Spring rate testing orgy

[Perpetuus]

Hmm.. I work in automotive engineering, but I certainly don't have all the answers, or know everything inside out about vehicle dynamics. My job also doesn't make my understanding of the topic any better than others necessarily. Anyway, here are the premises of I was basing my statement on. Of course, this is vastly simplified: (Alot of them from Milliken's Race Car Vehicle Dynamics book)

For making the following less complicated, I am assuming straight line driving.

1) Vehicle tires generate more grip as more force is exerted on them, up until the ultimate limit of grip the tires are capable of with respect to the road surface.

2) For driving over typical roads with undulations, dips, and potholes, etc., the car with the best grip is the car whose tires can track the road surface irregularities the best (i.e.: tire is following the surface of the road 95% of the time vs. 80% of the time with an overly soft spring/damper setup).

3) The car with the best grip is the car i) whose sprung mass (which is invariant), is the least upset by the suspension/tire combination heaving the car up and down over bumps, and ii) whose tires can track the road surface irregularities better (or rather, a higher percentage of the time). Both conditions i) and ii) are necessary to generate the best grip the tires can deliver, assuming everything else is unchanged.

An overly soft spring/damper combo or an overly hard spring/damper combo will both have drawbacks on grip and handling.

a) In the case of the car with the overly soft spring/damper combo, the sprung mass stays relatively undisturbed, especially over high frequency bumps in the road. This is good for helping keep the weight of the car on the tires as consistent as possible. However, if we look at the unsprung mass of the tire and suspension, we have an issue where the tires are not able to follow the contours of the road very well, and hence diminishing grip at the tires. Part of the reason for this is because the spring and damper combo allows the tires to ride easily over road irregularities during the compression stroke, but not necessarily during rebound, where the suspension cannot react fast enough (or with enough force from the soft springs) to allow the tires to ride on top of the road surface well enough to generate good levels of grip.

b) Now, consider the case of the car with the overly hard spring/damper combo. The combination of the hard springs and dampers will make the car feel bumpy (harsh on compression because of the spring rates might be too high for various road imperfections). When the spring rate is too high for the given road irregularity the tire is travelling over, this translates to a displacement of the sprung mass, heaving it up quickly, and hence, reducing the total effective sprung mass acting on that very tire. This is felt by a loss of control by the driver, possibly requiring steering adjustments to readjust the trajectory of the car, a loss of grip at the affected tires at that very instant, and unfavourable weight transfer induced by the road surface. The weight transfer is complex, and can be fore-aft, diagonally, or a combination of both. This unfavourable weight transfer upsets the balance of the car, the effectiveness of the tires to grip the road, and a higher possibility of the car to snap-spin during cornering. However, the unfavourable weight transfer induced the by hard springs, (which stems from the road irregularities), combined with the quick weight transfer means that the car will snap-spin much more easily during cornering, as compared to a car with less stiff springs/dampers.

Because of the increased inconsistency in vehicle grip and handling, and the higher tendency to snap-spin (especially in lower grip conditions), the car with the slightly lower spring rate/damper combo is probably better all around IMO.

Of course, the higher spring rate/damper combo makes the car more confidence-inspiring. The transient cornering response is increased, the car rolls less in turns (accompanied by less unfavourable camber change), and overall the car feels sportier to drive. From personal experience though, sometimes you don't necessarily need very stiff springs to make a car handle and grip well. In many cases, the driver will be able to feel the road alot more with the more stiffly-sprung car, which also gives the sensation of a higher speed as more vibration/bumps are upsetting the car at a higher rate, even when driving at the same speed as the same car with a softer spring/damper setup.

IMO, for the above reasons, the ideal case is a suspension system that has a variable spring rate and variable damping to anticipate the road surface ahead. This will help the tires track the road properly, while maintaining the highest possible level of grip on the road surface and translating little to no extra amplitude to the sprung mass, which tosses the body upwards and removes grip from the tire(s).

Sorry for the long post. This is my understanding of vehicle dynamics at a simplified level. We are all here to learn, but I hope this makes sense.

 

[the bruce]

This is my understanding of vehicle dynamics at a simplified level. We are all here to learn, but I hope this makes sense.

It does. Great write up. :thumbsup:


I'd like to ad when it comes to spring rates we should keep in mind sway bars.
It does make huge difference if someone runs stock bars or both front and rear
28 mm AWE/H&Rs of course.

 

[miamirice]

Hmm.. I work in automotive engineering, but I certainly don't have all the answers, or know everything inside out about vehicle dynamics. My job also doesn't make my understanding of the topic any better than others necessarily. Anyway, here are the premises of I was basing my statement on. Of course, this is vastly simplified: (Alot of them from Milliken's Race Car Vehicle Dynamics book)

For making the following less complicated, I am assuming straight line driving.

1) Vehicle tires generate more grip as more force is exerted on them, up until the ultimate limit of grip the tires are capable of with respect to the road surface.

2) For driving over typical roads with undulations, dips, and potholes, etc., the car with the best grip is the car whose tires can track the road surface irregularities the best (i.e.: tire is following the surface of the road 95% of the time vs. 80% of the time with an overly soft spring/damper setup).

3) The car with the best grip is the car i) whose sprung mass (which is invariant), is the least upset by the suspension/tire combination heaving the car up and down over bumps, and ii) whose tires can track the road surface irregularities better (or rather, a higher percentage of the time). Both conditions i) and ii) are necessary to generate the best grip the tires can deliver, assuming everything else is unchanged.

An overly soft spring/damper combo or an overly hard spring/damper combo will both have drawbacks on grip and handling.

a) In the case of the car with the overly soft spring/damper combo, the sprung mass stays relatively undisturbed, especially over high frequency bumps in the road. This is good for helping keep the weight of the car on the tires as consistent as possible. However, if we look at the unsprung mass of the tire and suspension, we have an issue where the tires are not able to follow the contours of the road very well, and hence diminishing grip at the tires. Part of the reason for this is because the spring and damper combo allows the tires to ride easily over road irregularities during the compression stroke, but not necessarily during rebound, where the suspension cannot react fast enough (or with enough force from the soft springs) to allow the tires to ride on top of the road surface well enough to generate good levels of grip.

b) Now, consider the case of the car with the overly hard spring/damper combo. The combination of the hard springs and dampers will make the car feel bumpy (harsh on compression because of the spring rates might be too high for various road imperfections). When the spring rate is too high for the given road irregularity the tire is travelling over, this translates to a displacement of the sprung mass, heaving it up quickly, and hence, reducing the total effective sprung mass acting on that very tire. This is felt by a loss of control by the driver, possibly requiring steering adjustments to readjust the trajectory of the car, a loss of grip at the affected tires at that very instant, and unfavourable weight transfer induced by the road surface. The weight transfer is complex, and can be fore-aft, diagonally, or a combination of both. This unfavourable weight transfer upsets the balance of the car, the effectiveness of the tires to grip the road, and a higher possibility of the car to snap-spin during cornering. However, the unfavourable weight transfer induced the by hard springs, (which stems from the road irregularities), combined with the quick weight transfer means that the car will snap-spin much more easily during cornering, as compared to a car with less stiff springs/dampers.

Because of the increased inconsistency in vehicle grip and handling, and the higher tendency to snap-spin (especially in lower grip conditions), the car with the slightly lower spring rate/damper combo is probably better all around IMO.

Of course, the higher spring rate/damper combo makes the car more confidence-inspiring. The transient cornering response is increased, the car rolls less in turns (accompanied by less unfavourable camber change), and overall the car feels sportier to drive. From personal experience though, sometimes you don't necessarily need very stiff springs to make a car handle and grip well. In many cases, the driver will be able to feel the road alot more with the more stiffly-sprung car, which also gives the sensation of a higher speed as more vibration/bumps are upsetting the car at a higher rate, even when driving at the same speed as the same car with a softer spring/damper setup.

IMO, for the above reasons, the ideal case is a suspension system that has a variable spring rate and variable damping to anticipate the road surface ahead. This will help the tires track the road properly, while maintaining the highest possible level of grip on the road surface and translating little to no extra amplitude to the sprung mass, which tosses the body upwards and removes grip from the tire(s).

Sorry for the long post. This is my understanding of vehicle dynamics at a simplified level. We are all here to learn, but I hope this makes sense.

Thanks for the perspective, I will disagree with you on some of this.

Example: when exiting the banking at daytona (130-140mph) on soft springs feels a hell of a lot faster than when on stiff springs. The motion created is very difficult to deal with. When the car is stiffly sprung....you long for more speed.

Can you elaborate on your personal experience with making a softly sprung car handle and grip well?


Springs that are too hard or too soft cause the suspension to become ineffective because they fail to properly isolate the vehicle from the road. Vehicles that commonly experience suspension loads heavier than normal have heavy or hard springs with a spring rate close to the upper limit for that vehicle's weight. This allows the vehicle to perform properly under a heavy load when control is limited by the inertia of the load.


As a base line, there is some basic calculations that can be done (does not account for aero, tire grip, particular track...etc)

http://www.hypercoils.com/spring-rate-calculator/


I am not that familiar with the mkv as mine is stock. When I have tracked the car I think it is way to soft, when I see the pics it confirms that the body roll and dive is excessive (and mine is on street tires)

But my points above are trying to point out that if TRACK PERFORMANCE is the goal, the spring rates being discussed have got to be very low. I don't know anybody that runs an mkv in NASA/SCCA so I dont know where the set up tends to be. All the Hondas I am farmiliar with have a weight of 1800-2300 lbs and tend to run at a spring rate of 650-1200lbs.

For what it's worth, I also think the mkv needs much more bar and spring rate in the rear to get out the excessive understeer, you cant rotate the car with trail braking/left foot braking as the electronic nannies can't even be turned off. :mad0259:

 

[Bunnspeed]

Lots of words ITT. :bonk:

Great info though. +1 on the need to talk about sway bars when discussing spring rates. I like how you can use sway bars to compensate somewhat for lower-than-optimal track spring rates when you need to maintain a degree of compliance for street use or choppy track surfaces.

For example, you can maintain a desired ride quality/ level of compliance while still improving track performance by either: tuning around soft springs with thicker sways (for grip with a better ride but worse transitions) or stiffer springs with comparatively softer sways (for snappier transitions as compared to the stiff sways/ soft springs setup).

There are so many variables as to what constitutes a fast setup, and so much depends on the specific track you plan on running at. A smooth autox track with a ton of rapid transitions will likely require/ benefit from a setup with very stiff springs and sways. A choppy track with lots of straight line acceleration would need softer springs to allow the car to hook up over bumps under WOT in a straight line, and not be thrown off-line in bumpy corners. Tire choices and pavement type and weather are also major factors to consider in any suspension setup discussion. Let's not forget to think about how different cars change tire angles as suspension compresses. Some cars gain negative camber as suspension compresses, which can be beneficial, whereas other cars tend to exhibit bump steer or positive camber with suspension compression...you'd probably be happier with softer springs on a car that maintains a good contact patch, whereas you'd want to avoid much suspension movement on a car with poorly designed suspension geometry.

 

[the bruce]

Miami, no doubt - from a track perspective Boston's springs rates are on the soft side.

I understood them as a worthwhile compromise between track and DD. That's it for me
with ~ 300 lbs on a GTI Mk.V.

Keep in mind when running 500 lbs and above the car needs a roll cage for stiffness.
Most of us don't have any.
To me it doesn't make any sense running 800 lbs without it and the bodywork doing
more spring work than the suspension itself.

 

[Dynjo]

Spring rate is always a compromise. There is the obvious "body motion control vs. ride comfort" tradeoff on double-duty cars, but even pure race cars must strike a compromise sometimes.

An example might be the needs of aero devices. Splitters work in only a narrow range of ride heights. I don't know for a fact, but I'd bet that a car with a full splitter benefits from super-stiff front springs to control ride height. If the car did not have the splitter, it might be better off with something softer. So, in this example, the tradeoff is between aero grip and mechanical grip.

Another example is the old ground-effects F1 cars. I recall reading that the springs were so ridiculously stiff that the cars were almost undriveable on all but the smoothest tracks.

 

[the bruce]

Agree. But it's WAYYYYY easier to find a compromise for one single track.

On a race car the faster setup is better. And if there are two similar fast ones, the one
the driver feels more confident with is better (to perform more consistent lap times).

We all agree on a dual purpose vehicle we have to find a compromise. But which one?
The one that is fast enough? The one I only just can stand on barely reasonable roads?
What are my priorities? Track? DD? 50/50?

There is no objectivity. It's an individual decision. Period.



ps:
That's why I like adjustable dampers and sway bar. Still a compromise but helps
spreading the car between road & track.


pps:
Does anyone have experiences with these?

http://www.stockcarracing.com/techarticles/39758_what_are_spring_rubbers/

http://premium.nascar.com/2001/NEWS/08/09/stockcarracing/index.html

 

[miamirice]

Miami, no doubt - from a track perspective Boston's springs rates are on the soft side.

I understood them as a worthwhile compromise between track and DD. That's it for me
with ~ 300 lbs on a GTI Mk.V.

Keep in mind when running 500 lbs and above the car needs a roll cage for stiffness.
Most of us don't have any.
To me it doesn't make any sense running 800 lbs without it and the bodywork doing
more spring work than the suspension itself.

I assume your 500lbs is referring to the mkv chassis?

The s2000 with 255f/275r toyo R888 was way under sprung at that rate. The car does nicely at 700lbs rate and it's a convertible.

 

[telaio]

Keep in mind when running 500 lbs and above the car needs a roll cage for stiffness.

That makes sense. I guess Unibraces can help a bit as one can already feel them on the street with street tires.

 

[the bruce]

I assume your 500lbs is referring to the mkv chassis?

The s2000 with 255f/275r toyo R888 was way under sprung at that rate. The car does
nicely at 700lbs rate and it's a convertible.

Yes. I even feel the Mk.V chassis stiffness to be at it's limit with ~ 300 lbs and H&R bars.
I can hear some noises I don't like.

Sadly I don't know the S2000 that much. I just have read a suspension book with some
examples on a S2000. From what I remember the S2000 needs rel. high spring rates as
the suspension arm motion ratio is near 0.5. Can you confirm this?

Yes, I know the MK.V rear is also in the 0.6 area while the front being at almost 1. That's
the main reason why some like higher spring rates on rear on a GTI.

 

[daigo]

Well, it is said that Mkv chassis are lazer welded body not like the old spot welded one, so I though it is more tough... Currently using over 500lb spring for both front and rear, now I got scared lol.

 

[the bruce]

Indeed it is much stiffer than the Mk.IV, but there are still limits. Of course not fixed ones.

I know some mates running close to 500 lbs (KW Clubsports, H&R Clubsports RSA), some with,
some without a cage. Again, sway bars play a role. Neuspeed offers some reinforment braces
for the rear subframe. Does anybody have them fitted? If they make any difference? Sadly
they discontinued the trunk mounted reinforcement bars.

 

[GodSquadMandrake]

Well, it is said that Mkv chassis are lazer welded body not like the old spot welded one, so I though it is more tough... Currently using over 500lb spring for both front and rear, now I got scared lol.

They do say that, but it's still spot welded. Honestly the chassis does feel much stiffer, the MKV is remarkably strong, but maybe by laser welded they mean laser spot welded? I have never seen any evidence if seam or even stitch welding anywhere on the unibody.

 

[Dynjo]

From what I remember the S2000 needs rel. high spring rates as
the suspension arm motion ratio is near 0.5. Can you confirm this?

Yes, I know the MK.V rear is also in the 0.6 area while the front being at almost 1. That's
the main reason why some like higher spring rates on rear on a GTI.

Good point. What matters is the wheel rate, not the spring rate. The S2000 has SLA suspensions front and rear, so the motion ratios are most likely different.

 

[miamirice]

Good point. What matters is the wheel rate, not the spring rate. The S2000 has SLA suspensions front and rear, so the motion ratios are most likely different.

Yes but wheel rate is factored when selecting spring rate.