Too light = poor handling stats

[Fit4aking]

In FM2 I noticed that my TVR Sagaris wouldn't handle correctly when full weight reduction was applied.  Even with near full downforce it was just incapable of holding turns well and sweepers were out of the question.  Adding weight seemed to help but the cars stats didn't decrease along with the weight.  Actually they showed that the car statistically would have handled better with less weight.  That proved to not be the case.

I recently have started rebuilding a lot of my cars in F class and now into E class.  I've found that the same anomoly is happening again in FM3.  Too light = poor handling.  However, I can see that the stat's reflect the negative effects that too little weight has on the different chassis'.  So far it has only been on the lightest of lightweight cars and I think it has more to do with the ability of the car to load the applied tires instead of a general additon to the stat calculator that indicates lighter is better.

Has anyone else seen the stats on their car in handling and braking start heading in the wrong direction when weight reducing upgrades are applied?  Not just weight reduction but any part that reduces the overall weight of the car.

I have found this to happen when building up my '84 Rabbit, Datsun 510, and most recently on my Fiat 500 Abarth Essesse.  Perhaps the game is correctly indicating what effect the parts are having on the cars even if the debate over if the PI system is flawed still rages on.

[fndrbndr]

OK, probably a dumb question, but that's never stopped me before, so here goes.  Have you tried a softer setup with your ultra-light cars?  A lot of this is probably dependent on driving style, but if you use weight transfer a lot when you drive, it seems like a stiff setup with a light car will make those techniques pretty unwieldy. 

I rarely have this problem with cars that SHOULD be light, which are also my favorite cars.  It's hard/impossible to make a 1st gen Miata too light, for example.  I'm surprised you're having the issue with the F/E class cars.  I've seen this issue in upper classes, though, and (paradoxically) stiffer springs seem to help somewhat.

[Fit4aking]

Worked on a lightweight FWD car last night and dediced to lighten up the SWR to see if it ended up helping any.  I lowered by 2%, then 5%, and then again by 10%.  The tires weren't heating up correctly.  Fronts were in the hight 180's and the rears were in the 150's.  (Slicks)  The car would handle well but not great.  Corner entry understeer until the car settled in and then did well but would oversteer and I'd lose the rear quickly if I didn't stay on the throttle to keep the front end pulling it all around.  My lap times were on par with my F-classers and I was packing 75 extra PI in handling upgrades.  I may go the aero route but I was hoping to not "need" extra downforce in such low class cars. 

Extra weight would help load the tires up but it would take away from already low acceleration numbers.  Adding wider Y-rated tires would add weight and grip but can't match the advertised handling stat.  Adding the wings on the car would add downforce weight to the car without sacrificing acceleration, but would absolutely take away from an already low top end.  I'm just baffled as to why an E-class car would need aero to have usable handling that make it effectively slower than an F-class build.  Extra PI should lower lap times right?

[Blooze]

Here's a thawght or two on these things...

Re: Ranking and ranking values.  I have given about as much credence to the ranking numbers once they are altered as Turn 10 seems to have given effort in making the Standings a viable system.  I have found them to have value for when making decisions on what car to buy when choosing something for le Mans or such.  Past that, once you start adding parts, they make little or no sense to me and I don't pay much attention to them.

Re: Weight and Handling:  I have found that the ranking numbers do seem to make a little sense when dealing with weight and handling, and weight and accelleration/speed even more so.  The system does seem to recognize when mechanical grip has been affected.  However, the Benchmark system is more reliable and numerically useful in my opinion.

One thing to remember when taking weight off a production car and replacing it with aero parts that the replacement parts are no where near as efficient as the parts that are on similarly weighted Race cars.  An Acura ARX-02a weighs in at 2020 lbs and has the ability to generate 1110 lbs of downforce.  The Honda VTi Civic weighs nearly the same, but adding a wing to it will only get you another 200 lbs of downforce.  That coupled with the driven tire size does more to limit what can be accomplished with the Civic than anything else.  Sure, you can add turbos, cams, and what have you, but once you reach a certain point - it is just coals to Newcastle...

Chasing tire temps is (IMO) is not worth the effort if one is concerned with front/rear equality, especially with drive types other than AWD.  I think the only thing that you can realistically accomplish is to get the PSI to equalize after a couple laps, and monitor the temps across the individual tires - most usually a result of camber settings.  Efforts taken to get all four tires to the same temperature are going to be squirrelly at best, and counter productive to most other areas of handling measurement.  Again, this is my opinion...  I don't want to go pissin' in anybody's Post Toasties here...

I just read through this...  I doubt it was much help with the OP.  Sorry...

$

[Fit4aking]

Thanks for the insight Blooze.  I haven't taken the upgrades back and forth to the Benchmark to see how it effects the lateral G's at different speeds but it may be something I'll pay closer attention to moving forward. 

The tire temps were never intended to be the same front to back, not on a FWD car but the fact that they were staying as cold as they were was alarming.  I'm used to pushing closer to 200 with the FWD and the rears in the area of 170 to 175.  With them staying so cold in the back I was attributing it to too much front bias.  I was set up with 56.5 and lowered it to 54.5 but it only made the front more skitish while not helping the rear enough to justify the change.

To lower my frustration I've been running the factory spec races that I'd missed in season play to build up the bank account and get familiar with lower weight FWD cars.  The Yaris series is coming up next opportunity I have so that my help baseline an E class build to compare to my failing Fiat 500.

Just when I thought I had it all figured out and was ready to go full steam ahead up the classes I hit a speedbump.  Guess that's what makes Forza so interesting.

[Spiny Anteater]

Fit, those temps in the OP are pretty similar to what I often experience in my F Class cars, and not just the lightweight ones. My most recent one was a Mazda Axela which wasn't too far short of 3000lbs, and that one barely broke 150degrees Fahrenheit in the rear tyres either. Like Blooze, I pretty much came to the conclusion that that isn't much of an issue, especially in F Class where the tyres aren't worked as hard (after all, what does the back end do on a FWD other than swing around a bit to help with turn in ). Generally I find that pressures are a more reliable guide, although deciding what pressure works best is a matter for another thread.

One thing to remember when taking weight off a production car and replacing it with aero parts that the replacement parts are no where near as efficient as the parts that are on similarly weighted Race cars. 

Most of the time that's true, but some of the race cars which start in S Class have pretty puny aero which is less than even a Civic. I think the Volvo may be one of them.

[Drift2XL]

Something I noticed in the various Exodus series ran so far.

I am useless in any car below 2400lbs. Unless your talking stock tire, stock weight Miata. Then we're cool.

It seems that you have to be unbelievably smooth with these low weight cars. Almost MR smooth on the inputs. No full lock on the controller or immediate push. And forget about tires warming up. Do I go low pressure for grip, or high pressure for more temps?? Neither worked for me. High pressure was more consistant so that was the way I went.

I personally like A Class sport tire FR builds right now. Just as fast at Catalunya through the technical portions. Including the three "light" off camber turns. Same speeds from slicks and sports.

[Choccy]

What is one difference between a Bentley Continental GT and a Lotus Elan? Curb weight.

To improve performance of the heavy car you can increase tire width, improve tire compound, boost power and torque, etc.
But at the end of the day, the greatest gains can be made by reducing weight. Reducing weight to an optimal amount means the car becomes better in all areas.

To improve performance of the light car you can increase tire width, improve tire compound, boost power and torque, add aero parts, etc. The light car was already designed with low curb weight in mind. Removing more weight does not improve handling. Why?

What is the only means of contact between your car and the ground? The tires/contact patch.
What is the most signficant factor that keeps the car in balance on its suspension? The ratio between sprung versus unsprung weight.

You can exceed a car's optimal weight on either side of the scale.

The car needs sufficient static weight to apply useful downwards force on the tires. Aero parts only help at speed and will do little to improve matters on a low speed vehicle. Knowing this, you will need to use particularly smooth inputs to get the most from the vehicle.

[barumba]

I was watching this, the link is from my other favourite forum.
http://blogs.insideline.com/straightline/2010/05/corvettes-at-le-mans-gm-shows-off-rare-1960-documentary.html
After you have wiped the drool off your chin, fast forward to time mark 15:00. Quite a pertinent commentary regarding the light weight coupes out pacing the heavier GT's, but the heavier GT's exhibiting superior handling (time mark 16:10).
I'm going to post this link in the Drivers' Lounge Thread, too good not to share.
Cheers!

[barumba]

Re: Light weights and spring/shock loading
« Sent to: barumba on: November 10, 2010, 06:38:10 AM »
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Quote from: barumba on November 08, 2010, 08:32:38 PM
Choc, hey, bud, I hope you are well. I'm doing not too bad, but work is driving me nuts. Cut my staff from 5 to 2, so, well the work didn't get cut, so busy busy busy. Anyway, I'm trying to recall a discussion you and others had about light weight cars and spring loading (lack there of) and how you compensated. I think your test car was the Lotus, but could be mistaken. I'm going to snoop around to see if I can find it, but would appreciate if you could point me in the right direction.

Cheers!
Choccy wrote...
Hi Trevor! I'm doing fine. Still got a foot in Forza, cursing Winter's impending doom and anxiously awaiting some track decals via snail mail to add to my car before she gets put away until Spring.

Yeah, there's nothing like fewer staff to do the same or, typically, more work.

I think you're talking about the Bentley versus Lotus theories? That was some time ago. I've made more accurate observations since then.

What it boils down to is this:

- You can add Sport or Race ARBs to a lightweight car, but by doing so, you rob the car of its ability to generate additional downforce on the outside tires. A light car tends to not get the tire temps very high(light green tire indicator in-game). This is good because it means you aren't often crossing the limit of tire adhesion. This is bad because you really have to work the car hard to approach the tire's optimum temp to maxmize grip. That's where aero aids can help, but you still have to work the car hard to see benefits and overcome the deficits of the added aero drag.

A heavy car is almost always exceeding the optimum tire temp on one or more tires(outside and inside fronts typically).

What does this have to do with spring rates?

If you setup a light car with stiff springs, you're resisting side-to-side roll, pitch and lurch. Spring stiffness is adjusted to make the car more stable through all actions on track. But if the car is light, it should require less effort(more stiff than stock but less than required for a heavy car) to do so.

If I think back to my experiences at Calabogie Motorsports Park, my 10 year old Honda Civic(~2700-2800 lbs) handled relatively well on its OEM, non-performance springs. But, it required very smooth inputs to avoid upsetting the car balance in braking zones and through transitions. Why? Soft stock springs on a light car.

Same venue but in a 3400 lb Genesis Coupe on sport-tuned springs. With minor steering inputs, I could make and feel big changes in direction but the car remained stable. As well, abrupt braking or sharp steering inputs could be made without causing the car to dive onto its nose or roll onto its side. My Civic would roll so far, it would look like and feel like you were about to be flung out the windshield or thrown out the side.

So, taking a Lotus in Forza like the Elise. Uprating the springs between 25 to 150 lbs should net gains in handling. Maintain the stock spring ratio. Too stiff and you'll lose the "feel" of the car.

As I've invested more and more laps in FM3, I've found it really is about subtlety moreso then FM2.

Cameron

[barumba]

Re: Light weights and spring/shock loading
« Sent to: barumba on: November 10, 2010, 05:30:13 PM »
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Choccy wrote...
The nights have been hovering around freezing but this week has been quite good during the day. Might be the last week to enjoy the temps...

I'll see if I can dig up some tangible figures for my lightweight cars. Regrettably, I don't have anything in spreadsheet form.

But there is a definite sweet spot for reducing both sprung and unsprung weight depending on the cars default curb weight.

If you remove an excessive amount(the absolute lightest wheel you can purchase) of unsprung weight, you no longer "feel" the connection between the wheels and the ground. Through the controller, I lose all sense of what the wheels are doing, even if they're perfectly planted on the tarmac. What the calculation is(I'm assuming there's one out there somewhere) I don't yet know. But I avoid going beyond 20 to 25 lbs lighter.

I remember reading a copy of EVO years ago. One of the writers mentioned 2,600 to 2,700 lbs curb weight was a close to ideal target for the better-than-average sports car/coupe(excluding the likes of the Ariel Atom, Lotus cars, Caterhams and other featherweights).

In game, no matter how smooth your inputs, a 3,000+ lb car is a pig. One of the NSX models straddles the ideal target weight in stock configuration. Compared to its heavier siblings, you can really notice the difference. But reducing the weight of that light NSX only helps to a certain point.

Returning to your query on spring and damper rates. For springs, you're looking at chassis dimensions and curb weight. It's like choosing a mattress. Your weight doesn't change. So you have to find the mattress with a spring rate that suits your needs. A firm bed gives you absolute support, but the rigidity is uncomfortable for the weight(person) lying on top of it. If the mattress is too soft, even the slightest body movement disturbs your balance and you're all over the place.

So, for the weight of the car, you need to find a level of spring firmness that improves dynamic performance without sacrificing drivability. If the springs are so rigid that turning the wheel full lock has no result and the car understeers off the road, then you've passed the ideal zone. A light car has less overall weight to distribute and focus as downforce on the outside tires. The tires on a light car may never have to endure the downforce that would exceed their levels of grip, but within the radius of a turn, the car must have the lateral roll tolerance to get increase tire temps to establish optimal grip.

So an Elise with OEM tires may get full greens with focused driving at the limit. Replace the stockers with Sport tires and you might not see green at all four corners. Why? Not enough static weight to work up the temps, meaning you can still drive faster and harder to dynamically build heat in the tires to achieve optimal grip.

[barumba]

Choccy
Sheet Metal Fabricator

Posts: 97


GT=sCHOCOLATE


   Re: Light weights and spring/shock loading
« Sent to: barumba on: November 10, 2010, 10:53:59 PM » Quote Reply Remove   

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Just to show I'm not talking out my tailpipe, I'll cite a recent reference I picked up from Chapters. "Going Faster! Mastering the Art of Race Driving" The Skip Barber Racing School.

Chapter 13: Tires. Levels of grip. Lateral(cornering) and longitudinal(accelerating and braking).

"The amount of traction available from a tire is determined by a number of factors: some you can't control, but some that you have the power to manipulate...Coefficient of Friction(CF) is a ratio of the maximum force a tire can generate relative to the load pushing down on it...a tire, especially a street tire, changes CF with inflation pressure allows you to change the relative grip of the front or rear end of the car. Increases in inflation pressure increase the CF up to a point, and then it falls off again..."

I've witnessed this first hand in FM3. Increasing psi beyond the sweet spot fails to improve lateral grip resulting in slower lap times.

"Adjusting the front and rear pressures up and down isn't the best way to adjust cornering characteristics, since you often end up reducing the traction capability of one end of the car to get its cornering close to neutral. When you do this, the car may feel better, but it will have less overall traction than it would have if both front and rear tires were at the pressure that yielded the best traction. Once you had both fronts and rears at their optimum pressure, you would then trim the car's cornering balance by mechanical means-by adjusting sway bars, spring rates, ride heights, etc. But if you're driving a car that is non-adjustable, juggling tire pressures, within reason, will work to trim its cornering balance."

This is why I endorse the formula recommended by that tire engineer. The calculation still holds well for my builds providing near optimal tire grip and crisp handling response to any car I drive in-game.

"...This phenomenon of diminishing CF as (tire) downloads get higher is one of the foundations behind the fact that lighter cars can attain higher cornering, braking and acceleration forces than cars with greater mass. It also explains why designers try to keep the center of gravity of racecars as low as possible in order to minimize the amount of load transfer from the inside tires onto the outside tires under cornering. The greater the load transferred to the outside, the more the CF of the loaded tires will suffer. Less load transfer means a higher overall CF for all four tires and, consequently, greater traction and cornering speed..."

[barumba]

Choccy
Sheet Metal Fabricator

Posts: 97


GT=sCHOCOLATE


   Re: Light weights and spring/shock loading
« Sent to: barumba on: November 11, 2010, 02:13:04 AM » Quote Reply Remove   

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2005 Lotus Elise[C367]
Stock config.
1,777 lbs
39% weight dist.

Tsukuba Full - 15 laps
[30.0][30.0] front / [30.0][30.0] rear psi cold
[33.3][33.1] front / [32.3][32.1] rear psi hot
approx. gains = (+3 psi front / +2 psi rear )
Best lap = 01:03.044

Tsukuba Full - 15 laps
[17.5][17.5] front / [17.5][17.5] rear psi cold
[19.5][19.4] front / [18.9][18.8] rear psi hot
approx. gains = (+2 psi front / +1 psi rear )
Best lap = 01:04.497

Tsukuba Full - 15 laps
[19.5][19.5] front / [19.5][19.5] rear psi cold
[21.8][21.7] front / [20.9][20.8] rear psi hot
approx. gains = (+2 psi front / +1 psi rear )
Best lap = 01:03.686

Tsukuba Full - 25 laps
[20.0][20.0] front / [20.5][20.5] rear psi cold
[22.3][22.1] front / [22.1][22.0] rear psi hot
approx. gains = (+2 psi front / +2 psi rear )
Best lap = 01:03.622

At 30 psi cold, steering inputs are very crisp but tire grip breaks away very easily. 2.5 psi above the value derived by calculation yields more comfortable and consistent results.

The psi ratio was adjusted(+0.5 psi at the rear) to compensate for the inflated hot psi at the fronts. The goal being, to get the fronts and rears to close to equal psi at hot temperature.

The tire pressure calculation is simply divide the curb weight by 100. Since we can only adjust psi in 0.5 increments, rounding is necessary. I round down on the base psi calculation, then increase the front and rear psi by 2 psi and then add 0.5 psi if the base psi calculation was less than the curb weight figure.

Elise curb weight = 1,777
1777 / 100 = 17.77
Base psi equals 17.7 rounded to 17.5 and not 18.0.

This is my subjective interpretation, heating the tires increases psi, so having marginally less than the calculation would recommend is accounted for by driving hard and, if required for optimal steering "feel", that final 0.5 psi adjustment.

When tire pressure has been sorted out on a non-adjustable car, then and only then do I consider manipulating springs, dampers or ARBs. As the Skip Barber book indicated, tires first, then mechanical adjustments to "trim" the handling characteristics. They are not used for gross handling changes.

[barumba]

Choccy
Sheet Metal Fabricator

Posts: 97


GT=sCHOCOLATE


   Re: Light weights and spring/shock loading
« Sent to: barumba on: November 11, 2010, 08:30:51 AM »
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2005 Lotus Elise[C381]
Modified config.
- Sport tires
1,777 lbs
39% weight dist.

Tsukuba Full - 15 laps
[20.0][20.0] front / [20.5][20.5] rear psi cold
[22.0][21.9.1] front / [21.7][21.6] rear psi hot
approx. gains = (+2 psi front / +1 psi rear )
Best lap = 01:03.021

2005 Lotus Elise[C417]
Modified config.
- Street Intake, Fuel, Oil, Flywheel, Clutch and Driveline
- Sport Tires
1,775 lbs
39% weight dist.

Tsukuba Full - 15 laps
[20.0][20.0] front / [20.5][20.5] rear psi cold
[22.0][21.8] front / [21.9][21.8] rear psi hot
approx. gains = (+2 psi front / +1 psi rear )
Best lap = 01:02.126

*** After more trials I decided to increase rear tire psi by another 0.5 psi to "square" the bias(making the rears 1 psi greater than the fronts at cold temp). The front left tire has been consistently higher than all other corners due to Tsukuba's layout. While this didn't result in a faster lap(yet!) I was able to improve my consistency lap after lap, staying within the 01:03.500 to 01:03.200 range. On the limit, the car "felt" more neutral through turns. Understeer was easier to manipulate rather than simply manage(two different things from the driver's POV). It's this fine tuning per venue that separates a middling setup from a good one.

[barumba]

Choccy
Sheet Metal Fabricator

Posts: 97


GT=sCHOCOLATE


   Re: Light weights and spring/shock loading
« Sent to: barumba on: November 11, 2010, 07:39:41 PM » Quote Reply Remove   

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I tried plus sizing the tires by one increment. Left the wheel diameter at stock. Just made turning response feel a hair sloppy. To be expected. Times didn't drop with the additional rubber, instead they increased slightly with the reduced "confidence" in what the car was doing.

To compensate, the wheels would need to be plus sized as well, but of course that adds unsprung weight and rotational mass. It might be a consideration for the older model cars with their 14" or 15" wheels, but I'm skeptical that it would help a Lotus.

Now that I have figures for tire pressure, I'll swap in some Sport and then Race springs to see what helps and what hurts in terms of the established lap times.

The point of all the tire pressure guff was to demonstrate how much and how little impact psi adjustments have. Then, how much impact tire compound has. And thinking about it, I should really trial Race slicks on the earlier psi trials to see how much faster the car can go. As you could probably tell, it's a matter of diminishing returns; you have to keep the car on the boil and be spot on with your inputs to see results at this point.

Adjusted spring rates and damper settings would then be expected to provide for the minute gains of stability in cornering. Again, all things considered, you're already driving smooth as silk to manage 1:03s in a Lotus at Tsukuba. I made a few last call runs in the same car with Enkei rims(-7 lbs unsprung weight) and Sport springs. Shaved the best lap time down to 1:02. Had the heavy eyelids though, so will try again tonight. Then switch to slicks.

Geez, this is turning into a lot of fun work. Wish my real job was like this.