NVH - placement of gearbox in transmission tunnel

[JAL73]

In my continuing quest to reduce NVH in my car I had a look at the placement of the gearbox in the transmission tunnel and noticed an appendage on the LHS of the gearbox touching some lining material on the inside of the tranmission tunnel walls (see pic) and wondered if this might be the a big contributor to the NVH I feel and hear. I have a 420 with Duratec engine mated to Mazda box and BMW diff. I've had vibration analysis carried out on the car with sensors attached along engine & drive train and been told theres no obvious rotational imbalances detected and been advised the engine mounts are not great at isolation, however I'm wondering if my gearbox seating/ placement might also be compounding the problem. 

Space is incredibly tight in the transmision tunnel so wondered if others have managed to fit their gearbox without any part of it touching the inner tunnel walls.
Also wondeirng what the circled appendage on the Mazda box is for and whether it can be removed and blanked off to give a bit more room to avoid touching the walls.
Im assuming it serves an important function so unlikely to give me the breathing space Im looking for!

 

Edited April 27 by JAL73

[Ainsley]

My 420 is the same.  

But I think you are chasing a never ending tail.  You will sort that bit which then highlights some other bit, which you sort that then highlights some other bit...........and on and on.   

I would just wear ear plugs and drive the nuts off it so you dont notice any noise/vibration/harshness.

[JAL73]

I have been in other cars which seem quieter and less vibration than mine, so I know something on mine can be fettled to improve it. 
I love thrashing the pants off it on track but I also find unnecessary NVH irritating and dont mind the challenge of minimising it.

[Ainsley]

It may be worth documenting what you did find and do to sort it.  Others would benefit if you manage to reduce any NVH.

I will probably be asking for advice after my first European road trip 🙂

 

[JAL73]

Im about to go on a road trip hence my renewed focus on NVH...

I was talking to Ollie at R&R Transmission about NVH and he said that the relationship between incline of the engine and the incline of the diff can stress the UJs on the propshaft and cause NVH to be transmitted from the engine to the diff. That sounds plausible but no idea how to test whether my car suffers from that particular ailment. Vibrating elements touching the transmission tunnel also seem highly plausible but not sure if this can be avoided with Duratec/ Mazda applicaiton given the tunnel width.   

[ScottR400D]

24 minutes ago, JAL73 said:

Im about to go on a road trip hence my renewed focus on NVH...

I was talking to Ollie at R&R Transmission about NVH and he said that the relationship between incline of the engine and the incline of the diff can stress the UJs on the propshaft and cause NVH to be transmitted from the engine to the diff. That sounds plausible but no idea how to test whether my car suffers from that particular ailment. Vibrating elements touching the transmission tunnel also seem highly plausible but not sure if this can be avoided with Duratec/ Mazda applicaiton given the tunnel width.   

That’s yet more old news. The Duratec does cant down at the front to clear the hood. About 1.8 degrees. 
But the diffs also point downwards by a similar amount. As long as the output of the box and input of the diff are parallel there no problem. 

When we investigated this closely a few years ago the ‘out of parallel’ amount was negligible. 

What Ollie has told you isn’t true. But it’s a good excuse for the noisy as sin LSD. 
Search for a thread from ‘21, called PROP SHAFT ALIGNMENT AND VELOCITY

I’m assuming you guys have LSDs? If not then it’s probably as good as it gets. If you do then look no further.  

Edited April 27 by ScottR400D

[Drakman]

This is a good view of what happens with out of alignment tail shafts and velocity.

 

[ScottR400D]

1 minute ago, Drakman said:

This is a good view of what happens with out of alignment tail shafts and velocity.

 

Yes that's in the ‘21 thread. 

[Drakman]

This might be of interest as well.

https://www.youtube.com/watch?v=GuI0hH2TuxQ

 

[Jonathan Kay]

34 minutes ago, Drakman said:

This might be of interest as well.

https://www.youtube.com/watch?v=GuI0hH2TuxQ

Thanks

This is about an app for diagnosing NVH issues. I haven't seen it before.

https://vibratesoftware.com

Jonathan

[SM25T]

Loosen the gearbox mount and see if you can push the gearbox away from that contact

[Ainsley]

I tried that when installing mine, it does not move.

[Jonathan Kay]

I don't know about modern Sevens, but slackening the engine mounts might allow some play at the gearbox mount.

Jonathan

[JAL73]

It’s going in to HWM tmrw and they are aware of the issue, will report back if they have any success in reseating it to avoid contact with the tunnel walls and checking the engine and diff incline angles. 

Surely Caterham have not engineered the width of the inside of the transmission tunnel to be less than or equal to the width of the widest section of the gearbox. I appreciate the tolerances maybe very fine but I’d hope it’s possible to seat the gearbox so that the only NVH transfer point to the chassis is via the gearbox mount. 

Edited April 28 by JAL73

[SM25T]

I suspect you'll find it didn't get changed when they started using the Mazda gearbox. This is Caterham.

[Ainsley]

It touches,  but is it not just touching the padding ?

The bell housing does the same.

[SM25T]

It's a heavy contact somewhere, as the NVH in this car is really harsh !

[Ainsley]

You have eliminated the Diff cradle touching the boot floor support ?

 

[ScottR400D]

2 hours ago, JAL73 said:

It’s going in to HWM tmrw and they are aware of the issue, will report back if they have any success in reseating it to avoid contact with the tunnel walls and checking the engine and diff incline angles. 

Surely Caterham have not engineered the width of the inside of the transmission tunnel to be less than or equal to the width of the widest section of the gearbox. I appreciate the tolerances maybe very fine but I’d hope it’s possible to seat the gearbox so that the only NVH transfer point to the chassis is via the gearbox mount. 

I’d love to hear what HWM have to say. 
 

Caterham didn’t change the chassis when the Mazda box began to be used. Just like when they changed the radiators, diffs and anything else. Engineered? 🤔
 

By the way, back in 2020 I asked one of CC’s ‘experts’ to confirm that the Duratec sloped forward to clear the hood. His answer? “I have no idea”  

As I asked, do you have a Titan LSD? Sintered plates? You’ll never enjoy low levels of NVH with that if so.

A final comment. The accumulated knowledge and experience in these pages will outweigh that of HWM by orders of magnitude. I would be wary of how much time I wasted.    

 

[JAL73]

2 hours ago, Ainsley said:

You have eliminated the Diff cradle touching the boot floor support ?

 

Im not sure on this, ill have to recheck, thanks for pointing it out

[JAL73]

2 hours ago, ScottR400D said:

I’d love to hear what HWM have to say. 
 

Caterham didn’t change the chassis when the Mazda box began to be used. Just like when they changed the radiators, diffs and anything else. Engineered? 🤔
 

By the way, back in 2020 I asked one of CC’s ‘experts’ to confirm that the Duratec sloped forward to clear the hood. His answer? “I have no idea”  

As I asked, do you have a Titan LSD? Sintered plates? You’ll never enjoy low levels of NVH with that if so.

A final comment. The accumulated knowledge and experience in these pages will outweigh that of HWM by orders of magnitude. I would be wary of how much time I wasted.    

 

All fair very points. If Mazda boxes are wider than their predecessors then i guess they would be more prone to NVH than earlier cars, unless being fitted with millimetre precision. If that's a fact, I've never heard it widely being cited, but you learn something new every day!

I guess it just comes down to knowing the width of the box and bellhousing relative to the appetures they sit in.
Im sure some knowledgeable person on these pages can prove or disprove the hypothesis with knowledge of the measurements in question.

Edited April 28 by JAL73

[Graham Hutton]

When I fitted my engine and gearbox it touched the side just like yours but I managed to get it nudged over slightly by loosening the engine mounts at both the chassis and engine ends on both sides and then pulling it over with a strop before tightening it up, gearbox end first. It only moved a millimetre or two but it was enough.

[AlastairA]

There is a spacer used between the gearbox and the gearbox mount on currrent kits. My understanding is that the spacer was introduced to get better angular alignment from the gearbox output shaft to the differential. Might it be worth checking whether this is fitted?

[ScottR400D]

32 minutes ago, AlastairA said:

There is a spacer used between the gearbox and the gearbox mount on currrent kits. My understanding is that the spacer was introduced to get better angular alignment from the gearbox output shaft to the differential. Might it be worth checking whether this is fitted?

There was/is a spacer that does something but not what you allude to. Not much can be done about engine/box alignment, the front of the engine needs to be down to clear the hood. 
In any case any angular misalignment is negligible. It’s a red herring.
 

Do we have any words of wisdom from HWM? 

 

[AlastairA]

50 minutes ago, ScottR400D said:

There was/is a spacer that does something but not what you allude to. Not much can be done about engine/box alignment, the front of the engine needs to be down to clear the hood. 
In any case any angular misalignment is negligible. It’s a red herring.

I was thinking of ChrisC's comments here: