Welding up diff mounts... bad idea?

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Welding up diff mounts... bad idea?

Postby MWP » Mon Feb 01, 2016 1:41 pm

Hi all,

I was thinking about making some poly rear diff mounts, which is a bit of a pain to do.
So then i thought... since poly is quite solid (and other solid mounting solutions exist), why not just weld them up completely?

As in, remove the rubber mounts and weld up:
Image

Can anyone foresee any issues with doing so?
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Last edited by MWP on Mon Feb 01, 2016 3:15 pm, edited 1 time in total.
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Re: Welding up diff mounts... bad idea?

Postby underscore » Mon Feb 01, 2016 3:02 pm

If you weld up those three but leave the 4 mounts that go to the chassis with at least a tiny bit of play I'd imagine it would be fine, it'll be harder on the drivetrain when you're launching it though.
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Re: Welding up diff mounts... bad idea?

Postby MWP » Mon Feb 01, 2016 4:17 pm

I just had a good look through RaceFab's photos... they solidly mount most rear diff's and subframes to the chassis.
So it doesnt look like it'll be a problem.

Image
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Re: Welding up diff mounts... bad idea?

Postby Insanity-74 » Mon Feb 01, 2016 4:45 pm

There are a few considerations to make when welding up diff mounts or adding solid mounts to anything, not just diff mounts.

1. Wheres the stress going to go to next?

Allowing parts to move helps eleveate stress to all parts of the car, if you remove that abilty to move the strain has to go somewhere. Usually to the next weakest part. Its figuing out what that part is and can it cope? Its ikely that it will be one of the bolts somewhere as they will be corroded and stressed already, or any of the pressed sheet metal components that may be weakened by corrosion.

2. Diff deisgn.

Do you thing that the diff casing is strong enough to take the extra strain? I would have thought so as its pretty big, but who knows? Cast material does like to go with a bang.

3. How goods your welding?

With the constant strain placed on the diff carrier, will the welds be able to cope?
Last edited by Insanity-74 on Tue Feb 02, 2016 8:24 am, edited 1 time in total.
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Re: Welding up diff mounts... bad idea?

Postby ___Scott___ » Mon Feb 01, 2016 6:12 pm

The place I would be concerned about is the sheet metal the subframe bolts to. The extra loads and vibration may cause fatigue in the thin metal and cause it to crack eventually. Additional reinforcement may be necessary.

The easy way to avoid that problem is to just use a flexible mount, rubber/poly etc...
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Re: Welding up diff mounts... bad idea?

Postby GT4times2 » Tue Feb 02, 2016 3:55 am

___Scott___ wrote:The place I would be concerned about is the sheet metal the subframe bolts to. The extra loads and vibration may cause fatigue in the thin metal and cause it to crack eventually. Additional reinforcement may be necessary.

The easy way to avoid that problem is to just use a flexible mount, rubber/poly etc...




Agreed, plus

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Re: Welding up diff mounts... bad idea?

Postby MWP » Tue Feb 02, 2016 12:38 pm

Because not complicating things is boring ;)
... and its actually simplifying things. It'll be a tad lighter too.

___Scott___ wrote:The place I would be concerned about is the sheet metal the subframe bolts to. The extra loads and vibration may cause fatigue in the thin metal and cause it to crack eventually.


Thin metal?
I gather youve never dropped off the subframe and looked at the chassis under there then?
They are built like a brick shit house.
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Re: Welding up diff mounts... bad idea?

Postby Mr Alltrac » Tue Feb 02, 2016 5:04 pm

Interesting idea, should be a fun project! I don't doubt that it would hold, my only concern would be if your not an engineer it would be hard to guess where the stress would be and where to make sure to brace.

Has that other rear subframe been ran in a rally or is it a road race subframe?
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Re: Welding up diff mounts... bad idea?

Postby MWP » Wed Feb 03, 2016 9:09 am

This is what is the TTE rally 185 rear subframe looked like:
Image


The work that RaceFab does is here. Most of that does end up in tarmac and gravel rally cars.
https://www.facebook.com/RaceFab-402451 ... tos_stream
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Re: Welding up diff mounts... bad idea?

Postby lumbercis » Thu Feb 04, 2016 12:51 am

What are you looking to use the car for?
Caldina powered...
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Re: Welding up diff mounts... bad idea?

Postby MWP » Thu Feb 04, 2016 5:34 am

Occasional weekend drives, club cruises, and mainly track/hillclimb/motorkhana events.
Ie: https://www.youtube.com/watch?v=DNqdvFOQDco
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Re: Welding up diff mounts... bad idea?

Postby MWP » Mon Feb 08, 2016 8:00 am

Its now rather solid :P :D

Just a few progress photos from today.
Should get it finished off tomorrow.

Getting it into the right location was a hardest bit.
The diff is now in millimeter accurate position :)

Image

Image

Image
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Re: Welding up diff mounts... bad idea?

Postby MWP » Wed Feb 10, 2016 8:02 am

Almost done. Just needs black paint now :)

Image

No way is sub-frame strength going to be an issue.
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