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Thread: Physics of the Fun Pack DLC

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    Vehicle Technical Lead Casey Ringley's Avatar
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    Physics of the Fun Pack DLC

    We did these in the first game and you guys seems to like it, so here we go again. Below is a collection of some notes covering production of the handling models for the Fun Pack cars. It's a long one, I know , but this was a really cool group of cars for us to work on with so much variety and research rabbit holes to dive down. Seems like you guys have been enjoying the cars over the holiday break and that is wonderful to see. Fire away with any questions below and I'll try to keep up with answers where I can.


    Ford Bronco RTR 'Brocky': Drivetrain for this guy is a classic off-road racer setup. Engine is a Ford Performance Z427 cranking out 600hp@6500rpm and 560lb-ft@5000rpm. Power goes through a stock TH400 three-speed automatic and transfer box to give either RWD or 4WD -- You can use the center ‘diff’ Spool to toggle this in setup. Spool=Yes for 4WD and Spool=No for RWD. The final drive ratio has a setting for the transfer box low range gear, but really this is only useful for rock crawling. There are also some optional TH400 gear ratio sets you can run, but the real thing is stock as is default setups with the 2.48, 1.48, 1.00 gear ratios. Typical differential setup Vaughn uses is a spool at the rear and ratcheting locker in the front.

    We had the real Brocky scanned for art production and this proved ideal for drawing up the long-travel suspension geometry, even the 4-link solid rear axle works great over its 18" range of travel. The complete truck weighs in around 4500lb with about a 50:50 weight distribution. Motion ratios of the suspension mean it balance well with a 650lb/in front main spring and 200lb/in at the rear. Dampers are matched to some similar King units, but there is a big adjustment range in these things depending on the type of terrain being driven. We’ve gone fairly stiff and digressive for the rallycross tracks present in game, and that seems to work pretty well.

    Thing is maximum fun at the RX tracks. It's about 4-5s slower than the RX cars, but being so big and changing direction so much more slowly makes it all feel like a real event as you slide around. It becomes a beast in RWD mode; not fast at all, but great at doing donuts and it drifts like a champ. It's neat how responsive it is to setup changes. You get something completely different with AWD, max ride height of 40cm and balanced anti-roll bars versus lowering it some, changing to RWD, and setting up the ARBs like a circuit racer. Very big toy truck.


    Audi S1 EKS RX: This one was fun to assemble as Audi sent over some wonderful reference details. The suspension geometry - struts all around as usual - has some interesting anti-dive/lift/squat properties which combine with springs which are a bit stiffer than some other RX teams use. Set up with 400lb/in springs on all corners and big front anti-roll bar makes it handle almost like a soft, tarmac road course setup. That matches up pretty well with how we can see the chassis move in recent race videos. No complaints here; it drives great with all that information applied.


    Audi Sport quattro S1: Great fun researching this car; it's famous enough and cloned enough that there is quite a lot of information for it on the web. The more you read about it, the more amazing it is that it worked at all! They basically started with an Audi 80 and kept cutting bits out of the middle until the wheelbase was only 2.2m to try and make the handling more nimble. That works in theory, but being an Audi you still have the engine slung way out beyond the front axle, so early homologation versions had horrible weight distribution like 62% front. Moving the cooling system to the rear and various other tweaks eventually helped get it to a 51:49, but all articles I've found still say that the chassis really works best if you are aggressive with it to get turn-in rotation.

    Engine for the S1 version is generally described as peaky with quite a bit of lag, and that comes through here - and it makes sense when the dashboard gauge reads all the way up to 2.5bar (36psi) boost! It is a smooth 450hp, but you'll find yourself waiting a bit for the power unless you row gears and help hold it high in the rpm range. They even ran it a few times with Porsche's early PDK gearbox - also used in some Porsche 962C factory cars - to stay on boost better. It's neat, in some old videos you can clearly hear the difference in the PDK car shifting and how smooth that unit flows through geares. Impressive to think that our Audi 90 IMSA GTO is an evolution of this same basic engine 4 years down the line and it was then doing 700+hp with 10psi less boost. Turbo tech came on very strong in the '80s.

    The early quattro system as in the race cars used a kind of planetary/Torsen/viscous combination center differential. It let them adjust fore:aft power balance, do some torque biasing on slip and still lock up if one end loses grip completely. Works really well when copied into our systems. Feels a lot more typical of AWD road cars than something like the modern RX designs and reminds me a lot of our time in the Subarus at DirtFish. Front diff defaults to a viscous LSD and rear to a typical ZF-type Salisbury LSD, which appears to be how it ran most of the time.

    Suspension is simple MacPherson strut all around. Aero is nothing special but does help stabilize the car nicely. Gearbox is the 6-speed manual it ran most of the time with ratio set pulled from the old FIA homologation forms. Have let it use the same tire set as the modern RX cars. Probably a good bit better than period rubber (Or maybe not! Group B was pretty wild and using F1 rubber compounds in period wouldn't surprise me) but it's a good drive on them out of the box and good fun to slide.

    So...it's a chassis that understeers until you push it hard enough, aero that works mostly to keeping things pointed in the right direction once a slide does start, plenty of power but only above 6000rpm and with some lag, and a great AWD system to put that power on the ground and pull out of corners. Racing this thing is fantastic once you get the hang of keeping it on power and how it likes using pendulum turns for initiating turn-in.


    Renault 5 Maxi Turbo: Ceci n'est pas un rallye. The Maxi was Renault hanging on to an aging format in a time with AWD was clearly the way forward in rallying. Mid-engine, RWD, double wishbone suspension, 60% rear weight bias, and 2.5kg/hp may as well be specs from the Renault R.S.01 GT3 car, and this comes through in how it drives. This car was a tarmac specialist in 1985 and it really is one terrific circuit racer. On the whole, it matches up quite well with the Audi quattro, with the per track favorite being all down to how much gravel there is. Daytona is 85% tarmac and gives the Renault a 0.5s advantage per lap; Loheac flips that and the Audi is a 1s favorite; circuits like Hell or DirtFish are more balanced and the match-up is nearly even.

    Like the Audi quattro, there is a great deal of information available about it in the form of old homologation papers and people who have restored and maintained the cars since their heyday. The chassis is a very simple design as far as suspension geometry, with the measured kingpin and caster values of 15° and 11.5°, respectively, giving good steering feel especially during opposite lock situations and powering out of corners. Drive comes from the C7K 1,527cc turbo 4-cylinder for 360hp at around 3bar boost absolute, through a 5-speed manual and 40% lock clutch & plate differential on the rear axle. The turbo had a very early anti-lag type of system which recirculated boost airflow to keep it spooled up so long as you don't spend too long off throttle; once boost does drop, it takes a good few seconds for it to come back on.

    The restoration folk were a big help in having already measured typical wheel rate of the rear springs and torsion bar front suspension. It runs significantly stiffer than the Audi - again, showing its focus on tarmac events - and with a chassis balance that is far more typical of GT cars than pitching it sideways into gravel hairpins. What's especially telling of differences in the two cars is the chassis moments of inertia that fall out from our models. While both are similar overall dimensions and the Renault is 18% lighter than the Audi (900kg vs 1090kg), the moments of inertia are upwards of 60% lower for the Renault! That's the value of having most of the mass concentrated within the wheelbase; the car is much more eager to turn and respond to steering inputs. Nimble chassis via basic design rather than through modifying a car to make up for deficiencies.

    So it's a much nicer handling car, but where the challenge arises is putting power down on the gravel. Power per kg is similar with the Audi, but going to only two tires means you have to be doubly careful about its application. This car doesn't like being tossed around and backed into corners for you to then plant the throttle and power out; that's far more likely to just end in a spin or the engine bogging down. How it gets speed is by using the brakes to rotate the car into a tight, late apex, use just enough power to keep the rotation from stalling, and then feed it back in through the exit with opposite lock to control direction if necessary. It's a lot more difficult than the Audi where anything goes to help fight the natural understeer of its chassis-drivetrain design; that car is all about being rough to make it do what you want while this one is about being smooth to take what it will give you. That's the trade-off of being a tarmac specialist. Loose stuff is pretty tricky, but you can more than make it up by treating it like a flyweight GT on any pavement sections; brake super late, smooth fast apex, power down hard through the exit. Sweet little car.


    Ford RS200: Like the other Group B cars, this one proved great for research as 1.) there is a ton of interest in the car and 2.) people who have restored them share tons of old documents and information. Scanning through the original owner's manual is particularly cool, with a whole section on starting and driving the car written by Jackie Stewart.

    This car is a classic example of huge potential which didn't have a chance to develop. Ford had spent a ton of time developing the front-engine, rear wheel drive Escort RS1700T for Group B only to throw it all away before entering a race as they saw AWD was the future. And so work started on the RS200 in mid-1983, well behind the rest of the competition and using a lot of parts which were left over from existing projects.

    The engine is a Cosworth BDT 1.8L turbo 4-cylinder developed for the RS1700T and is a fine engine, producing upward of 440hp@7500rpm with a wide powerband that runs from 5500rpm right up to the rev limiter at 8500rpm. The parts-bin special nature of the early RS200 cars meant it was fairly heavy, though, at 1050kg and this put the engine at the small end of the displacement scale for its weight class. Evolution versions of the BDT-E, had they competed, were enlarged to 2,137cc and people who have since tweaked and tuned them can see upwards of 800hp!

    Clever positioning of the 5-speed gearbox meant an even weight distribution of 50:50 (ideal, in theory) and a simple double wishbone suspension controls the chassis well. The AWD system is comprised of three Ferguson viscous limited slip differentials left over from Escort and Sierra projects. No exact rating for these RS200 diffs online or in old books, but we know from the Sierra Group A car that they tended to run in the 100-500Nm range. Standard setup is for 63% power to the rear with the viscous center controlling differential slip between the axles. It's a good configuration for stability, viscous diffs all around, but does generate a fair amount of understeer. Steve Millen once said "It's quickest to drive the car into a turn very hard, pitch it in with oversteer and maintain that attitude through the corner with the throttle." and that definitely works; it's a car that takes well to the Scandinavian flick technique and chassis rotation follows the steering quite directly. Still, it tends to drive a little tight unless you are rough with it; not slow by any means, but less exciting than the other two above.

    This 1986 version of the car sorta feels like it lands squarely in between the Audi and Renault. It has the mid-engine and tarmac handling of the Renault combined with the AWD traction (and inherent understeer balance) of the Audi. Further development probably would have made the car significantly lighter or more powerful and solved any handling issues to produce a truly strong competitor. But Group B was cancelled in 1987 and the RS200 project ended after only a 1-year run.


    Ford Mustang RTR Spec 5-FD: Star of the show in this pack, I suppose. Fundamentally, this thing is pretty simple and works almost like a dirt track racer. Take a basic Mustang shell, strip it down to basics, add a Roush-Yates V8 making stupid power levels, drive it through a 4-speed Andrews dog box to a ratcheting locker rear end, and change up the suspension geometry/setup to make the handling dominated by those rear tires.

    Neat aspect of the Formula D rules is that the chassis-side suspension pickup points must remain stock but wheel-side is free. Up front that means a custom control arm for the strut to push the wheels out for wider track and a significant increase caster angle. The rear gets more interesting. Standard Mustang rear end uses a funky multi-link suspension which is surely great for road car use, but probably a little finicky to deal with when tuning a race car. So to get around this, they’ve used the stock pick up points mixed and matched with a custom upright and control arms to make a pure double wishbone configuration with the right kind of anti-squat and roll center properties to help with drifting. There are some good pictures on the interwebs showing what they did.

    The trick in drifting it really is mostly about finding a setup that suits your style. We’ve included a few baseline options to get you started. The competition setup runs super low tire pressures and softer rear end to lift the inside front tire like you’ll see Vaughn’s car do in FD events. It’s a good way to get huge drift angle at high speed, but takes massive commitment to hold that edge without over-rotating or straightening and spearing off in the other direction. The ‘drift’ setup takes more of a demo day approach with higher pressures and more rear end stiffness to balance with the front. It is easier to get into a drift but generally slower through turns as a result; which is fine, we’re doing this for fun and not to impress the judges during a tandem run. I’d recommend playing with both setups to see which feels better as a starting point for your personal style then mainly tweak tire pressures and anti-roll bars from there. Small changes to handbrake strength can have a good effect in tuning the car, as well, for those of us without full analog handbrake hardware.

    Being late in getting these notes written up, I’ve already seen some of your videos drifting the car and it’s great stuff to see. Love the tandem runs and seeing others get on with such a different driving style. Try it in VR if you have a chance!!


    Ford Mustang RTR 1966: There's a lot in common between this one and its main competition, the 1969 Trans-Am Camaro: 5L V8, control arm front with Hotchkiss rear axle, ~2800lb total weight, and decent aero for the time.

    For the engine, we took the GT350 302ci V8 as a reference point. There were claims of race-prepped engines cranking out 480hp@8000rpm for Trans-Am, but this is a fairly big number considering the homologation street car (not all that different) was generally agreed to produce 350hp and the Camaros it raced against were dyno-tested to show something in the low-400 range...and those won most of the races. Our model uses dyno data from similar cars and balances it in the 415hp range at 7800rpm to fit well with the rest of the class. FIA's historic database came through again with lots of good information, including a wide range of the gearbox options used in 302ci Mustangs from that era and all are present here.

    This car benefits from an improved model of the Hotchkis rear suspension design. Many thanks to you guys for isolating the issues our old version of this geometry had on the Camaro. The investigation into why that one would fall apart resulted in a much more accurate model within the chassis solver limitations (no flexible links) with a roll center that moves more correctly with chassis motion. The Ford Escort models have been revised for this new approach as well.

    All of these racers from the Trans-Am era went through some intense modification to cut weight and become race ready; coolest of these processes being an acid dip to thin out sheet metal. They commonly got the cars so far below the 2800lb minimum weight that large amounts of ballast could be placed around the car to get an ideal weight distribution, taking the car from maybe 55% front to balanced at 50:50 or even a little extra on the rear if it suited the track. Suspension setup leans heavy on vintage GT350 data and it works out very well here. It drives like an old muscle race car. Good power but nothing compared to modern cars, loose bias-ply tires so the chassis moves around a lot but never really snaps away suddenly, aero that is fairly neutral, very average brakes and good cornering from wide tires in an era where rubber was just starting to get really sticky.
    Haste Is Slow

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    WMD Member Madhun67's Avatar
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    thats some great reading,thank you !
    Last edited by Konan; 05-01-2018 at 10:54.
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    WMD Member dodge33cymru's Avatar
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    YEEEEY!!!

    Thanks Casey, love these

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    Superkart Pilot banner77amc's Avatar
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    If you have notes like this on every DLC I will read it Good stuff here!
    "Straight roads are for fast cars, turns are for fast drivers." Collin McRae
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    WMD Member Invincible's Avatar
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    Awesome read Casey, as always. Thank you! Only one minor thing... You forgot the Citroën DS3 RX
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    WMD Member bmanic's Avatar
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    Supremely awesome as always Casey! Fantastic write-up and an amazing job on the DLC itself! I'm thoroughly enjoying all of the cars.
    .. please SMS, make the AI faster in rain before shelving the game for good. Please! Love this game dearly!
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    Superkart Pilot
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    Love these - thanks for taking the time to do them again

    gives a real sense of character and history when you step in the car after these read ups !
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    WMD Member David Slute's Avatar
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    Excellent info and read Casey, thank you!

    This part really interested me and i am so happy you can disable the 4wd on the Bronco.

    "Power goes through a stock TH400 three-speed automatic and transfer box to give either RWD or 4WD -- You can use the center ‘diff’ Spool to toggle this in setup. Spool=Yes for 4WD and Spool=No for RWD."

    Does disabling the spool on any other vehicles change it to rwd....likey say the F150?

    Of all the dlcs ive purchased in my time....the Fun pack is probaly my favorite to date!

    Cheers
    Last edited by David Slute; 05-01-2018 at 02:19.
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    Moderator Konan's Avatar
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    WMD Member Azure Flare's Avatar
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    Quote Originally Posted by Madhun67 View Post
    thats some great reading,thank you !
    Please don't quote very long posts.
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