dampers (shocks to you non acoustic types) and autocross

I don't get it.

I mean, I do. Digressive versus regressive curves, damping ratios, angularity and resulting efficiency, motion ratio, I get all that.

How does that equate to fast autocross times?

If I take a well sorted random autocross car and toss the existing dampers in the toolbox, what will happen if I make the following four runs? 1 with Penske racing dampers adjusted by Penske engineers, 2, Some good Afcos or something, adjustable by me at the track, 3, Out of the box bilsteins, 4, Monroe $17 specials.

Why?

I remember that the mag had an article where they filled the body with motorcycle fork oil and did a dyno on the results, but I don't remember a times result on a track, I'll have to dig that up.

I was sure I understood this better than I did, but then as part of the truck restoration I was searching through catalogs looking for a damper to fit the back, but could only find one for another application. I called them (small aftermarket company) and they said it would work fine. I asked what the difference in damping would be and they said there was none. I inquired further, apparently they aim to be roughly the same damping curve as a Bilstein, which implied (and I confirmed) that they all had the same damping, whether for the rear of a light pickup or the front of a Cadillac. I was amazed, as I always thought that the damping was something tuned by the OEM front to rear and mimicked by replacement aftermarket places, so I am really out on this one.

Shocking, eh?

Help

As a non-engineer here's my $0.02.

Any low-end "street" shock is going to be engineered as much for comfort as anything else. Even lower-end "sport" shocks often offer way too much dampening in jounce and way too little in rebound in an effort to feel "sporty".

On the opposite end of the spectrum, true high-dollar racing shocks are focused on two things: keeping the contact patch...contacted, and allowing the ability to fine tune the suspension for a variety of courses/conditions.

Rather than make(any more of) a fool of myself, I'll allow the MEs to wow us with pie-charts and PowerPoint presentations filled with all the minutia the rest of us cannot grasp.

petegossett wrote: Rather than make(any more of) a fool of myself, I'll allow the MEs to wow us with pie-charts and PowerPoint presentations filled with all the minutia the rest of us cannot grasp.

pssst, I'm a mechanical engineer - I still am not sure I get it.

Well the Penske shocks will have four way adjustments allowing for different dampening in high speed and low speed in bounce and rebound allowing you to tune better as ideally you do not want the same high speed and low speed damping ratios in bounce or rebound. Also the dampers will be tuned by the OEM for different applications and I would be hugely surprised if the rear of a light pickup and the front of a Caddy are the same because it would result in very poor ride. Also the dampening ratios may be very similar between similar types of shocks (ie comfort versus an off the shelf performance shock) but the damping coefficient will be different between shocks for different vehicles because your critical damping coefficient will change based on the weight of the vehicle.

I am not an engineer of any sort. Sadly, all I got from this excellent write-up was that my own shocks are 'crap'. Perhaps you might learn more.

Shock discussion

Take your shocks off some time and try driving or autocrossing it. You'll soon understand.

Bouncing tires off the pavement isn't a way to drive fast. Airborne tires have no traction. That's one thing that shocks do, help keep tires on the ground.

A pitching and wallowing car is darn scary and uncomfortable feeling, causing you the driver to slow down. It also wreaks havoc with the alignment and tire angles, slowing you down some more.

Millikin has some interesting charts that don't often get referenced in Race car vehicle dynamics about the relative INSENSATIVITY of lap times to critical damping of sprung mass. It was a sort of Get it in the ball park and your difference between $1400 penskes and $140 Koni's will be measured in tenths and hundredths not seconds.

Glancing through the page linked by Bluedroptop seems to give a good rundown of the types of shocks and advantages of each. The reason nicer shocks are better is ultimately a tire will produce a given lateral/longitudinal force for a corresponding verticle load. This curve for most tires has a fairly flat peak grip range however it is a non-linear curve that always reduces grip as normal force increases. This non-linearity results in lost time through inability to maintain peak cornering if the normal force on the tire cannot be maintained in a smooth fashion. Spikes in normal load will result in sudden loss of grip (Drive a car that hits it's bumpstops during mid corner you suddenly loose grip on that corner, this is why). The job of the shock is to reduce these spikes as much as possible. The better a shock is the better it will keep the tire close to peak grip when suspension movement needs to occur.

nocones wrote: Millikin has some interesting charts that don't often get referenced in Race car vehicle dynamics about the relative INSENSATIVITY of lap times to critical damping of sprung mass. It was a sort of Get it in the ball park and your difference between $1400 penskes and $140 Koni's will be measured in tenths and hundredths not seconds.

This is exactly what I was hoping to hear. I can't understand how a Penske whatever could be that many times better than a Bilstein of Koni. I get they may be more controlled, unit to unit, and may be more tunable, and have advantages when F1 cars are involved, they may even be more consistent over time/heat/age. But an XJ won the autocross competition last time, ya know? It can't be that critical.

I'll read that link next.

They are definitely better bar none and if you are competing for the win in a class that has the money in it that the fast cars have the fast shocks tuned right with the alien drivers you quite simply will not beat them with lesser shocks. However I feel the "benefit" has been exaggerated and quite a few people are running around with $2500 wrapped up in shocks that they "need" when they could drive as fast as THEY are with $$$ shocks with $ shocks.

Sometimes it's not about the outright measurable speed, but there's also something to be said with how "easy" the shock makes it for you.

Yeah, i might not go any faster with the $$$ shocks than i did with the $ shocks, but it's going to be easier to go that fast, less work, and something i can do more consistently.

Well, in theory, that is.

I feel like people spend too much money on shocks for autocross. And the street.

But if i were about to do endurance racing on a slightly beat up track? Yep. Give me the expensive E36 M3, thanks.

Its all about the camber curve. If your suspension keeps more of the tire in contact with the road, it will grip more. Better shocks allow you to compensate for track conditions.

The pinnacle of weird shocks: High end stock cars use front shocks with very high rebound damping. Brake into the corner, the nose of the car sits down on the bumpstops, and the shocks hold it down there until after the race, when the tech guy needs to see 4" of ground clearance. If there were not rules stating the front suspension must consist of an upper and a lower control arm, I'm confident super late models would have a beam axle up front, and if the ride height rules left, they would have Kart suspension.

With a large enough damping coefficient, you can "fake" having a higher spring or bar rate through a transient maneuver.

Autocross is composed of transient maneuvers.

Most cars need a hell of a lot more rate in ride and roll than the manufacturer provides to be any good at autocross. They also need to have a lower center of gravity.

My CRX, on stock springs, is only competent with the Koni Yellows turned almost all the way up on rebound damping - which makes each corner pretty much unable to rise up once it's been pushed down in a corner.

In many classes, the SCCA won't let you spend $500 on springs, so you have to spend $5000 on dampers.

Didn't Colin Chapman say something about not offering adjustability in his cars since most people will adjust it wrong?

The winning XJ reminds me why I'm constantly haunted by the idea that most of the time most of us who are not actual race engineers do as much harm as good as we tweak our vehicles.

That vehicle had substantial compromises that should have made it slower than a lot of other vehicles as a baseline: It was tall and heavy. If it wasn't nose-heavy, that's only because it was carrying a bunch of weight up high in the rear.

If by getting a few things like springs, dampers, and anti-roll bars "more not wrong" than a bunch of other competitors who were in what should have been seriously better platforms, it makes me wonder just how wrong we normally get things with the usual bolt-ons...

One other thought on high-end dampers: I don't know, but I suspect that some of the features like the ability to damp accurately and meaningfully over very small motions and so forth aren't meaningful until you've removed all the compliant bits from your suspension (i.e. you've only got rose/heim joints for pivots and especially damper mounts). Just seems like that Nth degree of control is somewhat meaningless when you've got measurable undamped motion in your bushings...

This was my logic for not going for the adjustable Vmaxxes from FM... I figured if the regular ones were good enough for the original Targa, they're already adjusted "right enough" for me :)

The winning XJ isn't what i could call "tall," nor "heavy."

In reply to Swank Force One:

Well, fair enough. I need to go look back at the info on it, but all the things done to that vehicle were fair game for vehicles that started less tall and heavy... My point is that it started with a handicap relative to... a lot of stuff.

I may be missing something important about the XJ, though, so pardon my ignorance if it was a papier mache XJ body over a tube chassis or something

ransom wrote: In reply to Swank Force One: Well, fair enough. I need to go look back at the info on it, but all the things done to that vehicle were fair game for vehicles that started less tall and heavy... My point is that it started with a handicap relative to... a lot of stuff. I may be missing something important about the XJ, though, so pardon my ignorance if it was a papier mache XJ body over a tube chassis or something

No, it was legitimately an XJ through and through, but it started life as a 2wd XJ, which meant it probably didn't start any heavier than almost half of the Challenge cars anyways.

And it was quite low. A bit jarring to the senses when compared to my daily driver XJ 4x4 Sport.

It really just ended up as a low-ish and light-ish car with an antiquated suspension with a metric crapload of rubber. (Much smaller than stock diameter rubber, at that.)

Myself, i just subscribe to the "Stiffer is better" and "more rubber the better" schools. Right up until it makes it too hard to drive, then i dial it back a notch.

Swank Force One wrote: Myself, i just subscribe to the "Stiffer is better" and "more rubber the better" schools.

Those are very good rules to live by!

The CJ was a 2600 lb vehicle on 275 Hoosiers. Yeah, it was massively stiffer and lower than a stock XJ (and hey, you can lower the snot out of a solid axle car without affecting roll center!), but just saying 2600 lb car on 275 hoosiers puts you in some crazy company like really serious S2000s and FD rx7s, for example. Almost any such vehicle will turn hard, even if it does it while listing 20 degrees and riding the bumpstops around turns with stock suspension (which that jeep didnt and wasnt).

Since im a cheap bastard towards my cheap cars which have no aftermarket, i've figured out a few ways to swap in dampers from other vehicles and i've always been a little curious about what 'should' happen when you swap the same damper between vehicles of very different weights.

For example, im currently quite tempted to try some $80 Koni Strts for the REAR of a 2400 lb neon, on the FRONT of a 3200lb caravan..

I'm just saying that the performance of the Jeep wasn't all that surprising once you stepped back and took a look at it.

An S2000 or an FD with the same rubber would have destroyed it, no doubt, but that's more a testament of the cars that show up to the Challenge rather than the Jeep being surprising.

Anyways... mndsm's Challenge MX6 is getting Koni Yellows from a MKii Supra up front, so...

And actually... my MX6 doesn't run 1st gen MX6 struts or springs.

Yep, and i've got a bunch of mixed and matched stuff too, and neither one of us probably has anything to say about any of it that isnt based off the butt dyno or a tape measure.

OP would be much better served reading the entire website linked here by bludroptop (which i just did.. http://farnorthracing.com/autocross_secrets6.html) than by listening to anything i could say about my damper-mixing. My primary motivation has simply been to lower the car and keep 'enough' compression travel, with absolutely no real data on how it affected any car under track conditions.