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MartinC 02-27-2014 07:06 PM

Interesting facts about the 9 speed gearbox
I came across a very interesting article in another forum which goes into great detail regarding the design of the 9 spd ZF gearbox. I do not wish to flout this forum's rules by providing a link but I will identify the article which is titled "ZF's 9-Speed 9HP Transmission Puts Dog Clutches On The Leash" by Alex Dykes. It can be found by visiting TheTruthAboutCarsdotcom. The article explains the difference between the conventional friction clutch used in most of the gear shifts and the dog clutches. Let me extract a small piece to give you an idea of what's involved:

" To save space and increase efficiency, the 9HP uses two multi-plate clutch elements, two friction brakes and two electronically synchronized dog clutches. ....The way the gearsets are arranged inside the case, shifts from 1-2, 2-3, and 3-4 involve only the traditional friction brake and clutch elements. As you would expect, aside from 1st being fairly low and somewhat distant from 2nd, these shifts feel perfectly “normal.” Under hard acceleration there is a momentary reduction in engine torque (courtesy of the computer to reduce clutch wear) and the shift occurs quickly and smoothly. The shift from 4-5 however is different. The transmission has to disengage dog clutch “A” in addition to engaging a friction clutch. This shift takes slightly longer than the 3-4 shift and the car’s computer makes a drastic reduction in torque to prevent wear of the dog teeth. Shifts 5-6 and 6-7 again happen with the only the friction elements at which point we need to disconnect the final dog clutch for gears 8 and 9 so we get the same kind of torque reduction in those shifts. The result is a transmission that has two distinct “feels” to its shifts, one that has only a slight torque reduction (1-2, 2-3, 3-4, 5-6, 6-7, 8-9) and one that has a more “manual transmission” feel where torque is cut severely (4-5 and 7-8)."

It goes on to say "Downshifts are where the 9HP truly feels different. Because of the design, if you’re in 8th gear and want to pass, the transmission will often need to drop 4 or 5 gears to get to a suitable ratio. (Remember that 4th gear is the first ratio going back down the scale that is lower than 1:1.) To do this the transmission has to accomplish the harder task of engaging two dog clutches. To do this the transmission doesn’t use cone synchros like a manual (too bulky) it uses software. Engaging dog clutches requires a longer and yet more severe reduction in torque than the disengagement because the transmission has to align the clutch and then engage it. In most automatics when you floor the car you get an instant feeling of acceleration that improves as the transmission downshifts. Although there would be moments of power reduction (depending on the programming) during this time, the engine is always providing some force forward. The 9HP’s software on the other hand responds by cutting power initially, then diving as far down the gear-ladder as it can, engaging the dog clutches and then reinstating your throttle command. The result is a somewhat odd delay between the pedal on the floor and the car taking off like a bat out of hell."

There is a lot more in the article that bears reading.

In the same forum, Alex has also posted a very comprehensive and fair review of the Cherokee (including a 30min video) which makes interesting reading.

I commend these references to you all.


ChadRizor 02-27-2014 10:54 PM

👍👏. Great information that you've provided. It has answered some of the question I've been pondering over. IMO this is a very efficient design. My father (who is a process engineer), was extremely impressed with the transmission for the jeep Cherokee. He has design many transmission,rear ends, and other parts for well known semi trucks & auto companies before retiring from Dana corporation. So for him to take notice and spend time researching the web tells me this is no slouch jeep. Thanks again for the write up provided.

WK2 Rover 03-03-2014 06:12 AM

Re: Interesting facts about the 9 speed gearbox
The 9-speed complexity times 2WD and two 4WD options times two engines times different driving modes (Auto, Sport, etc.) is why the vehicle was delayed. Fortunately firmware updates can continue to improve shift smoothness, performance, and gas mileage.

Mr_Taz 03-04-2014 11:12 PM

Re: Interesting facts about the 9 speed gearbox
Thanks for the information within the article by Alex Dykes and the video on the Jeep Cherokee Limited also has loads of useful facts and evaluation!

Can't wait to get my hands on my Limited :D

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