Blog Fast Cars Random Stuff Explained by Robert Pepper 2 January 2023
Retired engineer slow-roasts commenter
My little corner of the Internet is generally a quiet place.
The comments tend to be well-written and interesting. Sometimes they’re questions, sometimes constructive criticism, and other times they are so useful I make blog posts out of them.
But there is one rule which seems to hold true, and that is:
“The level of knowledge of a commenter is inversely proportional to the politeness of the comment”.
So, the ruder the comment, the more likely it is that the commenter is a lot of smoke but no fire. This is particularly true when the commenter is asked to elaborate on their initial point. However, I always ask for more information, as I did when H T commented on my heel and toe shift video:
You’ll note two points:
a) the part about no mechanical engineering sense – that’s an insult
b) the comment doesn’t actually say what’s wrong.
Already I know the commenter is likely to be a person challenged not only by automotive understanding but decency. But before I had a chance to go too much further, Contributor replied and what a reply it was! I reproduce it in full below.
“@ H T If you are going to make ‘clever’ comments on the YouTube channel of a very well credentialed automotive journalist like Mr Pepper, you should be aware there will people like me who follow his channel. Before I retired a while ago, a good part of my job was providing expert advice for investigations involving complex engineering and to put expert evidence before our court system – this after many years of working hands-on as an engineer and research engineer. I have taken the liberty of copying and pasting your original comments into my response, that way should you choose to try to go back later and delete or edit them to correct some of your incorrect statements you have made they will stand here in the form I replied to.
“H T The gearbox to road wheel is a fix ratio. You have to match the engine speed to the gearbox. In a car you may or may not have synchronisers to match any speed differences. In a lorry it is harder where the mass of the gearbox slows down the gearbox speed so much once you push in the clutch. That’s why you have to double de-clutch.”
I’ll come back to your first point.
Your comment that the ‘mass’ of heavy vehicle gearboxes ‘slows down the gearbox so much when you push in the clutch’ is utterly wrong – the complete opposite is the case. The rotational inertia of heavy vehicle gearbox internals is far higher than on light vehicles, so the gearsets in a heavy vehicle gearbox take far longer to slow down or build revs (the reason an engine with a heavy flywheel behaves the same). Anyone who has studied basic school-level physics knows this. You double de-clutch heavy vehicle gearboxes to control that increased rotational inertia using engine speed, and because synchromesh is rarely fitted to anything other than light trucks due to the fact it struggles with such high rotational inertia. This is the reason why many manual car drivers ‘crunch’ the gears even when selecting first gear from rest in a heavy vehicle – the gearsets haven’t been given anywhere near long enough to dissipate their rotational inertia.
“The gearbox to road wheel is a fix (sic) ratio”.
Wrong again. A modern constant-mesh gearbox is comprised of input, intermediate gearsets (sometimes referred to as ‘lay gears’), and output gearsets. All of the gears are in mesh at all times; except most commonly reverse intermediate gear. You select a ratio by locking one of the output gearsets to the gearbox output shaft via the selector hub, selector ring, sprags (or other detents) and, if fitted, synchromesh – all the other ratios continue to rotate but they are freewheeling on the output shaft. ONLY the output shaft, selector rings, selector hubs, detents, and synchro rings (if fitted) have a fixed ratio relationship with the drive wheels and there are exceptions to this too; for instance two speed differentials, some very modern front-wheel-drive transmissions that use two different final drive gearsets and four wheel drive vehicles with a secondary transfer case. If you select neutral while a car is in motion you are unlocking all gearsets from the output shaft. Incidentally the only time PARTS of the gearbox geartrain are operating completely independently, unlike your comment elsewhere, is if you have selected neutral AS WELL as depressing the clutch – otherwise gearsets remain connected via a selector ring to the output shaft and these are being driven by the wheels via the final drive.
You aren’t “matching the engine speed to the gearbox”, you are matching the engine speed and the input and intermediate gearsets of the gearbox to the speed of the gearbox output shaft (mostly dictated by wheel speed). When you engage the clutch with the gearbox in neutral, it is to attempt to use engine speed to match the speed of the input and intermediate gearsets to the selector ring and hub, and therefore the output shaft. On heavy vehicles this helps avoid damage to the dog teeth the selector ring engages with, on light vehicles it makes for reduced synchromesh wear and smoother gear changes. Mr Pepper’s statement was quite correct – yours is wrong on pretty much every level.
“H T Yes I have a question? How did you produce that video without any mechanical engineering sense”.
Mr Pepper will understand all of this very well but simply won’t have had the time to write such a detailed and rather boring response – not to mention redundant, explaining all of this in a video where he no doubt assumes people will understand the basics of gearbox operation. Retired nuisances like me, however, don’t have to worry too much about that. So next time you feel tempted to insult Mr Pepper’s level of ‘mechanical engineering sense’, I suggest very strongly you know your facts first. Have a good one – I have no doubt you will.”
So a few things I’d like to say after reading that. First, thank you Contributor for your explanation. I didn’t know all of that, so I learned something.
My reason for not going into great detail on the topic above was because it wasn’t necessary. The video is about heel’n’toe shifting as a practical technique for drivers, not a detailed technical explanation of how and why it works for engineers. Most people understand that, but there are a few people who try and show off their knowledge by nitpicking semi-relevant points of order.
This brings me to another dilemma; the degree of simplification. Often I will simply things, and am picked up on them; for example I’ll talk of ‘drive’ where I should use torque, refer to kg of force not newtons and so on. Generally I find that the people who notice such simplifications don’t need the explanation in the first place, so it works out, and those that do notice and point it out, actually do need the explanation and a quiet word to the effect that a little knowledge is a dangerous thing and trying to show off ends in tears.
Anyway, thanks again to Contributor and all the other experts who comment on my work; it is appreciated!
Here’s the video in question: