Manual Trans. Thud when shifting

[aeropaul]

There have been a number of posts here about this "issue" and frankly I feel like there is a lot of unproductive drama surrounding this, mostly coming from a lack of understanding of how manual transmissions work, and the engineering tradeoff between stiffness/compliance and NVH, and I'd like to steer this into a positive contribution to the discussion.

I feel like I've fleshed out the "how synchros work" and there are a multitude of videos and gifs that explain it, so I'm going to let that one lie.

With the remote mounted shifter, if the mounting scheme has compliant enough bushings installed, very little NVH is transmitted back to the body. Obviously that produced a lot of issues with differential movement between the body and transmission on the 11-14 cars, causing missed shifts. Yes, that shift linkage was way too compliant, and looking at it with my engineering glasses, the reasons were obvious.

However, the shifter assembly is much improved in the S550. It is much more rigid, there is less compliance in the mounts, and the shifter box-to-transmission link (the cast aluminum truss thing) is much more rigid than the previous design and is cast directly to the shifter box (one less joint), meaning less compliance. There is the large rubber isolator at the back of the shifter box mounting the shifter to the body (look at the MGW video about the shift knob). A bushing with that large will require some stiff rubber to not be mushy. Also, its proximity to the shift box is fairly close, meaning easy motion transmission fwd-aft.

I do think that the MGW design will reduce NVH transmission because it has two close-tolerance slider rails at the back of the shifter box to mount it to the body, which should allow for better decoupling of any fwd-aft motion without relying on the compliance/NVH balance of a single rubber biscuit. They also employ a single linkage between the box and transmission. They can do this because they have inserted another joint in the tie-rod between the box and transmission (the stationary one). Personally, I think it's a great design (however I feel like their product is able to stand on its own merits without slamming the OEM design unnecessarily).

You also need to realize that in order to keep cost and weight down, the OEM wants to keep part count low. The all-machined MGW unit has a lot of individual parts, and they're all machined. Yes, its a stiffer assembly, but it's also heaver, more labor intensive to install, and machined parts are more expensive to make than cast parts at a high production rate.

The 370Z, fox body mustangs, and the 1970 have transmission-mounted shifters, which means any NVH in the transmission stops at the shifter assembly. I drove a 2012 GT500, and was not impressed with the shifter (subjective evaluation, I know). It's a different assembly as well (two tie rods instead of one). Go shift any RWD manual transmission, and you'll feel a good amount of feedback in the shifter. If the shifter is also mounted to the body, and the mounts are stiff enough, all that feedback you feel from the synchros will go straight into the body structure and resonate thoughout the transmission tunnel.

It comes down to a tradeoff. Do you want the customer to have a solid feeling shifter, which clearly communicates all its motions though to the users hands? This requires little compliance which produces a lot of feedback. I feel like they achieved this adequately in an OEM solution. Or do you want it quiet? That requires a careful balance of compliance/NVH which may produce inconsistencies at the ends of the spectrum of use. I personally (as an engineer and a user) like the former approach.

If you want to go talk to your dealer/service dept about it, go right ahead. Just be armed with the logic behind the design. If I'm proven wrong and all the MT82s in S550 mustangs lunch themselves, I'll publicly eat my words, I'm not above that.

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[Ruby305GT]

Now I am worried and my excitement has diminished some. I hope there is a fix for this because I am very picky about these sort of things. I like things working right, thuds and grinds are not something I am willing to live with on a car that cost nearly 40k.

Additionally, if there is a fix, I hope it's something easy and not requiring crazy downtime as in a full transmission replacement. I do agree although it may not be an issue at all, everyone should record it and report it.

I remember the g35 vq35 engine and the weak piston rings which brought on thousands of g35 with rev up engines to consume oil. It took plenty of fighting and complaining for Nissan to fix this, they ended up changing long and short blocks in the long run. Most were out of warranty by the time this came around, as was mine... Nothing is perfect and there are challenges, I just feel uneasy about anything coming from China. The stigma of the "types" of products and build quality coming from there due to x reasons seem to have a long track record. On a brighter note, my iPhone 6 plus has been great thus far, it is a Chinese product.

End of rant and blabber.

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[minicobra]

There have been a number of posts here about this "issue" and frankly I feel like there is a lot of unproductive drama surrounding this, mostly coming from a lack of understanding of how manual transmissions work, and the engineering tradeoff between stiffness/compliance and NVH, and I'd like to steer this into a positive contribution to the discussion.

I feel like I've fleshed out the "how synchros work" and there are a multitude of videos and gifs that explain it, so I'm going to let that one lie.

With the remote mounted shifter, if the mounting scheme has compliant enough bushings installed, very little NVH is transmitted back to the body. Obviously that produced a lot of issues with differential movement between the body and transmission on the 11-14 cars, causing missed shifts. Yes, that shift linkage was way too compliant, and looking at it with my engineering glasses, the reasons were obvious.

However, the shifter assembly is much improved in the S550. It is much more rigid, there is less compliance in the mounts, and the shifter box-to-transmission link (the cast aluminum truss thing) is much more rigid than the previous design and is cast directly to the shifter box (one less joint), meaning less compliance. There is the large rubber isolator at the back of the shifter box mounting the shifter to the body (look at the MGW video about the shift knob). A bushing with that large will require some stiff rubber to not be mushy. Also, its proximity to the shift box is fairly close, meaning easy motion transmission fwd-aft.

I do think that the MGW design will reduce NVH transmission because it has two close-tolerance slider rails at the back of the shifter box to mount it to the body, which should allow for better decoupling of any fwd-aft motion without relying on the compliance/NVH balance of a single rubber biscuit. They also employ a single linkage between the box and transmission. They can do this because they have inserted another joint in the tie-rod between the box and transmission (the stationary one). Personally, I think it's a great design (however I feel like their product is able to stand on its own merits without slamming the OEM design unnecessarily).

You also need to realize that in order to keep cost and weight down, the OEM wants to keep part count low. The all-machined MGW unit has a lot of individual parts, and they're all machined. Yes, its a stiffer assembly, but it's also heaver, more labor intensive to install, and machined parts are more expensive to make than cast parts at a high production rate.

The 370Z, fox body mustangs, and the 1970 have transmission-mounted shifters, which means any NVH in the transmission stops at the shifter assembly. I drove a 2012 GT500, and was not impressed with the shifter (subjective evaluation, I know). It's a different assembly as well (two tie rods instead of one). Go shift any RWD manual transmission, and you'll feel a good amount of feedback in the shifter. If the shifter is also mounted to the body, and the mounts are stiff enough, all that feedback you feel from the synchros will go straight into the body structure and resonate thoughout the transmission tunnel.

It comes down to a tradeoff. Do you want the customer to have a solid feeling shifter, which clearly communicates all its motions though to the users hands? This requires little compliance which produces a lot of feedback. I feel like they achieved this adequately in an OEM solution. Or do you want it quiet? That requires a careful balance of compliance/NVH which may produce inconsistencies at the ends of the spectrum of use. I personally (as an engineer and a user) like the former approach.

If you want to go talk to your dealer/service dept about it, go right ahead. Just be armed with the logic behind the design. If I'm proven wrong and all the MT82s in S550 mustangs lunch themselves, I'll publicly eat my words, I'm not above that.

You make some very good points, and looks like you definitely have some engineering experience in this area. Thanks for the feedback

Now, with that said, I guess the point that I was trying to make was that after 50 years (or more) of manual transmission development, there shouldn't have to be a trade off. A bunch of smart people from around the globe should be able to figure this out. I just find it hard to believe that they couldn't come up with a fix or solution within cost constraints, budget, etc.
The reason why I feel this way, is there are cars on the market right now, that don't do this while shifting. My friends Corvette shifts great, the Shelby shifted great (both Tremec) and I've owned over a dozen RWD cars that didn't do this either.

I'm a big supporter of save the manual transmission, it's part of the joy I get from driving. Why don't we have a perfect shifting, feeling, NVH manual transmission by now. After 50 years of Mustang production, from a billion dollar company :shrug: Sorry, am I asking too much?
Look at the PDK transmission in a Porsche, that's an engineering marvel, if they can do that with a transmission, they could surely make a simple manual feel and shift great, weather transmission mount, body mount, hybrid mount, muffler mount, IDK.
I know the bean counters in these companies try and save money everywhere they can, and the probably have a tough time trying to figure it out. For me, if I was in charge of the Mustang program, the shifter would be a priority.

Ok, done with rant :headbonk:

Now, what's the solution? If it has to do with the mounting of the shifter. MGW ? would there shifter mount cure this?

 

[Ruby305GT]

I cant wait for DemonCoyote to come back with the findings. I am hoping everything is smooth as butter when he is done and I will line up to purchase everything he did.

 

[D3adch1ld]

Mine makes a thudding noise when shifting into 1st and 2nd and sometimes 3rd, and I hear/feel a much lighter thud when letting the clutch out, but only under easy driving conditions. When I'm accelerating briskly, all of the noises go away. The thud while shifting is the synchros. I figured it would go away in warmer weather, and as it broke in more. My theory about the light thud sound/feel when letting the clutch out is due to stiff clutch damping springs on the clutch plate itself, and not driveline slack (can also explain away some of the bucking while shifting that others including myself have experienced).

I'm honestly not worried about any of it at all. I have not yet had a missed shift, or hard to hit gear, or any other nastiness yet, just what I would consider normal manual transmission noises. I've never driven a manual transmission that didn't make its fair share of synchro and clutch noise. They are after all, a box full of gears rotating at high speed.

Yes, it is in fact an inherent RWD trans signature. Press the clutch for 2-3 secs before engaging 1st and no more thud. It has to do with the input shaft spinning and the rest of the drivetrain is stopped, engaging the gear causes the input shaft to abruptly stop and making a noise. This isn't a problem, it's just the nature of it all. If you don't want to hear it, then give the inputs haft a sec or 2 to stop spinning.

 

[aeropaul]

Yes, it is in fact an inherent RWD trans signature. Press the clutch for 2-3 secs before engaging 1st and no more thud. It has to do with the input shaft spinning and the rest of the drivetrain is stopped, engaging the gear causes the input shaft to abruptly stop and making a noise. This isn't a problem, it's just the nature of it all. If you don't want to hear it, then give the inputs haft a sec or 2 to stop spinning.

Yup. Every time I grab 1st quickly after pressing the clutch in (like half second after pedal hits the floor), it thuds. The clutch disk will carry quite a bit of rotational inertia, so the input shaft is still spinning at engine speed or slightly slower until the synchro cones slow it down or the synchros engage generating a "thump." Even FWD cars make these kinds of noises, you just have to have the window down to hear them, and they don't feed back into the body so easily because most of them have cable shifters. The difference is that a FWD trans is in front of the firewall (thick steel) and insulation, the RWD trans is in a tunnel, right about calf-muscle position. It's like an amphitheater.

 

[Ruby305GT]

So if i depress the clutch pedal for at least 3-5 seconds prior to shifting into first the problem should not be there? Can someone confirm this with real life testing?

 

[aeropaul]

Yeah, I can. Cause I've done that. If you've developed the habit of having the clutch pedal in at a stop light, NBD. I've been driving a 1978 MGB with a carbon (graphite) throw-out bearing (not a roller bearing); the normal procedure is to leave it in neutral until light changes, then push clutch in and throw it in first to reduce wear on the T/O bearing. So I've carried that habit for a while now.

Which, BTW, you want to hear a noisy transmission, my MGB's transmission is like an obnoxious puppy. It wants you to know it's there and alive and kicking. Synchros are noisy, nibbles on 1-2 and 2-3 shifts when cold, vibrates and whines, tons of feedback through the shifter. The joys of a real gearbox. The rest of the car will rot away from the drivetrain though (and mine is one of the few and proud rust-free), its a very stout transmission.

 

[aeropaul]

Just for the sake of this conversation, I went out and tested it. If you slow-shift into first, no thud. If you slam it into first right after pushing the clutch in, slight thud. If you grab reverse too fast, more noticeable thud (actually quite a thud). If you slowly shift into reverse, no thud.

The only effect it had on me was that it made me want to go do a 2500 rpm clutch dump, which sadly I can't/won't because it's wet and rainy.

 

[Ruby305GT]

Just for the sake of this conversation, I went out and tested it. If you slow-shift into first, no thud. If you slam it into first right after pushing the clutch in, slight thud. If you grab reverse too fast, more noticeable thud (actually quite a thud). If you slowly shift into reverse, no thud.

The only effect it had on me was that it made me want to go do a 2500 rpm clutch dump, which sadly I can't/won't because it's wet and rainy.

Paul you da man! Thanks buddy for the quick test. I have gotten used to leaving the clutch in with brake while neutral. I will sometimes leave it in first with the clutch pressed and brakes as well, rarely I'll slam the gear in as soon as I press clutch. It seems it might have been the reason I did not come across of it when test driving. My driving habit helped avoid it


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[aeropaul]

No problem dude.

 

[D3adch1ld]

Yup. Every time I grab 1st quickly after pressing the clutch in (like half second after pedal hits the floor), it thuds. The clutch disk will carry quite a bit of rotational inertia, so the input shaft is still spinning at engine speed or slightly slower until the synchro cones slow it down or the synchros engage generating a "thump." Even FWD cars make these kinds of noises, you just have to have the window down to hear them, and they don't feed back into the body so easily because most of them have cable shifters. The difference is that a FWD trans is in front of the firewall (thick steel) and insulation, the RWD trans is in a tunnel, right about calf-muscle position. It's like an amphitheater.

Couldn't put it better.

 

[D3adch1ld]

Ok, this is great to know, and is a quick work around for the 1st gear input.
What about from 1st to 2nd gear when driving slowly, like in a parking lot, or heavy traffic. That's probably the thing that annoys me more.

Is an aftermarket shifter or trans mount going to help dampen this, or will it make matters worse. I'd be ok if it was a little less of a thud, mine is pretty loud.

The return spring mod will probably take this away. Its all in the modulation of the clutch, for some reason the spring really makes modulating the clutch unpredictable and guessing it feels wrong, but if play with the clutch as you enter 2nd it softens it up.

 

[Ruby305GT]

If it's synchros then why is it not happening on all gears?


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[aeropaul]

:shrug: Without examining the design differences between the synchro hub assemblies, I won't even begin to formulate a theory as to why it doesn't do it in all gears.

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