Rank: Driver's License
 2 users liked this post.
#1 Posted : Wednesday, April 29, 2015 6:02:04 AM(UTC)
I've touched on the effect of a car's gearing setup on the overall handling in another thread, now I'm going all out with a full guide as to what on earth I'm actually talking about when I say I use my gear ratios to tune the handling of my car!
Many gearing guides talk primarily about adjusting the ratios according to the max torque and max horsepower curves; these guides are good general outlines for how to tune a production car in S700 class or below (maybe some street legal R3 cars). However, when tuning full-fledged GT and LMP class racecars at the R3/R2/R1 level, much of the tuning philosophy those guides present goes clean out the window in favor of ultra-close-ratio boxes that you could potentially upshift two gears at once and still not drop down to the rpm max torque where the engine creates max torque. So if the theory of tuning the ratios to drop the engine speed to max torque then shift just after max horsepower is no longer effective, how exactly should you be tuning your racecar's gearing?
In order to follow along with the example I'm going to use (which I would highly recommend as I'm going to use extreme ends of the spectrum to fully demonstrate just how wildly I can change a car's handling with gearing alone) you're going to need a #40 Ford GT Mk7 for my example section.

Direct drive- A gear ratio of 1.00 so the only ratio in effect is the final drive.
Overdrive- A ratio that is numerically lower than 1.00 that is generally used more frequently in road cars than racecars, though very short overdrives of 0.85 or numerically higher are sometimes used in modern racing machines. These can also be called an "economy gear" as most modern production cars now have very deep 5th/6th/7th gears that drastically lower engine rpm for the sole purpose of fuel mileage (General Motors and Dodge have used a 6th as low as 0.50 in most of their T56/TR6060 applications). These often serve no performance purpose and using excessively low overdrives on the track will bog the engine down; transmissions with double overdrives are generally used only with large, low-revving engines that have massive low-end torque (read: mostly big OHV American engines). In the majority of cases, a direct drive top gear or sometimes a single overdrive where necessary is the preferred racing setup.
Draft gear- These are sometimes employed in racecars on long tracks where the engineers know the car will likely be in traffic on longer straight sections. Basically, this is where the top gear is set so that the car sits below the rpm where it achieves max power and is incapable of hitting redline in this gear under its own power. If a racecar is on the redline in 6th gear when it catches a draft on the straight it can over-rev (damaging the engine in real life) or bounce off the rev-limiter; the car is setup so that it only uses the first 5 gears under its own power and if it catches a draft on the straight the driver shifts to 6th draft gear. With the decreased wind resistance of drafting the car is capable of achieving 15mph or more than it could without drafting and can close a gap with the car ahead much more quickly than if the car was setup to be near the redline in 6th gear under its own power. Draft gears are most commonly used on tracks like Le Mans, Road America, Mugello, Catalunya, and ovals where speeds are high enough to justify the longer top gear for drafting. You probably would not want a draft gear on Tsukuba or Positano where your car is not going to be close to its top speed and won't hit its top speed with or without a draft.
Launch gear- This isn't a technical term, just something I use to specific a numerically high 1st gear found most often in manual transmission road cars. These make city driving easier by allowing for easy launching from a stop without excessive slipping of the clutch; BMW has been using very high launch gears for decades and the concept is starting to carry over even to the TR6060 6-speeds found in most newer V8-powered American performance cars. These are commonly referred to as a "granny gear" in the truck world, though for trucks these tend be much higher numerically and often the gear can be skipped entirely (launching in 2nd gear) unless the truck is loaded down heavily or pulling a trailer. While there are some older '50s and '60s cars that used a launch gear to compensate for the extremely low final drives used on tracks like Le Mans, they serve little purpose in the modern racecar and you won't see much over 3.00 used for 1st gear in most racecars these days.
Overall ratio- The overall ratio is the ratio of the current gear multiplied by the final drive. So if I have a 3.00 final drive, with a 2nd gear of 2.00 my overall ratio is 6.00 and if I have a 4th gear of
1.00 my overall ratio is 3.00 (simple stuff I think). In the case of an overdrive you still multiply but the overall ratio will be less than the final drive, so with an overdrive of 0.50 and the same 3.00 final you would have an overall ratio of 1.50 (hopefully that makes sense).

The Real World
If you've spent any time examining/comparing gear ratios in your racecars versus your road cars (disregarding the Japanese Super GT cars as many use ratios straight from the production cars) you've likely noticed your racecars will generally use much closer ratios and lack the launch gears or economy gears of their street-legal variants. Getting the car moving in traffic, extending clutch/transmission life, and top gear fuel economy are not primary concerns in racing (fuel economy is optimized in other ways, deep overdrive top gears are not preferred as the sacrifice in top speed hurts the car more in the long run with slower lap times). Combined with racing engines that are tuned to operate entirely at higher revs than a street engine would be, you're going to find a 1st numerically much lower than most road cars and a top gear numerically much higher with the overall ratio spacing far closer together in order to achieve maximum performance.
So why those exact ratios in that car? For that we actually look back to the old American muscle cars for the best analogy. Overdrive gears were actually seen somewhat frequently (often they were optional) in more expensive American cars such as the Thunderbird through the late '40s and '50s, but during the power-crazed '60s virtually every car resorted to a direct drive and overdrive gears were dropped almost entirely. The reasons for this were that overdrive gears are bulky in terms of physical size, can add substantial weight to the transmission, and can lack strength compared to numerically higher gears. From a racing point of view we want the smallest, lightest, and strongest transmission setup we can fit in the car that provides optimal ratio spacing and can handle the power; in GT cars you will often find the production transmission is used, either with factory ratios (Vipers mostly) or custom ratios retrofitted into the production casing. Sometimes a manufacturer contracts out to an aftermarket company such as Hewland (Michelotto uses modified Hewlands), X-Trac, Emco, Ricardo, Quaife, etc to fit custom adjustable gearsets into the production case or do a comets ground-up design specifically for racing; generally prototypes use clean-sheet designs from X-Trac or Hewland engineered for that specific application (several mechanisms such as drop/spur/cluster gears are employed to make the ratios adjustable to the track separate from final drive ratios changes).
The majority of race designs will use either a direct drive top gear or a short overdrive of 0.85 or numerically higher with a numerically low final drive ratio. When talking about a standard RWD differential separate from the transmission, a numerically higher final drive often means a physically larger ring and pinion which is obviously going to be stronger than a smaller ring and pinion that has a numerically higher ratio. There are other advantages (and a few drawbacks) associated with numerically low final drive ratios that I'll get into below.

Final Drive
In a standard car we have one remotely adjustable gear ratios (the transmission) and one foxed ratio (the final drive). The final drive affects all gears simultaneously, making it numerically higher will cause your car to be at a higher rpm at the same speed and therefore it will redline quicker (lowering your top speed).
Adjusting the final drive alone can have an enormous effect on handling! How does it do this? For starters the final drive is directly related to the LSD settings, changing one often requires changes to the other.
A numerically higher final drive ratio makes the car more less agile at corner entry, more stable through the corner, less prone to oversteer on corner exit, can have a large impact on acceleration, can slightly shorten the spacing between gears, and can noticeably shorten braking distances before the corner. The disadvantages of a numerically higher final drive are a lower top speed, the car becomes more difficult to turn (similar to increasing the LSD decel setting), you could be shifting more often, and the car is going to be more likely to spin the tires in lower gears or at lower speeds.
A numerically lower final drive is generally preferred for racing as it allows the car to turn in easier and allows for adequate top speed even with a direct drive or short overdrive top gear. The drawbacks are decreased braking performance, the car being more responsive can make it more unstable at all points through a corner, it will usually require higher LSD settings to compensate for the lack of natural driveline lockup, and may become more prone to sudden oversteer on corner exit particularly at low speeds (the outside rear tire will break loose more quickly when applying the throttle), and in event of the car destabilizing it can be more difficult to regain control without coming to a complete stop (this effect is more pronounced in LMP cars than heavier/less powerful GT cars).
Much like suspension tuning, the balance between natural driveline lock and a "loose" more responsive car is largely driver preference. I personally tend to lean more towards a numerically higher final drive than most others. Some of this is probably because I never ABS/TCS even with LMP cars so the extra driveline lockup helps keep the car stable under braking and the gearing helps slow the car down more without me having to use more brake pressure that could potentially cause the car to skid off the track; this also keeps the car more stable through the corner as I roll into the throttle without TCS, in the event that the rear tires start to spin I can quickly let off the gas to make the driveline lock up and prevent the car from spinning out. In general, lowering the final drive ratio numerically will require you to raise both the accel and decel settings of the LSD to compensate for the lower driveline lockup and, with more extreme changes, can require you to get on the brakes earlier as the drivetrain will not be helping slow the car down as much.

The Transmission
Manufacturers and aftermarket companies such as Hewland and X-Trac spend more money on R&D for their transmissions than most normal could possibly imagine. A tough number is anywhere from 50-90% of what is spent on the engine, so when merely parts and labor of some these engines can easily exceed a million US dollars each not even accounting for what was spent on R&D you almost would not believe the price tag of a custom clean-sheet design from X-Trac or Hewland. The engineering and development is often in the millions and each transmission can cost upwards of $100,000+ a piece; figure that most teams keep 2-3 spares on hand with the assembly line is on speed dial for more and you can start to get an idea for the amount of money invested in these gearboxes. My point here? They paid teams of crazy smart people millions to develop the closest possible thing to perfect ratios for the application, chances are you won't be able to do much better than what was already designed for the car. Not saying improvements can't be made as teams adjust their ratios for every track and there are always design limitations that don't exist in Forza Land, but for theory part the guys in white lab coats got the stuff figured out better than some casual video gamers haha!
With that in mind, you'd be very wise to somehow document or chart real world ratio setups from performance gearbox companies. Hewland, Quaife, Richmond, and Rockland are some of my preferred sources when the manufacturer's own design is not being used; you won't have much luck with X-Trac so don't waste your time there, stick to Hewland for the most part at the highest levels and you'll be pretty well off.
For racecars, only use the road going model's transmission setup when absolutely necessary as it is otherwise lacking compared to what can be accomplished with a custom setup. The exception here is primarily the Porsche 911 GT3, the Cup and RSR variants come from the factory with gearing optimized for track use and they'll usually only need final drive changes for the current track. Although, in cases where I've added quite a bit of power or otherwise find the Porsche setup unsuitable, I do swap in Quaife close-ratio gearing sets (again with creating a small personal database of transmission ratios, it is REALLY worth the time investment once you get to the point that you're seriously tuning the gears of all your racecars).
There are so many variables to adjusting individual gears and how close to set your ratio spacing that it is best explained in the examples rather than trying to detail the information in an overall fashion. The bottom line is it boils down to a case-by-case basis when dealing with racecars; you can tune similar cars similarly but a GT2 Viper and a Ferrari F333 are going to be worlds apart in the way you go about dealing with them.

#40 Ford GT Mk7
The first example in going to present is one of the easiest to understand: it has a wide, stable chassis and a broad powerband that allows for a large variety of gearing combinations that could all work effectively. Unless you
Do something drastically wrong like having it redline in 6th gear at 80mph, it's hard to mess this one up too much.
Remove all upgrades
Tire pressure: 28.0/28.0
Front Camber: 1.1
Rear Camber: 0.9
Toe: 0.0/0.0
Caster: 5.5
Front ARB: 21.0
Rear ARB: 20.5
Front Springs: 677.0
Rear Springs: 827.5
Ride height: 3.4/3.4
Front Rebound: 8.2
Rear Rebound: 10.0
Front Bump: 5.4
Rear Bump: 6.6
Front Downforce: 253
Rear Downforce: 677
LSD: 40%/40%
I'm well aware these settings are far from an optimal suspension setup, however using them gets everyone on the same page so that way my description of gear ratios affecting handling don't do something entirely different for someone who has their car setup differently. We're not setting record laps here, just seeing how the gears change the handling and feel.
The stock gearing is 2.73/2.29/1.94/1.70/1.50/1.35 with a 2.24 final. I'll admit this is a little on the goofy side with the 1.35 6th gear and a bit unorthodox compared to what I'd personally select, but just run the stock setup to get a feel for a very low final with numerically high transmission ratios across the board.
I suggest taking the car to Laguna Seca. As you take a few preliminary laps you'll likely find 1st bogging down in the corkscrew and last corner while the gears seem just overall too close together. On Laguna Seca you'll pretty much want to be redlining 5th or barely get into 6th on the straight in anything R3 or above. Trying to ring out all 6 gears here can cost you time as you'll be packing them together ultra-close to the point that you're just shifting excessively with no real benefit; plan to only realistically use 5 gears on this track with 6th mostly there if you catch a draft on the straight during a race.
The default Ford GT gearing is a case where your overall ratio is good for the track but the gears are too close together. You drop below the powerband in 1st while the rest of the time the spacing is so close you don't get much chance to use the broad power curve of the engine and you end up shifting more than you need to.
How do we fix this problem? Simply raising the final ratio could fix 1st gear but will ultimately make the excessively close spacing that much worse. That means we need to start adjusting individual gears, and in the case of this car I think the default setup is total garbage so we're going to adjust all of them.
For the Ford GT and GT40 I use an RBT 6-speed transaxle ratio chart, this gearbox is intended almost exclusively for GT40 replicas/kits (being an evolution of the original ZF unit from the GT40) so it obviously fits this application very well with lots of ratio options as icing on the cake.
The default setup has the overall ratio in 1st at 6.115 and I want this numerically higher to keep the engine from bogging down, but the overall ratio of 3.36 in 5th gear is right where I want it so I'm going to try to preserve that number as I make adjustments. I installed a 2.89/2.06/1.47/1.18/0.96/0.85 setup with a 3.44 final that keeps a 3.30 overall ratio in 5th, jams 6th close so if I should need it I'm already at relatively high engine speed to maximize any short draft I might catch, and makes a 9.94 overall in 1st gear that should really help coming out of those slower corners.
Alright, after a lap or two we find 1st gear is now
Almost useless as it maxes out too quickly and the gears are now a little too far apart causing the engine to drop below the powerband on upshifts. Let's switch to a 2.42/1.72/1.32/1.09/0.96/0.85 setup and keep the 3.44 final. 5th and 6th a short but we aren't too concerned with either of those gears on this track, let's get the first 4 where we want them and we can revisit 5th and 6th later. As I drive it more I'm feeling like there's more in this engine, it just needs more final drive to wake it up!
Leaving the transmission alone, I took the final drive up to a 3.77 and even though 1st gear became relatively worthless again, the acceleration almost feels like a different car entirely at this point. I took the LSD accel up to 45% to help stabilize the rear when I slam the gas pedal coming out of corners and I lowered the decel to 30% so the carb turns in a little easier. At this point you'll likely notice a car that is more prone to under steer on entry but is less likely to go into a slide mid-corner when coming into the throttle as well as notably quicker in acceleration. You kind of have to muscle it into the corner at first but as you get on the loud pedal it is more planted, more stable, and gets to the next bend quicker. By now my lap times are right around 2 full seconds quicker than when I started and that's from only gearing/LSD adjustments! 2 seconds without touching a single suspension setting!

I have other cars I can do if you guys would like me to, especially cars with peaky high-revving motors and/or turbochargers as they tend to respond to gear changes a little differently.

Edited by user Wednesday, April 29, 2015 6:27:11 AM(UTC)  | Reason: Not specified

Rank: S-Class Racing License
#2 Posted : Wednesday, April 29, 2015 6:35:46 AM(UTC)
Very nice write up, thanks for sharing!

Bigtime Muscle Member | Now on One!
Rank: A-Class Racing License
#3 Posted : Wednesday, April 29, 2015 10:57:10 AM(UTC)
enzo guy. does this car come with a race gearbox (fully adjustable)?

If it does then i like what you are doing. If not then theres a little bit i would do differently. I wouldn't add the race gearbox and would do it all through the sport gearbox and only adjust the final drive.

You were saying 2-6 were too close together but 1st is too long. I would lengthen the final drive to open up gears 2-6 which would really lengthen 1st gear making it completely useless. Which is fine, I will use it at launch and never see the gear again. Also lengthening the gears will probably make 6 no good for the track, also fine by me. Then i would go back and adjust the final drive ever so slightly after a few laps to make sure im not shifting right before my braking point or requiring a shift mid turn, and done.

PTG Home

Race Team
Rank: Driver's License
#4 Posted : Wednesday, April 29, 2015 2:28:14 PM(UTC)
Originally Posted by: PTG Baby Cow Go to Quoted Post
enzo guy. does this car come with a race gearbox (fully adjustable)?

If it does then i like what you are doing. If not then theres a little bit i would do differently. I wouldn't add the race gearbox and would do it all through the sport gearbox and only adjust the final drive.

You were saying 2-6 were too close together but 1st is too long. I would lengthen the final drive to open up gears 2-6 which would really lengthen 1st gear making it completely useless. Which is fine, I will use it at launch and never see the gear again. Also lengthening the gears will probably make 6 no good for the track, also fine by me. Then i would go back and adjust the final drive ever so slightly after a few laps to make sure im not shifting right before my braking point or requiring a shift mid turn, and done.

Yes, since the #40 Ford GT is a GT2 class racecar it comes with a fully adjustable race box by default. II think the only GT racecar that does not is the GT3 class #66 Porsche 996 GT3 Cup but it still has an adjustable final by default and also, being a Porsche GT3, comes from the factory with a gearbox optimized for track use so the ratios shouldn't need much (I believe Porsche Motorsport used their 3.17/2.14/1.72/1.35/1.12/0.93 in the 996 GT3 Cup but knowing Turn 10 I wouldn't be surprised if they mistakenly used the GT3 RSR 3.15/2.44/2.00/1.64/1.38/1.16 setup in that car, either way the 996 GT3 Cup does well with a 3.44, 3.77, 3.88, or 4.00 final depending on track).
This guide is primarily intended geared (no pun intended) towards GT1/GT2/GT3/GTS/LMP racecars since they're much more sensitive to gearing changes than a standard road-legal car and (minus one Porsche and a couple Group C cars) have fully adjustable gearboxes as standard equipment. Most road-legal production cars in S700 class or below can get by with the stock transmission setup (not to say there isn't room for improvement but it isn't always worth the point for the race box unless it's a drastic case like the Bel Air and '54 Corvette with their 2-speed Powerglides) or if you have a race box on a road car you can often just use performance transmission setup from the manufacturer or an aftermarket company that caters to that car; the final drive can usually be set to the stock value or something otherwise available as an option, from a similar car, or on the aftermarket. So if I have a '54 Corvette I'm going to almost always use a close-ratio 2.29/1.61/1.21/1.00/0.80/0.68 setup from the T56/TR6060 with almost endless final drive ratios to choose since it's a GM rear end. Something like an E34 M5 will likely be left alone because even though the 1st gear is a worthless launch gear, it's otherwise an excellent transmission setup that is not worth the PI hit to reconfigure beyond a simple final drive swap in some cases (although dumping the E34's lazy stock final for a 3.73 or 4.10 can really wake that car up in A600 class).

Either way, road cars and racecars are two totally different animals when it comes to tuning the gear ratios. Road cars generally follow the "shift just past max horsepower and let the rpm drop to right around max torque" philosophy; racecars run ratios spaced much closer together in order to keep the car close to max horsepower at all times (among other reasons such as not losing time having to downshift should you be a gear too high, the close ratios can often save you there). I mean you can find very closely-spaced road car gearboxes like the Muncie M21/M22 (2.20/1.64/1.28/1.00), certain Getrags in older BMWs, some of the NISMO units, etc but generally speaking the ratios that would be adequately competitive in a road-legal S700 or lower class car would likely leave your R3/R2/R1 true racing car trailing behind the pack (unless of course it's a Viper, the ratios don't mean quite as much when you've got a 8.0L V10 making over 500ft-lbs at 3000rpm or lower and the Tremec 6-speed from the road-going model is pretty much the only thing around that won't explode when hooked to that engine).

To quickly address merely changing the final drive ratio on the GT2 class Ford GT, changing the final drive only would not really yield great results as the transmission ratios themselves are the bigger problem. With a 1.35 6th gear you can't do a whole lot of shortening with the final before you find yourself redlining in 6th and with that default 1st gear and final drive combination the car doesn't even really get into the powerband until about 50mph or so. When you're talking about GT2 class racecars they're close enough in performance that bogging down out of a corner can result in you getting passed; I used Laguna Seca specifically to somewhat exaggerate the effect knowing that it would bog through the corkscrew, makes for a good example of 1st gear being too wide. Shortening up the final would help 1st gear (although not really feasible on a longer track without losing top speed due to that 1.35 6th), however it would make the problem of the excessively close 2nd-5th gears even worse as you'll be shifting even more often than previously. If you brought the final drive up to somewhere around 2.65-2.90 with the default gearbox you could potentially just start skipping gears on the upshift (going straight from 1st to 3rd or 2nd to 4th). The gears would be so close you are starting to get close to the point at which you're actually spending more time shifting between gears than actually accelerating in each gear, definitely not a good thing as you end up losing time so only shortening the final would not truly fix the problem with that car and could potentially cause you to run slower lap times.
Widening out the final drive (not really much room to do that as it's already almost at the lowest end of the slider bar by default) would make it below 60-70mph your car would have about the acceleration of a C425 car at best. As you said, 1st gear would be useless, but instead of it just being that it redlined too quickly it will be too wide with no lower gear to help it on Laguna Seca where having no acceleration out of low-speed corners is going to have you in last place rather quickly

Edited by user Wednesday, April 29, 2015 2:49:19 PM(UTC)  | Reason: Not specified

Rank: C-Class Racing License
#5 Posted : Wednesday, April 29, 2015 8:59:33 PM(UTC)
For the Ford, just run gears 1-4. Make 1st go up to around 80, then spread the last 3 out evenly, and then adjust the final drive for the track.