Q: “I’m currently reading Skip Barber racing school’s book “Going Faster” and came to the chapter about shifting. The author wrote that race cars usually don’t have synchros, and thus require double-clutch for downshifting. Since the book was written in the ‘90s, I’m wondering if it’s still true for race cars nowadays? Is it a required technique if I plan to develop my racing career in the future?”
A: Double-clutching is not needed, even in a non-synchro “dog” gearbox. Absolutely, heel and toe is, but there’s no need to double-clutch. Having said that, knowing how to do it does help a driver understand the reason and timing for rev-matching. But it’s not important.
For anyone reading this who are not familiar with double-clutching, here’s how it works: Approaching a corner where you have to downshift, you depress the clutch, move the shifter to neutral, let the clutch out, blip the throttle (rev the engine), depress the clutch again, move the shifter to the next lower gear, and then let the clutch out.
This technique was needed when downshifting in old cars without synchromesh transmissions, so unless you’re racing a vintage car, don’t worry about it. Even if you’re driving a car with a dog-ring (non-synchro) gearbox, don’t use it. But you absolutely need to rev-match on the downshifts, which requires heel and toeing.
What happens when doing a single clutch heel toe in a dog box? Would there be a jolt from the layshaft speed mismatch? Is it the case that it’s not significant enough to lock the wheels?
The reason for blipping the throttle when heel and toeing is exactly for that reason – to match the revs of the engine with the drivetrain so there will not be a “jolt.” It’s the same for a dog box or synchromesh transmission. The throttle blip makes it smooth in either gearbox. At least, it should if you time it right and give the right amount of blip. And this is the same whether single of double clutching.
Hey thanks for the reply Ross! I guess what I mean is, if the purpose of double clutching is to match the layshaft speed to the gear speed so they can mesh smoothly without synchros, how does the gear manage to engage without double clutching and does it result in less smooth of a shift? I understand that the blip matches the engine speed so when the clutch re-engages, the inertia of the engine doesn’t create a jolt. But when blipping with the clutch pressed, if the layshaft isn’t connected to the engine, it’ll be out of sync with the gear won’t it?
A dog box doesn’t have a lay shaft. That’s where the dog rings come in (and I’m not able to explain how a dog box works without illustrations or actually opening one up). The blip matches the engine revs with the output shaft (which, essentially, are the driving wheels).