M-DCT Manual Mode - Deceleration

[Lodaka]

Just have a quick questions about the topic, as it has been bothering me a while.

When I am in a Manual Mode, e.g. S3, when I decelerate from 5th gear or higher, the DCT drops me down from 5th - 2nd - 1st.

However, when I am in 4th gear or lower, it goes through 4th - 3rd - 2nd - 1st.

Is there any reason for this? The reason why I ask when the gear drops from 5th to 2nd, there is a bit of abrupt deceleration, which feels funny. At this time, I am manually downshifting to at least 4th gear for this reason.

I tried with various keywords but I couldn't find a thread that discusses this.

Does anyone have any feedback on this? Thanks.

[Scoobe]

I noticed this too and I think it's just the programming. Manual mode attempts to keep you in the selected gear as long as possible before auto- downshifting. For some unknown reason, some gears tend to hang in there longer than others on the lower manual settings (S1-S3).

I do have a theory though that the software is trying to be predictive to you're decelleration. I haven't tested it, but I think if you decellerate quickly it skips gears, if you decellerate more slowly is goes sequential.

[Lodaka]

Yea, I should test your theory as well. I don't think I particularly slow down faster than an average driver. I normally drive with S3, but different settings may result in different results.

[skierman64]

I always shift down to third manually to take advantage of engine braking. I let the tranny shift to 2nd or 1st on it's own. I find this is a very smooth way to drive the car on a daily basis.

[klammer]

^+1... love the control of the seven gear selection for both acceleration and deceleration...great car

[mkoesel]

Interesting, didn't know this. Maybe its a function of driving conditions. So, if you are cruising at a given speed at low RPM and declerate it assumes you would rather the more leisurely, lazy downshift behavior. But, if you downshift to 4th at that same speed your RPM will go up obviously. So then maybe it assumes you want a more aggressive downshift behavior so it runs down through every gear. Just a thought.

So I guess what I am saying is, suppose you are cruising in 5th gear at 3500RPM (or something like that) and then decelerate. Will it go to 4th in that case?

[grieverr]

I, too, assume that's a behavior of the computer predicting. If you're (for example) at 50mph in 5th gear and start decelerating, the car can stay in 5th until you get to 30mph, at which point, switching to 2nd allows faster engine breaking. No need to go through 4th and 3rd. Being in 2nd also allows for faster acceleration, should you need to speed up (if you had slowed down for a curve or something). I think the computer tries to find that gear which will be most efficient for the situation.

[jodtx]

I wonder if it's a function that even gears are handled by 1 clutch and odd ones by the second one. A transition from 5 to 2 could be handled by simply selecting the second gear and engage the second clutch. Versus 4 to 2 involves the same clutch and so the software is forced to use the second clutch as a step in between.

[Lodaka]

Originally Posted by jodtx

I wonder if it's a function that even gears are handled by 1 clutch and odd ones by the second one. A transition from 5 to 2 could be handled by simply selecting the second gear and engage the second clutch. Versus 4 to 2 involves the same clutch and so the software is forced to use the second clutch as a step in between.

That's a very interesting observation. I suppose going from 4th to 1st may be a bit drastic in this sense; so, it goes to 3rd.

At any rate, I tested the theory up there, and the conclusion is... regardless of the rate at which I slow down or whether I am in S3, S4 or S5 (haven't tried S6), it's always the same result.

Just to throw a curve ball at this, I think if you are in D mode, i.e. D3, D4, etc. I think the transmission actually uses all gears regardless of whether you start at 5th or 4th when slowing down. I still need to confirm this but I am fairly certain that this is the case... I suppose not a huge deal but I don't quite get it.

[MysticBlue]

Originally Posted by jodtx

I wonder if it's a function that even gears are handled by 1 clutch and odd ones by the second one. A transition from 5 to 2 could be handled by simply selecting the second gear and engage the second clutch. Versus 4 to 2 involves the same clutch and so the software is forced to use the second clutch as a step in between.

That sounds like the right answer to me.

[skierman64]

Originally Posted by jodtx

I wonder if it's a function that even gears are handled by 1 clutch and odd ones by the second one. A transition from 5 to 2 could be handled by simply selecting the second gear and engage the second clutch. Versus 4 to 2 involves the same clutch and so the software is forced to use the second clutch as a step in between.

Yes, that's exactly why.

[sor]

Except my impression is that it doesn't have to jump to the other clutch just to change gears, it can simply disengage, change gears, and then re-engage the same clutch. I don't have any experience to back this up, just older discussions on the same topic.

[SenorFunkyPants]

Originally Posted by jodtx

I wonder if it's a function that even gears are handled by 1 clutch and odd ones by the second one. A transition from 5 to 2 could be handled by simply selecting the second gear and engage the second clutch. Versus 4 to 2 involves the same clutch and so the software is forced to use the second clutch as a step in between.

Sounds favourite...I don't think there is any "predictive" element to the downshift when in manual mode as the downshift is simply an anti-stall feature not any attempt to put you in a "better" gear.

[M3 Pilot]

Originally Posted by jodtx

I wonder if it's a function that even gears are handled by 1 clutch and odd ones by the second one. A transition from 5 to 2 could be handled by simply selecting the second gear and engage the second clutch. Versus 4 to 2 involves the same clutch and so the software is forced to use the second clutch as a step in between.

I always thought 1 clutch handles even and the other, odd gears.

Eg.

Clutch A = 2,4,6
Clutch B = 1,3,5

Neither clutch can swap between even or odds.

Hence in the scenario you describe (4th to 2nd), in theory, I would expect a certain amount of lag mainly because Clutch A has to come out of 4th, and go into 2nd.

However when I tried going from 4th to 2nd in Manual mode, with a quick double tap, the gears went from 4th to 2nd without delay.

To my knowledge, "I think" thats how the R35's Getrag box works. I'm not sure if the M3 differs in this aspect.

Having said that, in regards to OP's query, I experienced the same absurb deceleration from 5th to 2nd in manual mode when coming to a full stop.

While in 5th, you can feel deceleration with a slight hint of engine brake and maybe 10 metres or so before you come to a complete stop it feels almost as though the car lurches forward.

That lurch forward motion is similar to what you would experience in a manual car as you clutch in before coming to a stop.

[mkoesel]

Originally Posted by Lodaka

At any rate, I tested the theory up there, and the conclusion is... regardless of the rate at which I slow down or whether I am in S3, S4 or S5 (haven't tried S6), it's always the same result.

Interesting indeed - thanks for testing it out. You've got me curious now - I am going to try and remember to test this out myself and see what I can find. It'll be hard though since I am so used to downshifting myself.

Originally Posted by jodtx

I wonder if it's a function that even gears are handled by 1 clutch and odd ones by the second one. A transition from 5 to 2 could be handled by simply selecting the second gear and engage the second clutch. Versus 4 to 2 involves the same clutch and so the software is forced to use the second clutch as a step in between.

The software isn't forced to do this, since the transmission is not a true sequential manual transmission. I.e. it can shift from any gear to any other gear, although shifts involving the same clutch and layshaft take longer than those that don't of course.

I am thinking along the same lines as SenorFP - in S mode there is no effort on the part of the trasmission logic to keep you in the best gear. Instead, it downshifts for you only to avoid a forced mechanical underrev (stall). So, they just programmed it to do drop you a few gears in order to be on the safe side and not put you right back into another situation where an underrev is about to occur.

[UltimateBMW]

Try S1 and see if the most passive setup won't result in the smoothest overall ride. I don't see how S6 would ever be beneficial in this type of situation. S6 is ment for tracks, where you are always accelerating whether its positive or negative.