I just re-read that. Did everybody catch that? Eight seconds? That's a huge improvement. Usually you're fighting for tenths here and there. That's almost unbelievable.
____________________________________
1986 944 NA - Guards Red - 78k miles
1984 911 Carrera Coupe - Guards Red (gone, but not forgotten)
"If you're not living on the edge, you're not living"

 

Funny. It's not the size - it's how you use them. Btw, I will be upgrading the brakes on my track car over summer.

 

I think I have seen that Evo flipping. I am not sure if it is THAT one but I have an outside video of a white evo doing a barrel roll in the air, it was insane.

 

uhhh....even with my street tires treadwear 300 my car oversteers....and its stock

 

"The gain in negative camber is counter the roll, right? So the end effect is a tire that is vertical and has the maximum contact patch on the pavement?"

Yup, that's correct. Regardless of what the body of the car's doing, you want to keep the tyre vertical and the contact patch flat. Due to the flexing of the tyre carcass, you actually want a little negative camber due to the squirming.

"Why would you put stiffer front springs on a car that already understeers? If you want a minor improvement, go with better swaybars with solid chassis mounts. Just to point out how awful the stock ones are, I could barely tell the difference between no swaybars at all and a 20/14 stock setup. That will improve your car's response much better than some rice springs."

There's a difference between "handling", which is subjective feel, and "performance", which is objective numbers. Understeering/oversteering effects are things that street-cars & drivers talk about a lot. It's a subjective "feel" quantity and it's often mistakenedly believed that a perfectly "neutral" car is fastest... Well, in an idealized lab with a single circular corner of constant-radius with the car driven by a robot, yes, neutral may give you the fastest cornering speed.

Understeering/oversteerng effects are only noticeable AFTER you've gone OVER the adhesion limits of your tyres, that is... you're sliding a bit. If you drive up to, but not over the adhesion limits of the tyres, you won't notice much in the way of understeering or oversteering. For example, if you take a cloverleaf on-ramp at 50mph and below the limits of the tires, you go around smoothly and everthing's fine right? But if you take it at 60mph, the front tyres will start sliding and understeer becomes every obvious as you're sliding ever closer and closer to the outside edge no matter what you do with the steering?

But on a racetrack with a race-car driven by racers, who are humans, things are much different. We don't do things perfectly and there's a variation lap after lap. The ultimate measurement here is the stopwatch, and teams that are meticulous with record-keeping and data-mining has figured out quite a few tricks.

First, a front-end that keeps the outside tyre vertical and contact flat has more grip than one that's got positive camber. So let's take the case of the car sliding on the cloverleaf on-ramp. By upgrading the front-end only so that there's less lean and flatter contact patch, you'll get more front-end grip, while the rear remains the same. Overall total grip goes up, even though ALL of the extra grip is on the front-end. However, the stiffer front end takes more of the lateral weight-transfer under cornering and the extra weight negates the increase in traction.. this is why there's more understeer even though you have more traction.. So now, you can take that on-ramp at 60mph without sliding like before... BUT... at 65mph, you'll end up sliding again... But this time because of the larger ratio of front-grip to rear-grip, the understeering will be worse... you'll slide off the corner instead of just edging towards the edge... However, it will require a higher speed to overcome the adhesion limit of the front tyres. So this is an examples of where objective improvements in "performance" numbers seems counterintuitive to subjective "handling" feel.

Another factor that a lot of teams have discovered is straightaway speeds are more important than cornering speeds. This is due to breaking down the lap-times to percentages of time spent in a straightaway vs. percentage of time in the corners. On most racetracks, like Willow Springs and Laguna Seca, even Sears Point, you spend more time going in a straight line than in the corners. So faster laptimes can be achieved if you go faster down the straights.

This invariably leads to a suspension-setup that causes lots of understeer when you go over the limits of adhesion, such as something with front spring-rates that's twice as stiff in front as in the rear for a 50/50 weight-distribution car. Check out how they set up the ALMS or IMSA-GTO cars. Why such a disparity in spring-rates? That's to give as much drive-traction in the rear as possible for maximum-speed down the straightaway. The line that's used through the corner is also modified away from the theoretical fastest-cornering curve with an apex at mid-way through the corner, into one with an late-apex. The effects of the suspension setup and cornering-line is that you have an understeering car that goes around the corner at only 98% of the speed of a perfectly neutral- car at the adhesion limits of its front-tyres, and this understeering pig will also take late-apex that's 10-30 ft later than the fastest line.

Why? Well, the understeering suspension with stiff front springs keeps the body flat, so that the rear tires press evenly on the track for maximum drive-traction. I'm sure you've all had the annoying situation where your inside tyre spins as you're laying on the gas. With a stiff front-end, when you're at the cornering limit of the front-tyres, the car will be flatter and you have less weight-trasnfer to the rear, and you may only be at 75% of the traction limits of the rear tyres review friction-circle). This allows you to get on the throttle much sooner than a perfectly neutral car. Thus the late-apex also starts the straightaway in the corner as well. So while the neutral car can only get on the gas after mid-way 50% through the corner and can only go 100% once it's fully drifted to the very outside with no cornering needed, the understeering car get on the throttle at 33% through the corner and can be at full throttle 50% mid-way through the corner. By sacrificing the cornering speed a bit through suspension-setup and the line taken, you can end up going 5-10mph faster down the straitaway. You lose 1-second in the corners to chop off 10-seconds on the straights.

A third factor in suspension set-up is the driver. Humans are not perfect, we draw a sine-wave curve on both sides of the theoretical maximum that the car can do. Sometimes we're below the limit, others, we're over the limit. Going over the adhesion limit of the tyres in an understeering car costs you a lot less in laptimes than in an oversteering car. With the understeering,if you start sliding, you can just unwind some steering to reduce the arc of the curve the car's carving and let off the gas a bit and you'll regain traction. In an oversteering car, it's a hairy situation as you can't let off the gas at all and steering corrections won't have as much of an effect because it's the rear tyres that's sliding, not the front. So you gotta hold it steady and let the slide scrub off speed, then get back on it again. Or you may not catch it at all and the car will spin, which costs you a tonne more lost time than a little understeer .

"I just re-read that. Did everybody catch that? Eight seconds? That's a huge improvement. Usually you're fighting for tenths here and there. That's almost unbelievable."

Well, it's not ALL in the suspension... I improved as a driver during all those upgrades to, so it's hard to exactly quantify how much of the improvements were from the suspension mods and how much was due to the dr iver. Overall, I've chopped off over 35-seconds from my lap-times at Willow Springs from the first time I was there. I guess I'll have to hop into a bone-stock 951 with SP8000 tyres and go around again to see what the difference would be. At least I can say that in a modifed-951, I'm faster than Derek Bell in a stock TurboS... heh, heh..

 

944cs I remember watching you drive at pocono raceway this spring and I never saw any oversteer. The car looked like it was sticking pretty well.

 

Hey Danno, on the topic of balanced handling, have you ever seen this chart?



It says that moving the weight distribution more rearward causes more oversteer / less understeer. This makes sense, considering more weight in the rear means that the rear tires have to handle more cornering force than the front, and so they will mostly likely brake loose first. However, isn't moving the weight distribution rearward similar to increasing the front spring rate / reducing the rear spring rate? With more weight in the rear, you will get a lot more force that causes body roll back there. Similarly, less weight up front means less body roll in the front. That sounds a lot like having a stiffer front and softer rear to me. However, according to that chart, stiffer front / softer rear results in more understeer / less oversteer. This is contrary to the original statement that moving the weight distribution rearward causes more oversteer / less understeer. What's up with that?

 

IMO, if you're interested in tuning your suspension, check out Herb Adam's book "Chassis Engineering."

Trust me, you'll feel better about what youre doing when you understand how every little part of your suspension affects it overall. It'll be much more useful than being force-fed advice by people who aren't quite sure themselves...

 

This was basically my question. Won't doing this upset the balance of the car?

I want to get rid of a little fender gap, and that's all. Nothing more. It's a band-aid for me until I have the money to build the suspension and drop it properly (with increased spring rates all the way around).

Is it possible to get rid of a little fender gap without sacrificing handling/balance? And without having to buy new torsion bars?

 

If you want to merely lower the car, get the Ground control coil over kit ($200) and use 150# springs. Seems like too much money to me for a ride height reduction though.

 

" put some weltmeister 1.5" drop/ 200lb springs-$100- and lower the rear eccentric bolts...
This was basically my question. Won't doing this upset the balance of the car?"

Nah... you'll end up with more cornering grip and will be able to go around corners faster. Just that once you're over the limit of adhesion, the car will understeer more than stock. You guys should see how much understeer is dialed into NASCAR and Indy cars...

" check out Herb Adam's book "Chassis Engineering.""

Oh yeah, forgot about him. He deals primarily with the hot-rod muscle-car market, but he's got some ideas that may appear radical to our cars. But they really are valid and some of those ideas can really benefit us. Along with Carrol Smith's series.

"Hey Danno, on the topic of balanced handling, have you ever seen this chart?"

Yup, those are general guidelines, and some of them reinforce and some seemingly counteract each others' effects. Keep a detailed log-book of your settings and don't get too focused and extreme on the damper settings. They really affect the transition phase in between straight-line to full steady-state cornering. Once you're at maximum cornering, the dampers won't affect it too much, unless you're hitting bumps. So it's actually possible to rig a car setting up the dampers such that you have oversteer on the turn-in to the corner, which is helpful in an autocross, yet once you're at the steady-state cornering limit, the car will actually understeer.

"It says that moving the weight distribution more rearward causes more oversteer / less understeer. This makes sense, considering more weight in the rear means that the rear tires have to handle more cornering force than the front, and so they will mostly likely brake loose first."

The actual mass of the car is a two-edged sword. It provides vertical loading on the tyres squeezing on the contact patch and gives you grip. Yet, this mass also has to be laterally accelerated into a corner. Problem is, even though the tyre-grip goes up with more vertical loading, it doesn't go up linearly. So by adding 10% more weight over the rear tyres, you may only increase their extra traction by 7%. Check out this chart:



This is why adding sacks of concrete to your back seat won't help you go faster. Sure, it'll give you more grip, but the added weight hurts you more than what you gain in grip.

"However, isn't moving the weight distribution rearward similar to increasing the front spring rate / reducing the rear spring rate?"

Nope, the stiffer spring makes that end of the car take MORE of the weight-transfer under cornering even though static weight-distribution remains the same. It's like making a clay door-**** and putting it 1" above the normal door-**** on the door. You push on both evenly with a flat hand to open the door and the majority of the force required to open the door goes through the stiffer solid doorknob, not the softer one.

"With more weight in the rear, you will get a lot more force that causes body roll back there. Similarly, less weight up front means less body roll in the front. That sounds a lot like having a stiffer front and softer rear to me. "

Note that spring-rates NEVER change. A 200lb/in spring is still exactly the same rate with a 100lb load on it or a 500lb load. Note that body-lean is the result of weight-transfer, it does not cause the body-lean. That's where you're getting confused. The weight-transfer under cornering is ONLY determined by the radius/curvature of the turn, the COG height over the roll-centers, and the track-width. Body roll is then only in RESPONSE to this weight-transfer. A 3000lb car will have an 800-lb transfer to the outside at 45mph going around a turn of XYZ-curvature regardless of what spring-rates are used. It's just that the stiffer springs will result in less body-roll for the SAME weight transfer. This results in a more vertical tyre and more total grip.

Next, let's go into dynamic lateral weight-transfer under cornering:

"However, according to that chart, stiffer front / softer rear results in more understeer / less oversteer. This is contrary to the original statement that moving the weight distribution rearward causes more oversteer / less understeer. What's up with that?"

No, you have to distinguish two factors here that's not the same and that chart lumps them together. There's:

1. static weight distribution F/R, this is determined by how the car's mass is apportioned over the wheels
2. dynamic lateral weight-transfer from cornering, this is weight that's additive to the car's mass on the outside tyres, and comes from the outside tyres pushing in on the car to make it go around a corner.


Let's say we have a car that's got 50/50 weight distribution and it goes in a straight line the following loading on the tyres:

_L__|__R_
100__100
100__100

And you go into a right-hand corner and the tyres apply and external force to the car to make it go around the corner. You've got two forces on the outside tyres, an external force from cornering and a force from weight transfering from the inside tyres to the outside. The resultant load may look like this:

_L__|__R_
150__90
150__90

You can assume that this force acts on the center of gravity of the car . Here's a picture of what that looks like:



Note that this cornering force has more to do with how hard the tyres are pushing in on the car and based upon the stiffness of the front vs. rear suspension, this weight-transfer can be split any number of ways, 60/40, 50/50 or 40/60, etc



So you've got TWO goals in optimizing suspension performance:

1. One is to maximize grip, this involves alignment and adjustments to keep the outside tyres as vertical as possible in all conditions to maximize the total amount of grip. That's why using stiffer suspension with only 1-degree of lean gives you more grip than stock with 5-degree of lean; the tyres stay flatter on the ground, even though you're going around the same corner at the same speed with the same lateral-weight transfer.

2. The 2nd goal is how to split the lateral weight-transfer under cornering between the front & rear so maximize that tyre grip. Review the friction-circle and you'll see that a tyre's total grip can be split between accelerating, cornering or braking forces, and any simultaneous combination of two (except for accelerating+braking at the same time).



So under cornering, stiffer front-end from springs and/or swaybars will take more of the lateral-weight trasnfer and results in understeer. Stiffer rear-end will take more of the weight and results in oversteer.

Keep those two concepts separate and adjust them separately:

1. maximizing tyre-grip by keeping tyres vertical under cornering and body-lean. This is done through using wider and stickier tyres and extracting the most from them by using alignment and spring-rates to keep them vertical.

2. spreading lateral weight-transfer under cornering between front & rear wheels in a way that loads both evenly. Depending upon the amount of power your car has, the course and percentage of times in the corners vs. straights, you may set up a car with a little oversteer, near neutral, or with a lot of understeer. Depending upon the car and course, these wildly different settings will end being the one that gives you the fastest laptimes.

 

Do remember that willow Springs is a bit ***** track. What I mean is that you need a large set tyo go fast. In many cases anything that gives the driver a bigger set makes him go faster.

Too much body roll can make one go slow because they feel uncomfortable.

Stiffen the car to make feel a little better and you can easily pick up seconds at that track.


On the flip side I saw a 944 spec driver go 4 seconds faster at willow by softening his sway bars. He found the car too stiff for comfort and by softening the bars gained more confidence and went faster.

 

It says in that chart that increasing tire pressure in the front and conversely lowring rear pressure gives more oversteer. Is that correct? Seems counter intuitive.

 

Just some personal experience here.

Firstly.
Too much understeer sucks. Period.

I have been driving my 83 944 at autocrosses. It has bone stock suspension with lots of body. It however is quite balanced and also one the fastest cars on street tires there.

Reason is balance. In 90% of the corners the car is sliding to some degree. As a driver I manage that sliding to effectivly make the car go faster than it other wise would. The reason is that most folks don't use all their grip. So if you drive it until you feel a little sliding and slow down. They you will be slow. Drive the car to manage the sliding and you will in fact me much faster. Now the trick is that to be able to mange the sliding takes balance.

Really is based on tire grip vs slip angle. Most folks can maintian a the optimum grip producing slip angle. Even good drivers can maintian a proper 4 wheel slip angle without a balanced car.

Now I have leared this through much evolution of 944 driving.

My 944-spec started stock, but stripped out. It was fast. Then in Danno school of thought I added in 250lbs front springs. I made the front stiffer and the car turn into an understeering pig. The car got no faster and burned through fronts pretty quick. What I did learn was to trail brake, because I needed to do things to get the car to rotate alot.

Well over time I tweak and adjusted things and gradually added more springs to front and rear. I have come to car with 100 lbs more front spring and 200 lbs more rear spring. The car is balanced well and is quite fast. Due to it being balance and being able to slide it around in very controlled fashion. In this car I do all my "driving" at corner entry and under brakes. A little trail braking in most corners helps me get the rear end to step out just bit so that I can modulate the throttle and brakes to toss the car around the corner.

I strong disagree with Danno feelings that the straighs are were time is gained. Nope time is gained by not slowing down for the corners. When you race with 134 rwhp slowing down too much for any corner costs you big time. Now you also need to put what little power down you have as soon as possible, but you need to find the right balance. Usally a late apex in 944-spec car makes you damn slow. You need much earlier apex and must carry massive speed into the corner. The trick with this is to not carry so much as to prevent from getting the car settled so you can apply power when you need to.

Being fast does requrire grip, but too much understeer will only make you slow. You need to have lots of grip and balance and I personally am much faster trading a bit of overall grip for proper balance. Simply I can generate more cornering speeds (slow down less for the corner) from a well balanced car than one with more front end grip and poorer balance.

 

That depends on where you are in tire pressure vs grip curve. To much air and too little air are both reasons for low grip.

I rarely use tire pressure to change balance. Better to find the optimal pressure to give you the best grip and then use other things like sway bars to tune balance.

Remember that tires are your number 1, 2, 3, and 4 most important things in keeping you on the road. The first thing you want to do is to optimize tire pressures for max grip. Then balance the car around that. Really spring rates them selves only impact balance, but not overall grip. Body roll does impact grip based on how it change the shape of the contract patch and springs impact body roll, but more so when moving from stock springs to stiffer ones. Once you have stiffer springs the main effect is to increase the resonse rate of the car and impact balance.


Couple thing I forgot.

Lowering a car lowers the CG. This reduced the weight transfer and is very good thing. There are only 3 things that impact weight trasfer.

1) overall weight
2) car width and length
3) CG location.


Nothing else does that.

Now lowering just the rear of a 944 has in the effect of causing understeer. Not sure exactly why, but in every 944 I have driven it does. Lower rear = more understeer.

Also adding weight to the rear of the car addes more understeer assuming no change to anything else.


Again I know this from autocrossing my 83 944 & 951 and racing my 944-spec. This impact is most evident when comparing full tank to empty and spare tire and tools vs removed. Where I believe it comes from is the change in ride height as the impact seems to be proprtional to rear spring rates. Softer springs means this has more impact. Even so my 944-spec noticalble has more understeer and feels less lively with full tank vs empty tank.