SundayDriver

While much of this does not apply to many of you, I think it is interesting, none the less:

By William Hewland.
July 2000

The following is some info regarding shifting gear and face dog wear. I am in the fortunate position where I have a good amount of knowledge on the subject, as I understand the mechanical side and the user (driver) side equally well.

N.B. For succesful gear shifting, remember that it is critical to ensure that all mechanical elements between the drivers hand and the dog faces are in good order and properly set. This includes the gear linkage in the chassis!

Successful up-shifting, (defined as fast and non dog-damaging) will be achieved by fully moving the dog ring as rapidly as possible from one gear to the next, preferably with the engine's driving load removed until the shift is completed. (The opposite is true of a synchromesh gearbox as used in passenger cars, where slow movement helps). It should be remembered that it is not possible to damage the dogs when fully engaged (in gear). The damage can only take place when initiating contact during a shift, (the `danger zone`) therefore this element must be made as short as possible. If a driver moves the gear lever slowly, or if the linkage is not rigid and effective, dog wear will occur. We always recommend lightweight yet solid rod linkage, not cables ideally.

I list below the different methods of up-shifting that are used in racing most commonly. The best at the top, the worst at the bottom:

Automated (semi automated). The movement of the dog ring is powered and the engine is cut / re-instated in a co-ordinated manner. Gear-shifts take milliseconds. This system produces zero dog wear when set up well. It is not applicable to most cars, but it illustrates that speed of shift is a good thing.

Manual with engine cut. This system is almost as good as an automated one as long as the driver pulls the lever very quickly. Again it is not applicable to many cars, but it illustrates that speed of shift is a good thing. A `cheat` version of this is to shift on the engine rev limiter, which can work well. With this system it is especially important to move the lever ultra fast, otherwise the engine will be reinstated during partial dog engagement, causing damage. The damage can usually be felt by the driver.

Manual.
Best method: With no assistance from the engine management, the driver must lift off the throttle sufficiently to allow the dog ring to be pulled out of engagement. He should then stay off the throttle long enough to allow the dog ring to engage with the next gear. In practice, the driver can move the gear lever faster than he can move his foot off and back on to the throttle. Therefore the effective method is to apply load to the gear lever with your hand and then lift the throttle foot off and back on to the pedal as fast as physically possible. In lifting your foot, the loaded gear lever will almost involuntarily flick to the next gear before the foot is re-applied to the throttle.
Another method is to load the gear lever with your hand, stay flat on the throttle and dab the clutch to release the dog ring. The overall effect on the gear shift is similar to the above method, but clutch wear may become a big issue.
The worst method (most destructive and definitely slowest) is to attempt to change gear in a `passenger car / synchromesh` way, i.e. lifting off the throttle, dipping the clutch, moving the gear lever, letting the clutch up and re-instating the throttle. The method causes unnecessary clutch wear, does absolutely nothing to help come out of gear and usually causes dog wear whilst engaging the next gear. This wear is due to several reasons. Firstly, it is impossible for a driver to co-ordinate the complicated sequence of all five physical movements accurately. Consequently the engagement dogs often find themselves engaging whilst the throttle is applied. The lever is usually pulled more slowly as it was not pre-loaded, lengthening the `danger zone`.

Successful down-shifting, has similar rules applied regarding speed of shift. Unloading the dogs is done in the opposite manner obviously. Whilst braking, the dogs must be unloaded by either touching the throttle pedal or- my preferred method- by dipping the clutch. However, one sharp dab of clutch or throttle is appropriate per shift. Continued pressure on either will cause dog damage for different reasons. `Blipping the throttle` just before engagement is advisable if the rev drops between gears are over 1300 rpm, as this will aid engagement and stabilise the car.

TOP TIP for ease of downshifting: Make the downshifts as late as possible in your braking zone (i.e. at lower road speed), because the rev drops between each gear are then lower. So many drivers make the mistake of downshifting as soon as they begin braking, causing gearbox wear, engine damage and `disruption` to the driving wheels.

This is a subject which can be much expanded on, but I feel that these are the basics, which I hope are of use.

William Hewland

RedlineMan

Thanks Mark;

As you say, only applicable to you race car drivers, but interesting nonetheless.

Incidentally, we sedan drivers can experience the "jump" out of gears that Mr. Hewland mentions. By pulling on the shifter early before declutching, the shifter will FLY out of its previous engagement. Not a bad thing, according to him.

Interesting.

DanS911

My guess is we are not talking about my 915 tranny are we

SundayDriver

But it makes me wonder, do any of the sedan guys modify the trans - kill or eliminate the synchros and remove some of the dogs for faster shifting? Can you even do that to a synchro transmission?

Tom Tweed

You need straight cut gears to shift like Mr. Hewland says, the helical cut gears in our "sedan" transmissions need the synchros to engage easily. The teeth on the dog rings are TINY compared to a Hewland. If you started removing every other one or something I think the others would sheer under load. The only way to do it reliably is to put Hewland internals in the Porsche box, which is not cheap.

BTW, congrats on your excellent finish at the Runoffs, Mark!

TT

SundayDriver

I had not even thought about the helical cut on the gears - Duh. Thanks for the info and runoffs comment.

APKhaos

Now there's a true pearl of wisdom. Early downshifts are a classic DE 'bad habit' that seems to go uncorrected. We see it a lot in higher group drivers.
Repeating the mantra "Brake like you mean it, get the gear and go" helps.

earlyapex

There was a very interesting shot of a Porsche Cup driver going up through the gears on Speed TV. He kept the accelerator pressed to the floor and shifted without lifting. I've heard of this technique to reduce the ET's in drag racing, but I haven't seen it in road racing.

MJR911

Here is another question... a lot of my old DE instructors have said don't bother going 5-4-3 on downshift and instead going right from 5th to 3rd. However, watching the best Supercup drivers today on Speed, they all hit every gear on downshifts. Any ideas?

38D

Wevo does a non-synchro conversion for th 915. I assume it's big bucks.

mpaton

Hewland's shifting advice has nothing to do with straight cut or helical gears and everything to do with dog face gear engagement versus synchro ring engagement.

While it's true that dog face engagement in racing transmissions is frequently combined with straight cut gears, that is usually because straight cut gears can be made with simpler machinery, absorb less power, and eliminate axial loads on the gear shafts.

Dog face engagement is usually used in motorcycle transmissions, together with helical cut gears.

Michael Paton

Tom Tweed

Yes, you're right, for any constant mesh transmission vs. sliding gear engagement, the arrangement of the teeth doesn't make any difference in shifting, it is the gear engagement mechanism that matters. I just always associate the Hewland large dog ring engagement with straight cut gears and no synchros.

Helical gears are usually chosen because they are quieter, and the straight cut gears because they are robust, easier on bearings, and simpler. The large dog ring engagement was developed by Hewland to eliminate the complexity of synchronizers and make ratio changes quicker and easier in the field, for racing purposes, according to my understanding, and it did come from the motorcycle world, I believe, as Mr. Hewland was a motorcycle enthusiast. It is a design compromise not seen on street cars, though, which do not need to change gear ratios rapidly and easily, and value quietness, ease of use and reliability in their design. I don't know of any automotive racing dog boxes that use helical cut gears. Are there any?

TT

mpaton

I haven't been a student of racing transmissions for many years, but I suspect you're correct. There isn't much reason for a race transmission to use helical gears. There may still be street car adapted transmissions which replace some of the gears with straight cut but retain some of the original ratios. In 1976 I had a Hillman Imp 4 speed transmission with lower ratio 3rd and 4th and original 1st and 2nd gears, straight cut but synchro. 90mph in 4th went from 6000 rpm to 7200. It was a stage rally car, and the noise on the road sections was considerable.

Interestingly, I've also had a car with a face dog gear engagement. The Series 1 Land Rover had a 4 speed with synchros only on 3rd and 4th. 1st was a sliding gear engagement, but that method wouldn't have worked for 2nd, so the sliding 1st gear had dog teeth on the other side. neutral to 2nd had a much shorter throw than 1st to neutral. I hadn't assumed last night that William Hewland ran the Hewland company, but I now see that he does, the text being copied from the Hewland web site. I confess I didn't entirely take his advice on the Land Rover, usually double declutching both up and down to get the revs close, and then rapidly pulling it into gear (and that part is consistent with his advice). Most people just rammed it through and were rewarded with a substantial clunk, probably due to the substantial rotating inertia of the last 1st gear on the dogs. Never any grinding though.

Anybody ever seen the gearboxes on large turbines? Frequently double helical, in a herringbone pattern to make them quiet but not produce any axial thrust.

Michael Paton