ground without messing up elastic/kinematic proportions), stiffer springs will reduce unwanted (and wanted) alignment changes. In this scenario, what would the rational be to select softer springs? There was mention of tire characteristics... Anything else?

With a softer suspension you get more load to the outside and up to a certain degree: more grip on the outside tyres.
So with a softer suspension you can go faster through turns as long as you maintain in control. At some point the deformation will become too big and the tyre will wash out but in the sims you can clearly feel that with a stiffer suspension than the "good" default (in AC and rF2 GT3s for example), you will lose cornering speed!
But as mentioned if you go too soft you will lose control in quick left-right combinations and aero/ride height problems.

 

[Boobietrap]

With a softer suspension you get more load to the outside and up to a certain degree: more grip on the outside tyres.
So with a softer suspension you can go faster through turns as long as you maintain in control. At some point the deformation will become too big and the tyre will wash out but in the sims you can clearly feel that with a stiffer suspension than the "good" default (in AC and rF2 GT3s for example), you will lose cornering speed!
But as mentioned if you go too soft you will lose control in quick left-right combinations and aero/ride height problems.

Suspension stiffness cannot change overall load transfer. And more load transfer on to the outside tires will reduce overall grip, and thus cornering speeds, not increase it.

 

[RasmusP]

Suspension stiffness cannot change overall load transfer. And more load transfer on to the outside tires will reduce overall grip, and thus cornering speeds, not increase it.

It does because the cog rolls and moves towards the rear*. Edit: Wtf I meant outside...
Look into Motec tyre loads with stiff and soft suspension, it will show.
Also it's in all sims I know that you can go faster with a softer suspension but it's more difficult to drive.

 

[Boobietrap]

If a car is production-based and is camber-challenged (has McPherson struts), then stiff springs are used to keep the camber in a range that works for cornering because you typically have no way to legally alter the camber curve.

Damping ratio depends on downforce, too. If the car weighs X at rest, you end up with one damping ratio based on the sprung weight, however at 90-120 mph, the car's effective sprung weight increases by 10%-30% which changes the damping ratio. For oval track cars with high downforce at 200+ mph, you're looking at much larger downforce.

I'm not sure that dampers need to account for aero. While the load on the tire increases due to aero, the mass doesn't. And by extension, the natural frequency stays the same as well as damping ratios.

 

[Boobietrap]

It does because the cog rolls and moves towards the rear.
Look into Motec tyre loads with stiff and soft suspension, it will show.
Also it's in all sims I know that you can go faster with a softer suspension but it's more difficult to drive.

I'm not really following your logic. Load transfer is a function of lateral force, track, and CoG height.

The CoG may move backwards in roll if the roll center is higher in the rear, but that's a function of load transfer. Where the CoG is on the longitudinal axis doesn't change how much one side is loaded.

 

[RasmusP]

I'm not sure that dampers need to account for aero. While the load on the tire increases due to aero, the mass doesn't. And by extension, the natural frequency stays the same as well as damping ratios.

The suspension gets compressed though and becomes stiffer. The mass for the natural frequency stays the same but the spring force becomes higher as far as I understand this. So you get a stiffer car at higher speeds while the dampers will move towards underdampening

 

[RasmusP]

I'm not really following your logic. Load transfer is a function of lateral force, track, and CoG height.

The CoG may move backwards in roll if the roll center is higher in the rear, but that's a function of load transfer. Where the CoG is on the longitudinal axis doesn't change how much one side is loaded.

I might be wrong there. I'm totally fine with learning new things! If I come across any differently it's down to language barrier.

Explanation for my logic:
1. It's what happens for me actually on track
2. I see it in Motec
3. From my knowledge the cog is higher than the axles so when a car leans further due to a softer suspension the cog will move to the outside. The higher the cog is, the stronger this effect becomes.
So with the cog moving a few inch to the compressed outside, the outside tyre receives a higher vertical load (Motec shows it in Newton).
Due to rubber being rubber, the overall grip increases if you take the overall load and give the outside tyres more than the inside tyres. It's the grip curve over vertical load. In ac there's the formula visible although I don't really get it together yet. Pretty hidden and cryptic in the ini files...

Anyway, the maximum lateral g the car can take increases with a softer suspension as long as the geometry stays in shape.

 

[Stereo]

it's the opposite, the more evenly loaded the tires, the more total grip. Soft suspension lets the tire loads stay more even though since more of the spring is taken up by suspension instead of the tire rubber.

That's why rear ARBs have to be softer than front, so that the rear axle has more grip than the front (since front load is more uneven; it'll be like 800 LF, 200 RF, 600 LR, 400 RR)

 

[PhilS13]

Thread looking like a social experiment. OP seems to know the answer to what he was asking.

 

[Boobietrap]

Thread looking like a social experiment. OP seems to know the answer to what he was asking.

Au contraire, I've done plenty of reading but have yet to be able to answer the questions at the start of the thread - as you'll notice that my comments in the latter posts are exactly why I posted the original questions .

Anyway. Still interested in what you meant by your comments on dampers .

 

[Boobietrap]

The suspension gets compressed though and becomes stiffer. The mass for the natural frequency stays the same but the spring force becomes higher as far as I understand this. So you get a stiffer car at higher speeds while the dampers will move towards underdampening

Unless the spring is progressive, a compressed spring isn't stiffer, the spring rate doesn't change - not if it's well made at least. It does however, as you state, exert more force.