Practical Transmission Tuning: Gear Ratios

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Gear Ratios

13.59

00:00	We've already talked about gear ratios a little when discussing planetary gear set operation, as well as how we achieve different gears, but let's go a bit deeper on the subject of gear ratios, since they're central to transmission tuning.
00:12	When talking about transmissions, we use the word gear a lot, but depending on the context, it can mean different things, plus the numeric portion can become a little bewildering until you get comfortable with it.
00:23	So, let's work on clearing up any confusion about gearing now.
00:27	First, there's the use of gear that nearly everyone already understands.
00:30	When we discuss driving an automatic equipped vehicle, we might talk about shifting from second to third gear.
00:36	In that instance, we're not referring to actual gear ratios, but it's a simplified way to call out the second and third ratio options in the transmission.
00:44	This brings us to the next definition of the term gear.
00:47	In this case, if we look inside a transmission, we see physical gears, which when combined with other gears, makes up a gear set.
00:54	When we're talking about gear sets, we could be referring to a manual gear set, which is essentially a stack of individual gears like we see here.
01:03	Or we could be talking about a planetary gear set, which, as we briefly looked at in the previous module, is a combination of gears which can function together in different ways depending on which of the components are allowed to move or is held stationary.
01:18	Next, sometimes when people say gear, they're really talking about a gear ratio.
01:22	Gear ratio describes the relationship of rotational speed between the input and output of a component system.
01:30	For example, a three to one ratio means that for every three revolutions the input makes, the output completes one revolution.
01:37	Here we can see as I rotate, my input speed and the output speed are not the same.
01:43	On that subject, let's review under, over, and direct driving the transmission output.
01:48	The three to one ratio we just looked at is a perfect example of an underdriven gear ratio.
01:55	When calculating gear ratios from tooth counts, remember you divide the output tooth count by the input tooth count, but then the gear ratio is stated as input rotations to output rotations.
02:09	For example, an output tooth count of 60 divided by an input tooth count of 20 results in a three to one gear ratio where three revolutions of the input results in one revolution of the output.
02:24	The input tooth count is lower than the output tooth count.
02:28	So, when you divide the output tooth count by the input tooth count, the first number in the calculated gear ratio is higher than the second.
02:38	If we're talking about a rear differential in a rear wheel drive car, a three to one rear end ratio will make three rotations of the drive shaft and turn it into one rotation of the wheels and tires.
02:49	If we're talking about a transmission gear, a three to one gear ratio would take the engine input through the torque converter into the transmission, then reduce it to one third speed and send it out of the transmission toward the differentials.
03:04	A three to one gear ratio would be a low number if we were looking at it on a shifter, perhaps first gear, not fourth gear.
03:09	To highlight and hopefully avoid a point of potential confusion, when ratios are being discussed, a high numbered ratio, like say four to one, is generally associated with a low gear number, perhaps first gear.
03:23	The inverse is also true.
03:26	A low gear ratio value, like perhaps 0.8 to one, will be associated with a high gear number, perhaps sixth gear in a six speed transmission.
03:35	Underdrive ratios with high ratios and low gear numbers on the shifter have improved mechanical advantage, higher axle torque, and higher potential for acceleration, but they have lower maximum vehicle speed at maximum engine speed, higher engine speed during cruising, and are generally worse for fuel economy.
03:55	The lower potential vehicle speed associated with higher gear ratios is the reason these ratios are sometimes referred to as shorter gears.
04:05	It isn't particularly intuitive when you first hear it, but I suspect the slang may come from higher gear ratios being conducive to shorter racetracks with lower top speed potential.
04:16	Now, let's take a look at overdrive ratios.
04:20	Here we have a lower input tooth count than output tooth count.
04:24	So, the first number in the ratio is lower than the second, 0.8 to one, for example.
04:30	This means for every 0.8 revolutions of transmission input, the transmission output will spin once.
04:37	Again, this is what we'd refer to as a higher gear number, meaning when we talk about a six speed transmission, for example, fifth gear would be higher than third gear.
04:47	Again, this is not to be confused with the gear ratio of fifth gear, which has a lower input side number than third gear.
04:57	Overdriven ratios result in reduced mechanical advantage, axle torque, and acceleration, but support higher vehicle speed at maximum engine speed and they reduce engine RPM during cruising, which can improve fuel economy.
05:10	The higher potential vehicle speed associated with lower gear ratio values is the reason lower ratios are sometimes referred to as taller or longer gears.
05:20	Again, it's not intuitive at first, but you can think of it like longer ratios are helpful on longer tracks where you can get up to higher speeds.
05:27	The top speed possible on a particular track is one of the key factors in determining optimal gearing for a given application.
05:35	So, we'll be coming back to this.
05:37	Now, that we have a good understanding of under and overdriven ratios, let's quickly discuss the one-to-one ratio, sometimes referred to as direct drive.
05:45	This ratio causes equal input and output speeds.
05:49	Many years ago, when there were far fewer transmission options, people used to say third gear was the one-to-one in an auto and fourth gear in a manual, but with the myriad of options available today, we really need to look at the actual gear ratios to see which gear is one-to-one.
06:06	Especially if you're working with a transmission with a high gear count, such as an eight to 10 speed transmission, it's extremely unlikely that third or fourth gear will be near one-to-one.
06:16	Generally, transmissions have more underdriven gears than overdriven ones.
06:21	So, if we don't have access to the gear ratios, in a pinch, we can guess that the gear number, which is about three quarters of the total gear count, may be one-to-one or close to it.
06:32	For example, in an eight speed, three quarters of eight is six.
06:37	So, sixth gear might be the one-to-one gear in that transmission if I had to take a guess.
06:43	Three quarters of a 10 speed doesn't work out to a whole number, but it's seven and a half, and I guess seventh gear is possibly the one-to-one gear.
06:51	As always, it's much better to know than to guess.
06:54	So, if we have the ratios available, we should always use them.
06:58	But in a pinch, this will usually get us close.
07:01	You've probably heard the one-to-one gear referred to as the ideal gear ratio for dyno testing.
07:07	In truth, it isn't always the best option due to practical constraints.
07:11	For example, if we have a long final drive ratio, which we'll discuss in a moment, and a low horsepower vehicle, a pull in a one-to-one gear may take a long time and put unnecessary strain on the vehicle and cooling systems.
07:26	On the other hand, we might have a high horsepower vehicle, which is getting wheelspin on the dyno and running it in a longer gear may be required just to get the tires to stick.
07:36	So,me transmissions may not use a one-to -one gear ratio at all.
07:39	So, it won't be an option, but we'll likely be able to select a ratio in that ballpark for dyno testing.
07:45	Moving on, we've been talking about individual gear ratios and transmission and output speeds.
07:51	What's important to remember that this isn't the entire gearing system in many vehicles.
07:56	Between the transmission output and the vehicle's final output, whether that be to tires on the road, a propeller on a boat or plane, track on a sled, et cetera, there are often additional items which further alter gearing before the torque gets to the end of the powertrain.
08:12	For example, if engine speed is 6,000 RPM, the torque converter is locked and the transmission is in a one-to-one gear ratio, we have transmission output speed of 6,000 RPM.
08:25	If we provided that straight to the wheels without another gear reduction, we'd have wheels rotating at 6,000 RPM.
08:33	That might not sound wild offhand, but on a 28-inch tire, wheels going 6,000 RPM would have a wheel speed of about 500 miles an hour.
08:42	To further gear vehicles to more reasonable speed ranges, additional gears are present, usually in differential housings.
08:50	In a rear-wheel drive application, the rear differential gear ratio has a large impact on the speed of the vehicle for a given engine speed and transmission ratio.
09:00	Ratios used are most often between two-to -one and five-to-one, though there are certainly exceptions.
09:05	This ratio is called the final drive ratio and is the final gearing change in the torque path from the engine to the end of the drivetrain.
09:14	Here's an example of what a one-to-one transmission gear would look like on a 28 -inch tire with more common final drives of 3.0 or 4.11, which is 4.11.
09:26	The x-axis is transmission input RPM and the y-axis is wheel speed.
09:31	I've used transmission input RPM here rather than engine speed because transmission input speed is the one we can count on to make this calculation work.
09:40	In a situation where the torque converter clutch is locked up and holding, transmission input RPM will also equal engine RPM, but when the torque converter is slipping, engine RPM and transmission input speed are no longer equal.
09:55	Data logging both is always recommended to see what the difference is under various conditions.
10:01	That will help inform your gearing decisions and give you a better idea of how the two speeds relate.
10:08	Once you know the relationship between engine speed and transmission speed on a certain vehicle for a certain set of conditions, you can create a table like this, which shows approximate engine RPM versus wheel speed.
10:20	Here I've taken the same data and replaced logged engine RPM for a given transmission input speed.
10:27	For this example, the pull started at 1 ,300 RPM stall speed, max torque converter slip occurred as the vehicle takes off, and the slip reduces as the engine revs out until there is no slip at 6,000 RPM.
10:40	At 6,000 RPM, engine speed with a one-to -one transmission ratio, a 28-inch tire, and a 3.0 final drive results in a wheel speed of 166.6 mph, while the 4.11 final drive produces 121.6 mph wheel speed, both far more reasonable than 500 mph.
11:03	You may be wondering why I'm labeling these graphs wheel speed rather than vehicle speed.
11:08	Wouldn't wheel speed be the same as vehicle speed? In many cases they are the same, but wheel speed can differ from vehicle speed during a loss of traction, and for that reason, you'll notice I use each specifically as we continue through this course.
11:22	The most common examples of loss of traction are wheel spin during attempts at aggressive acceleration or lockups during braking.
11:30	This is important to understand because when we get to talking about shift scheduling, which is the process of deciding when to up and downshift, it's often written in software based on vehicle speed.
11:42	For example, at 20% throttle and 50 mph, shift from 2nd to 3rd gear.
11:50	The reality is, shift tables labeled vehicle speed are usually based on driven wheel speed, so when driven wheel speed doesn't match vehicle speed due to a loss of traction, just remember shift points are following driven wheel speed, not vehicle speed.
12:07	With that in mind, data logs of shifts occurring during a loss of traction will make a lot more sense.
12:13	It's also important to keep in mind that any error in the ECU or TCM's calibration of gear ratios, final drive, tire size, vehicle speed sensor, or related functions that cause incorrect calculation of presumed vehicle speed can lead to unwanted shift behavior because shifts will no longer occur under the correct conditions.
12:34	At this point, hopefully some questions about gear ratios have been answered, so let's review.
12:40	There is a difference between gear count, as in how many gears we can select in our transmission, gear number, as in the number of the gear the vehicle is currently in or shifting into, and gear ratio, determined by the input and output tooth counts, which results in a change in speed between gear input and output.
13:01	Underdrive and shorter gears, like a 3 to 1 ratio, improve acceleration through torque multiplication.
13:08	Overdrive and longer gears, like 0.8 to 1 ratio, allow for higher top speed and better fuel economy while cruising.
13:15	The final drive ratio is generally equivalent to the axle ratio, because the axle ratio is generally the last gearing change in the torque path from the engine to the end of the drivetrain.
13:27	Transmission gear ratio, final drive ratio, and tire size all play a role in the transformation of transmission input speed to driven wheel speed.
13:38	Torque converter slip causes a difference between engine speed and transmission input speed, but while the torque converter is locked up, both speeds are the same.
13:48	The relationship between wheel speed and vehicle speed generally matches while the vehicle has sufficient traction, but may not match during a loss of grip.