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 Post subject: 308 suspension setup
PostPosted: Sat Mar 02, 2019 8:21 am 
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New thread to talk about setting 308 suspension up properly....whenever that means.

I have spent the past week reading the internet and found lots of people claiming to know the secrets and that setup isn't black magic.....only to find the advice filled with "magic numbers" and I've litterally found NOTHING about sizing swaybars beyond a bigger bar will add addition roll resistance...yes thank you so much for that.

So thread about 308 setup. Please add any knowledge you have person experience is best but absolutely share the setup from anyway you spoke to.

My current setup is 450/400 springs, factory .0700(i think it is) front bar, no rear bar. caster/camber/toe all set to factory. Honestly it feels soft to me and rear grip is not all I think it could be.


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 Post subject: Re: 308 suspension setup
PostPosted: Mon Mar 04, 2019 8:28 am 
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mk e wrote:
New thread to talk about setting 308 suspension up properly....whenever that means.

I have spent the past week reading the internet and found lots of people claiming to know the secrets and that setup isn't black magic.....only to find the advice filled with "magic numbers" and I've litterally found NOTHING about sizing swaybars beyond a bigger bar will add addition roll resistance...yes thank you so much for that.

So thread about 308 setup. Please add any knowledge you have person experience is best but absolutely share the setup from anyway you spoke to.

My current setup is 450/400 springs, factory .0700(i think it is) front bar, no rear bar. caster/camber/toe all set to factory. Honestly it feels soft to me and rear grip is not all I think it could be.


Okay, I am watching, this should get interesting.

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 Post subject: Re: 308 suspension setup
PostPosted: Sun Mar 24, 2019 4:35 pm 
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Posts: 216
I've said for a while that any discussion of handling that doesn't begin and end with tires is a waste of time and breath.

So... tires are non-linear.
A graph of grip (or lateral force capacity; y axis; dependent variable) vs. load (x axis; independent variable) is concave DOWN.
That means that each additional pound of load results in less additional grip than the prior pound.
Thus the tire that is operating higher on the load/grip graph will have less grip available than a tire operating lower on that graph.

We can substitute contact pressure for load and have a metric that varies predictably across tire widths.

Thus the end of the car with the greater contact pressure will slide first in a corner.
The closer the contact pressure is to equal between front and rear, the more neutral the car will be.

There are three steps to equalizing contact pressure: Static, roll moment, spring rate.

Static:
First, the contact patch size should be in proportion to the weight distribution.
For a Fiero with a weight distribution of ~45/55, the contact patches should have the same ratio. Since 45/55 = 0.82, the front tire should be 82% as wide as the rear tire.
This ensures that as the car is sitting in the garage, you can push it at the CG and it will slide straight sideways without turning.

Roll Moment:
Second, as the car rolls, the contact pressure on the outboard tire at each end should grow at the same rate in order to stay constant front to rear.
If you cut a car along a series of transverse vertical planes, calculate the CG of each slice and calculate a best-fit line for all the CGs, the line will be called the "Centroid Axis".
The centroid axis is higher at the front of a front-engine car and higher at the rear of a mid- or rear-engine car.

For this discussion, I'll call the transverse vertical planes through the axle centerlines the "axle planes".
The suspension geometry results in a roll center about which the body rotates as the car rolls into a corner. The distance from the roll center to the centroid axis in the axle plane times the mass of that end of the car is the "Roll Moment" for that end. The line drawn through the two instantaneous roll centers is the roll axis.
If the suspension geometry is well designed (not a good @$$umption on a 308?) then the roll center doesn't move much; we'll @$$ume this ideal for the rest of the discussion. We'll also assume that the car in question is mid- or rear-engine and thus has the centroid axis higher in the rear.
If the roll center is at ground level at both ends of the car, then the rear mass is acting on a greater moment arm than the front mass. Because of this, the rear mass develops a greater roll moment per pound. Because of this, the contact pressure on the rear tire grows more quickly with lateral G than the contact pressure on the front tire and the rear tire will slide first.
If the roll center is the same distance from the centroid axis front to rear, then the roll moment per pound will be the same and the contact pressure will grow at the same rate front to rear. The condition of having front and rear roll centers the same distance from the centroid axis can be expressed as having the roll axis parallel to the centroid axis.

Spring Rate:
The foregoing discussion @$$umes that the car is ideally sprung such that contact pressure at each end of the car increases at the same rate with body roll. With similar track widths front-to rear, we can say that if the wheel rate is proportional to the vehicle's weight distribution, then front to rear contact pressures will stay balanced as the car rolls around the ideal roll axis.

All that being said, since the 308's suspension pickups are mounted to structural steel tubing, dialing in the geometry to achieve the aforementioned effects shouldn't be too hard.


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 Post subject: Re: 308 suspension setup
PostPosted: Sun Mar 24, 2019 9:01 pm 
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No question tires are the the most important item...70-80% probably?

I'm not sure the sizing formula is as simple as the the static weight distribution since the goal is specifically to be as nonstatic as possible :) When people are talking midengine they are usually looking at low polar ineria and sor this discussion more balanced tire loading under hard braking.....which of course leads to less balanced loading under acceleration but braking and turn-in usually matters more in a race if not maybe in ultimate lap times. Not sure.


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 Post subject: Re: 308 suspension setup
PostPosted: Sun Mar 24, 2019 9:06 pm 
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Static contact pressure is just step 1 :P

What I described would be the analytical starting point for tuning. Maybe the driver needs to be able to brake harder at turn X or get on the gas sooner out of turn Y or has understeer in turn Z but no where else... which then informs the development process.

You buy the track time, I'll help you tune! :D


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 Post subject: Re: 308 suspension setup
PostPosted: Mon Mar 25, 2019 7:47 am 
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I don't think it is setup 1 though is my point. Front tire size is about the needed contact patch under braking, rear is under acceleration and the rest of the suspension tuning deals with the mismatches in the after affects.

Also tire pressure controls the contract patch size, the tire size is more the shape of the contact patch. On paper patch shape is irrelevant but in practice it matters a lot....tall skinny tires are much better in the at accel/braking direction and tend to be better dealing with water, short wide tires are better for turning. Standard OEM tires on plain jane cars and trucks produce patches that ware pretty uniform front/back and side/side in an effort to balance performance.

With the 308 its about what's even possible....205/225 seems to be about the right front tire, but all out track use maybe a 245 depending. In back...285/295 is what fits. The 288GTO with its flare was only 225/255....I think today they'd go bigger in back as the 488 is 245/305 with a 41/59 weight split...I think the 308 is a little more rear biased but I'm not certain, I'll need to weight the car. Anyway, I think we can borrow all the track time ferrari bought and be pretty sure somewhere right in there is probably the sweet spot.


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 Post subject: Re: 308 suspension setup
PostPosted: Mon Mar 25, 2019 11:43 am 
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Cars turn too, Mark! :lol:

These will help your rear grip: https://www.tirerack.com/tires/TireSear ... iameter=15

I'm in the middle of mods, but I have 285/30-18's on the back of the Fiero now, with 245/40-17's ready to go on the front after I change out the spindles to upsize the bolt circle.
Goal is 315/30-18's rear with 255/40-17's front, according to static weight distribution. Rear heavy cars automatically get pretty good weight distribution for braking.

In general, I've had very good success with matching tires to static weight distribution. It works wonders for stock Fiero handling. I had a Pontiac 6000 SE AWD as well. Using Grand Prix wheels front and identical but narrower Bonneville wheels rear, I had 245/50-16's front and 205/60-16's rear. Car handled impressively well for what it was, but a turd can only get so shiny.

If you're worried about whether you have a long contact patch or a wide contact patch, I think you're well down into the <1% effects regime... How long since you installed your driver mod?


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 Post subject: Re: 308 suspension setup
PostPosted: Mon Mar 25, 2019 1:06 pm 
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TheDarkSideOfWill wrote:
Cars turn too, Mark! :lol:

These will help your rear grip: https://www.tirerack.com/tires/TireSear ... iameter=15

But sadly won't allow the brakes to work....or the fenders :o

I'm not saying you're math isn't maybe a good starting point but for sure its not the way ferrari decides what tire sizes to run....and there cars turn pretty well ;)

The contact patch size is determined by tire pressure and weight on the tire...if the tire is at 30 psi and there are 300lbs on the tire, the contact patch is 10sqin, it can't be anything else so air pressure is a pretty critical tuning factor.

Tire size determines the shape of the contact patch and is really important. A little antidote is I bought a little suzuki swift and at the last moment decided to go to the GT instead of base model. Some months later my sister bought a base model. Her car drove good, my car drove better on sunny days but was nearly undrivable in the rain, 55 on the highway was white knuckle driving fighting the hydroplaning...the difference was 165/65R14 tires on my car vs 155/70R13. I couldn't believe the difference that little change made....no doubt tread pattern came into the game too with the GT tires a sportier design vs standard and DEEPER all season tread on the base model and running the GT tires under inflated in the rain did help quite a bit but best dry handling was like 45/25 f/r psi at the autocross, which to your original point would match up the contact patch sizes pretty well.

...but I don't think its as simple as just matching section width to weight distribution....but I'm also not saying its a problem of any kind.


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 Post subject: Re: 308 suspension setup
PostPosted: Tue Apr 02, 2019 1:37 pm 
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mk e wrote:
running the GT tires under inflated in the rain did help quite a bit

I really hope it's over or my world is about to shatter to pieces.

I'd like to learn more about the impact of the contact patch shape on the dynamics on the car. Seems to me wider tires (so wider contact patch, although the surface will be equivalent) offer more longitudinal and lateral grip and that's sounds a bit surprising.

Maybe with the skinny tires you get the same contact patch but the load is less evenly distributed so the coefficient of friction (I'm sure I don't have the right to call it this but I stopped physics in high school) is lower than on the bigger less deformable tire?


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 Post subject: Re: 308 suspension setup
PostPosted: Tue Apr 02, 2019 3:11 pm 
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mk e wrote:
no doubt tread pattern came into the game too with the GT tires a sportier design vs standard and DEEPER all season tread on the base model and running the GT tires under inflated in the rain did help quite a bit but best dry handling was like 45/25 f/r psi at the autocross, which to your original point would match up the contact patch sizes pretty well.



The Swift is a tiny vehicle with low contact pressure to begin with. The fact that underinflating the tires (further reducing contact pressure) made it better in the rain says that was a problem with the specific make/model of tire you were using rather than a problem with the tire sizing. Of course you *should* have seen the opposite effect: increasing the tire pressure made it more resistant to hydroplaning. Tire carcass construction can have some really weird effects on handling. GM engineers went wider in front on the Grand Prix GXP with LS4 V8 engine. They were able to dramatically reduce torque steer by playing with the belting angles in the tires they specified for it. They had whichever tire company produce those tires specifically for the car. I'm not sure if the industry as a whole learned that lesson or not.

Rear heavy cars have very benign hydroplaning behavior, especially if they're running square or close to square tire fitment, due to having greater contact pressure over the rear tires. The front tires hydroplane and you feel this in the steering wheel. The rears stay on the pavement. Lift off the gas to slow down a smidge and the fronts touch back down. Easy peasy.

A front heavy car hydroplanes on the rear first, so when the driver tries to slow down, the dynamic instability of braking only with the front tires causes the car to try to swap ends.


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