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Fast Car Gearbox Terminology Guide

Fast Car Gearbox Terminology Guide

Posted by Glenn Rowswell on 21st November 2017

If you’ve saw our Fast Car Gearbox Guide last week, you should have a good idea on why gearboxes are important and the best one to suit your needs.

Although, once you get in to modified gearboxes, things get a bit more complex. So here’s a run down of some of the common terms, and why they could be good for your car…

Fast Car Gearbox Terminology Guide

What is a dog gearbox?
A dog box is a manual transmission, either conventional H-pattern or a sequential, that doesn’t have synchromesh, which helps prevent you from crunching your gears on a normal production box.

Synchros sound like a great thing to have, and they are on a road car. But they also slow down maximum shift speed, and forcing a box into gear faster than the synchros want you to means they rapidly wear out. So on a racecar, where all you want is the fastest gear change, a dog box is the thing to have.

It certainly takes some practice to use a dog box without constantly crunching the gears, especially when driving slowly, which is often why so many dog boxes fitted to road cars wear out so quickly. But once you get used to the precise shifting and rev matching needed, dog boxes transform your gear changing.

Fast Car Gearbox Terminology Guide

What is dog-leg gearbox?
This is something people often mix up with dog boxes. But aside from them both being most useful in race situations, they’re totally unrelated.

A dog-leg gearbox has the gears positioned differently to most conventional boxes. First, 3rd and 5th are at the bottom of the shift pattern, while 2nd and 4th are at the top. But because 1st is to the left of the others, so 2nd and 3rd are opposite each other, and 4th and 5th are too. (Unlike a conventional gearbox where 1st and 2nd are opposite, and 5th is on its own.)

This is because in most circuit racing, 1st gear is used for the standing start only and never used again. So 1st being out the way and a more direct shift from 2nd to 3rd, and 4th to 5th, is much more useful.

Fast Car Gearbox Terminology Guide

What are straight cut gears?
Production car gearboxes use helical gears, which means each tooth on the gear is at an angle. So the teeth of two gears mesh together gradually, making them smooth and quiet, especially when using a large number of small teeth – exactly what’s wanted for production road cars.

The main disadvantage of this setup is it’s not the strongest gear design, meaning they can be broken easily with big power and hard use. While having helical gears with thicker teeth mounted at a less extreme angle is certainly stronger than the standard ones, most gear upgrades tend to skip the semi-helical gears and go for straight-cut ones.

Straight-cut gears are exactly what they sound like, with the gear teeth positioned perfectly straight. This tooth style, combined with most straight-cut gears having thicker teeth than production-car helical gears, makes for a gearbox able to take a lot more power and abuse. Though that’s at the expense of a lot more noise – sometimes so much so it will even drown out a very loud exhaust system!

What are transbrakes?
While automatic gearboxes aren’t very popular in the UK tuning scene, there’s a wide variety of upgrades available for them, and the coolest is a transbrake.

A transbrake setup allows, at the push of a button, both a forward and reverse gear to be selected at the same time, effectively locking the transmission solid.

This allows full power and full load to be applied without the car moving at all, which while useful for launching in general, allows turbo cars to sit at full boost on the start line.

This not only sounds insane as the turbo spools up, but means the minute you let go of the transbrake button the car either launches like an absolute rocket if you have the grip to match. Or does the biggest, craziest, smokiest, rolling burnout you’ve ever seen in your life.

Log on to YouTube and type in “Transbrake power skid” and you’ll find no end of mad Aussies doing ridiculous rolling burnouts on the street.

Fast Car Gearbox Terminology Guide

What are transmission losses?
We’re sure you’ve seen rolling-road power figures quoted as power at flywheel, and power at wheels, with the wheel figure always being significantly lower. This is due to transmission losses – ie the transmission – particularly the gearbox, which absorbs some of the power the engine makes.

This is a controversial subject that always sparks huge debate, particularly as different rolling roads, different tuners, even different groups of owners, estimate hugely different percentages of transmission loss. The key word here is estimate. Rolling roads can only accurately measure wheel power, and flywheel power is nothing more than an estimate – albeit a complex one using various measurements from the rolling road run.

Most people tend to say that 2WD manual cars lose around 10-15 per cent through the transmission, and 4WD lose around 30-35 percent. But we’ve seen rolling-road graphs claiming losses as high as 30 per cent even for a 2WD manual car, which sounds more like artificially inflated figures to impress the customer to us…

The moral of this story is, don’t worry too much about numbers printed on a bit of paper. It’s only power at your wheels that makes your car fast, no matter how much is claimed at the flywheel. And regardless of any of it, it’s how fast your car is on the road, track or strip that really counts!