30 replies to this topic

[Andy69]

I ran across this thread on a Ford Contour board.  Not really the same car as a CRX but I think the same conversation applies here.

 

http://contour.org/c...e-setup-myths_=

 

Basically, he's challenging the idea that to reduce understeer, you reduce front or increase rear roll stiffness.  According to him, that only applies if your fronts have the optimal amount of roll stiffness, so you need to get that right before anything else.  If you try to balance the car without the optimal amount of front stiffness, you will be slower.  His argument makes sense in light of my experience.

 

But the question is, how do you determine what the optimal front roll stiffness is?

 

I recently upped the front bars from 23.5 to 27, and the rear from 350 to 500.  I also added a spoiler.  Observers and photos of my last event confirm little to no inside rear wheel lift.  It's noticeably faster around the course. 

 

I had the front shocks set to 1 and the rears to 5 (Tokicos).  I upped the fronts to 2, making them stiffer, and immediately dropped more time, getting faster to the point where I was having trouble navigating a tight slalom due to the slow steering and not the car's handling.  I didn't try bumping it to 3 or higher since I ran out of runs.  The car gripped better in the front but I did not increase oversteer.  The car actually oversteered LESS with more front roll stiffness as I was able to get around the course at a speed that was higher than one where I had previously spun.

 

I had done this last year too, with the weaker springs, and found on our local course I could up the front to 4, but when I got to Blytheville the car was very tail happy until I dropped the front shocks down a couple notches.  Perhaps due to faster speeds, or a different surface.  Not sure.

 

My take is, the rear is just along for the ride, and the front stiffness will determine how much rear stiffness you need - increasing front roll stiffness will increase front traction, and the rear must be increased along with it or the car will oversteer. 

 

But, only to a point.  I would imagine there is a point where adding front roll stiffness would actually reduce traction.  My question is, what is that point and how do you determine it?  Trial and error?

[rufusbob]

Too much front roll stiffness is when you get more understeer than desired for your chassis / suspension setup and track condition(s), and too little front roll stiffness is when you get too much oversteer......JMHO.   

[cbstdscott]

A very interesting question and an answer I can only guess at.

The first consideration is chassis stiffness. These cars are not very stiff so you would want to adress that.

The next consideration is keeping the tires in contact with the road surface. A suspension that is so stiff that it can not maintain the tires on the tarmac is not good. Rather than looking for the stiffest suspension, you want it stiff enough.

How does your car handle right now? Is it neutral, understeers, oversteers?

The front and rear are connected. What is your rear suspension setup? Have you locked the rear axle?

I do not believe in locking the rear axle. I know this is an emotional issue with many people but I have enjoyed excellent results by using an HFA style rear axle, located with a large external rear sway bar.

[Dirtcircle86]

I've tried for two years now to understand what Ive read about traction, weight transfer, spring rates, and roll stiffness.  I still don't. 

 

With my recent addition of traction, I do realize my car is ALL WRONG!  I am sure I have read this put more eloquently but with more traction, you need higher spring rates and more roll stiffness on that end, probably both ends.  My situation has gone extreme so it makes it easier to see and understand. 

 

Last year, the big track was super shinny rediculous slick and my ten year old polished tires (5.5" wide and durometer 75) would skate horribly.  I never stayed within any traciton circles...  Whether under acceleration or braking it would push to the point of all four locking up under heavy braking.  All four did stay on the ground and my RF wasn't bottoming out. 

 

Now, the track has new gritty clay and some "secret expensive chemicals" that makes it grip (it is a lot rougher though).  Car has 8" wide real tires that duro at 55.  Any braking into the corner causes the ass to step out.  Most call it snap oversteer but yes it was gone and I didn't expect it.  I can easily assume its due to having 5 times as much traction on the front than the rear, rear locks up now...  I can also stay high-mid throttle on exit and stay just under tire spin.  It's fast but im now bottoming out the RF and lifting the LR about 6". 

 

In my situation/traction level, I obviously need to increase both spring rate and roll stiffness on the front.  If I am lifting the LR, I am obviously transfering a lot of weight from LF to RF causing the bottoming out and unloading some traction from the LF.  Added spring rate will add some roll stiffness IIRC, but I may actually need an ARB re-installed.  Wish I had kept the CRX bar but I do have a TEG.

 

To stop rambling, I think the answer to perfect roll stiffness is traction related.  Unfortunately, this changes from track to track (and temp/humididty/price of beans) on asphalt  and minute by minute on dirt.  When I had no traction, I needed no stiffness.  With a little more traction, I need a LOT more stiffness.  I am apparently going to go against all dirt track conventional thinking and run a front sway bar. 

 

Traction related side question:  Do you prep/scuff tires before a race or just let the pavement do it for you?

 

EDIT: I re-read autoX-fil and I think my ramblings agree.  IF you have the traciton and rear roll stiffness to lift a rear tire, you need more front roll stiffness.  It may never end tho.  Yes, trial and error....

[cbstdscott]

Snap over steer can be driving technique.

It would be helpful if we had all the details of your build. Exactly, what is your current mods, engine size, wheel size, etc.?

[Dirtcircle86]

It was driver.  I didn't know what technique my new set-up needed. 

[cbstdscott]

Brake in a straight line. In slow, out fast.

[rufusbob]

Brake in a straight line. In slow, out fast.

Please forgive me if I'm missing something.....

 

.....but doesn't "in slow, out fast" apply better to cars with a rear weight bias (like the Porsche 911)?  The idea being to use power or braking to transfer weight to whichever end of the car needs more help developing cornering force (i.e. the heavier end of the car)?

 

Following this line of reasoning says a front heavy CRX will respond to being trail-braked deep into a corner...more like: "in fast, out slow".

 

Bob 

[cbstdscott]

The principles of physics applies to all cars.

[Andy69]

Please forgive me if I'm missing something.....

 

.....but doesn't "in slow, out fast" apply better to cars with a rear weight bias (like the Porsche 911)?  The idea being to use power or braking to transfer weight to whichever end of the car needs more help developing cornering force (i.e. the heavier end of the car)?

 

Following this line of reasoning says a front heavy CRX will respond to being trail-braked deep into a corner...more like: "in fast, out slow".

 

Bob 

 

It's 1000% better to go into a corner a hair too slow than a hair too fast, regardless of what car you are driving.  You can always add speed, but you can't undo a blown corner.

[Dirtcircle86]

Must be a perception thing. "Blown corner" wasn't in my thinking process. "Slow" in, feels that way, but could be quicker overall. "Fast" in feels quicker but may not be. "Oh shit" well.... Didn't cross my mind. But could also be an outcome.

[anjin]

I set up my crx with as much stiffness as I could get, which includes 31mm torsion bars and the wagon antiroll bar.  I then set the rear up with sufficient stiffness - 600 lb/in springs and a 14mm antiroll bar with adjustable arms, to get neutral steering. I then played with the struts - the tiens - to get the best behaviour at the lowest rate I could.

 I can adjust the tiens or the rear antiroll bar to dial in oversteer or understeer.  This is for a relatively high speed bitumin track.  On a slower track I'd have to alter the set-up. Autocross where you want the rear to rotate you'd do differently - but rotation in a 60mph corner is the last thing I want.

 

I followed your line of thought on this - maximise the front which does all the power transfer and almost all of the braking - and adjust the rear to match. Try it until you start going slower.

[Dirtcircle86]

Rotation in a 60 mph corner is exactly what I want...    It will make you hold you breath but it's way more fun than the straights.

[cbstdscott]

So sideways is the prefered attitude through a low traction oval corner? In a FWD car?

Isn't that like taking a knife to a gun fight?

[cbstdscott]

Get into the corner, tap the brakes to shift weight forward, get back on the gas, hope for the best.

There is no limit to front roll stiffness in this scenario.