Publishing my Autocross Secrets

[jtmcinder]

Without NF how do you know how close you are to critical damping?

Well, considering that the NF does not enter into the equation for calculating the damping factor, I'd say you can know exactly where you are with regard to critical damping without knowing the NF of either the end of the car or a single wheel.

With that said, I appreciate your bringing the issue of damping factors into the discussion. It's another place where the advice given by road-racers (including most texts) needs to be taken with some salt by autocrossers.

- Jtoby

 

[bjones18]

... I'd say you can know exactly where you are with regard to critical damping without knowing the NF of either the end of the car or a single wheel.
- Jtoby

You must know the mass & stiffness to calculate critical damping (NF=sqrt(k/m)). Whether it's the wheel end mass or the body mass you are trying to damp the motion of (SDOF).

http://www.efunda.com/formulae/vibrations/sdof_free_damped.cfm

 

[jtmcinder]

Thanks, but you're missing my point. You claimed that you have to calculate or know the NF to calculate the damping ratio. I noted that your claim was wrong. Yes, the information required would also allow you to calculate NF, but you don't have to.

- Jtoby

 

[bjones18]

I do not see where your claim that I am wrong is coming from... the efunda website listed above clearly states critical damping equal to 2*mass*NF.

You may be missing a point which is fundamental to this thread. Natural frequencies are fundamental metrics which are used to describe dynamic systems. If you have data on a Mustang, Talon, BMW, and a Civic and you are looking to compare between vehicles, the relationhip between stiffnesses and masses (NF) brings a more clear understanding than looking at wheel rates alone. As you have pointed out, NF is not the only factor. Load transfer (cg height, trackwidth, latac) and stabar stiffness (1-wheel vs 2-wheel bump rates) and many other variables must also be taken into account.

Experience is the only effective way to set up a car. Calculations, testing, and modeling are effective ways to extrapolate or interpolate on experience. The original post was claiming to reveal successfull autocross set-up secrets. I look forward to more specifics if the site remains "free." The information I saw has been published in many different forums & "how to" books.

 

[bjones18]

... increasing the toe-in will produce understeer, toe-out increase makes the car numb and unresponsive.

Rear toe-in produces understeer, which is generally thought of a numb and unresponsive. Toe-out in the rear is usually referred to as "lively" or in the extreme case dangerous.

Driver natural reaction to "oversteer" is to take your foot off the throttle or hit the brake. In many suspensions, throttle-on-toe-out is directly tied to throttle-off-toe-in. or visa-versa. Therefore throttle-off-toe-out is generally a no-no.

 

[jtmcinder]

There is more than one way to calculate the damping factor and some of them do not involve NF.

That web-site is cute, but I'm not going to pay for something I can already do. Note, also, that it seems to assume a perfectly linear force-velocity curve for dampers. No (decent) shocks act like that. I feed in about ten points from the rebound and compression curves and work from there.

- Jtoby

 

[Caithness]

In a similar vein, has anybody read the "Make it stick" articles in Sport Compact Car magazine? I thought they were pretty well done, starting with very basic stuff like tire choice and moving on to more technical stuff, with a similar but less thorough and less in-depth write-up on dampers in the last installment. It did, however, cover some of the basics like how to interpret a shock dyno plot and a more detailed description of the basic terms and how they affect handling.

 

[jtmcinder]

I have switched from SCC to Charmin. To quote Dr. Evil's son, it feels good, on the whole.

- Jtoby

 

[Caithness]

Hahaha, I knew I would get that sort of response. I don't know Kojima's credentials so I certainly can't say that he's writing truly great, definitive articles on the subject. In fact, just from comparing his latest installment to some of the stuff here and on DG's site I know he's not. I simply found them helpful as a collection of definitions of some of the basic terms and suspension geometries that confused me, with nifty diagrams and explained on the level of the complete ignoramus (my level, in this subject).

 

[eclipsh]

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Here are some well priced heim joints too. I was thinking the precision ones should be sturdy enough, but maybe going chrome moly.


What is everyone's opinion on going to solid, weld-in sub-frame mounts versus prothane? Pagosa DSM has been talking about making some weld-ins. Is it streetable after doing so or does it make the ride too harsh?


If this is getting too off-topic or thread-jacky I apologize

 

[jtmcinder]

Streetability involves (at least) two components: ride quality and durability. Both of these will suffer as you remove more and more of the rubber isolators. Are you ready to check every bolt on the car every 2000 miles? If not, then maybe heim joints and solid bushings aren't for you.

One company that has put a lot of effort into figuring out how to make a car solid while still being streetable is -- ready for it? -- Mazda.

- Jtoby

 

[eclipsh]

Every bolt or just the suspension bolts?

Why not just use some lock-tite in the right places? Just a thought but our flywheels take tons of abuse and vibration without the bolts backing out. So do our entire engines and transmissions for that matter. My gut reaction says you may be right, but do you have experience with solid subframes to back your claim?

 

[jtmcinder]

Lock-tite all you want, but you will still need to check them. I already spend two hours each April checking everything I can get to, especially after a winter I was stupid enough to do a rallycross or two.

- Jtoby

 

[Caithness]

Safety wire>Vibration.
http://www.whizwheels.com/Tips/safetywiring.html

PITA to do everything in the car though. But how much fun would it be to brag that every fastener in your car is safety wired?

 

[logic]

our flywheels take tons of abuse and vibration without the bolts backing out

There's at least a few members here who would disagree with that statement. Safety wire is your friend.

 

[eclipsh]

Pagosa DSM can't post in this thread but wants me to say that almost all his suspension is heim joints and he doesn't experience any harsher ride quality or decreased durability. He says it is because all his pieces were machined to very high tolerances and were press fit to avoid extra vibration. He also says he has no problem with bolts or nuts coming loose. All his nuts and bolts are factory and did not use thread lock.

Someone please give him some rep points for one of his posts so I don't have to speak for him

 

[DG-FNR]

Updated.

DG

 

[90AWDTalon]

Thanks for the update, I've got a couple of questions.

For those of us that don't have scales or access to scales. If you set your ride height to match left to right with the drivers weight, does this get you in the ballpark? For example with 240 pounds of weight in the drivers seat of my miata, I weigh 260, I set the fender to center of the wheel hub at 11.875 inches both sides front and 12 inches both sides rear. Are my corner weights in the ballpark, or impossible to tell?

Second on tires pressures. You set the front for maximum grip, and balance the car with rear pressures. Say you need to loosen the rear some, do you adjust the pressures up or down to gain the desired effect? Also I like the way you described how pressure affects grip. I remember at a novice school a few years ago being told to add pressure the end that looses grip first, on my talon this was the front. I did so and it made a huge differnece. I never knew why this worked, now I do.

 

[DG-FNR]

Updated, now with a suspension dynamics calculator.

Guess what the example car is?

DG

 

[slipstream808]

Back from the dead! And yes, I mean Dennis Grant, too!

 

[DG-FNR]

I'm not dead yet!

DG

 

[RipperXX]

Glad someone is keeping this thread going. encase no one knows though DSS is no more, and so as far as I know you can't get the Toe elem kits anymore. I think there was one vendor maybe on DSMTalk that was thinking about making some but duno if it ever happened.

 

[pagosa dsm]

I think Jay racing is making the toe eliminator kit.I made my own,I have some photos in my gallery.They might give some ideas.