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#41 stevo_gj

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Posted 04 November 2009 - 06:31 PM

You're right! And judging by what I've calculated, I would need to be using 4.5 kN at each hole to reproduce 400 lbft of torque, which is way higher than I previously estimated. Obviously this is not the loading condition supported by this bracket.

I think I might stick with the 1kN per hole figure so that it is easy to compare results.

#42 Zeddophile

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Posted 04 November 2009 - 06:39 PM

You're right! And judging by what I've calculated, I would need to be using 4.5 kN at each hole to reproduce 400 lbft of torque, which is way higher than I previously estimated. Obviously this is not the loading condition supported by this bracket.

I think I might stick with the 1kN per hole figure so that it is easy to compare results.


Out of curiousity, how did you get to that figure?  Without knowing the gearing used by the car that put the dyno figures down, you'd have to assume worst case scenario, which would likely be 1:1 gearing (4th gear) with a 3.5 final drive, and then assume that the car now is in a very short first gear (say perhaps around 3.8 or 3.9) with a 4.3 or 4.5 final drive.

Obviously the bracket shouldn't ever see the full extent of this loading anyway, unless it was solid mounted to the body - which from memory they aren't, they have a large rubber mount at either end which should absorb a portion of the torque transmitted....

So many variables when you start to actually work this stuff out on paper!

#43 stevo_gj

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Posted 04 November 2009 - 06:48 PM

I've attached the basic working. I am assuming 1:1 gearing which you have pointed out is incorrect, since the maximum stress will occur in first gear.

However, even with the rubber mounts on either end, if the other mounts for the diff are stuffed, then once the rubber has given as much as it can this full torque would indeed be transmitted through the bracket. I don't think it would cause immediate failure though, unless it was already fatigued. I'll post results soon.

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  • 11761_cf7cbb255fd84a98fa4a3b1eca6573524e07c044.jpg


#44 stevo_gj

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Posted 04 November 2009 - 07:07 PM

I'm not 100% on which 6061 aluminium alloy to choose, because as you can see here: http://en.wikipedia....aluminium_alloy the yield strength can vary significantly.

Anyway here are the results for the repositioned bolt holes. The first example is showing the 1kN forces and the second example is showing what would happen if the entire motor torque of 400lbft in 4th gear was transmitted to the bracket in first gear instead.

187 MPa = bang since yield (failure) is around 55 MPa for standard 6061.

edit: This does not mean the design will fail, since as Wax has stated, there are other mounts which will absorb some of the forces involved. It's just interesting to see that if 100% of the motor torque was applied to this bracket alone in first gear it would break,

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#45 waxhead

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Posted 04 November 2009 - 07:58 PM

Thats also assuming you have traction at that point to huh
I think trying to put 400ftlb down in first gear with out breaking traction would be a hard thing as well

#46 stevo_gj

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Posted 04 November 2009 - 08:17 PM

The changes made to V3 seems to have weaked the model, as V2 has slightly lower stresses under the same load.

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#47 waxhead

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Posted 04 November 2009 - 08:20 PM

Wow thats different huh
The v3 model as more metal in it than the v2 model

#48 stevo_gj

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Posted 04 November 2009 - 08:50 PM

This one performs much better.

Mass of V2: 2.006 kg
Mass of V3: 2.007 kg
Mass of V4: 2.135 kg

By increasing the mass by 6% the maximum stress under this particular loading condition has been reduced by 56%.

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#49 waxhead

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Posted 04 November 2009 - 08:54 PM

Thats awesome, that will be the design i get cut out then
Thanks heaps for that , i wouldn't mind a copy of that software you have

#50 mayhem

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Posted 04 November 2009 - 09:12 PM

sorry guys i just want to add something ... ive seen the models on the twisting of the bar ..... ive got 260kw atw... it doesnt want to twist it gets pulled down pretty evenly ......the front diff bracket wants to go up thats why its straped down.... ive got wear on the mustache bar from the down right brackets holding the A arms... the diff wants to spin nose over tail if there is same amount of force to each wheel ... so in my oppinion what you guys are doing will work very well but you guys know more about the structure than me clearly

#51 waxhead

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Posted 04 November 2009 - 09:17 PM

sorry guys i just want to add something ... ive seen the models on the twisting of the bar ..... ive got 260kw atw... it doesnt want to twist it gets pulled down pretty evenly ......the front diff bracket wants to go up thats why its straped down.... ive got wear on the mustache bar from the down right brackets holding the A arms... the diff wants to spin nose over tail if there is same amount of force to each wheel ... so in my oppinion what you guys are doing will work very well but you guys know more about the structure than me clearly

Yup thanks for that
Its the very thing i was thinking, it wants to lift the front of the diff up.
Im going to make a mount for the front of the diff with a rubber block over the  top to stop it lifting to far.
Your input is appreciated

#52 stevo_gj

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Posted 04 November 2009 - 10:11 PM

Ahh I see, so the rear of the diff is being pushed downwards like I initially modelled!

In that case, now that we have strengthened its resistance to torsion can I suggest you make a couple small changes to the model to increase its strength against transverse loads like the one in the picture I've attached.

I would suggest decreasing the size of the cutout near the yellow circles, and removing 'lazy material' where I've put the circle in red. This will allow you to add material to other areas without increasing the weight.

I think that since the stress is still concentrated in one location its worth editing the model a little bit more to improve its performance against this downwards loading

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  • 11773_99de78155e763e008443dbd383fa1bdbe7286ec7.jpg


#53 stevo_gj

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Posted 04 November 2009 - 10:46 PM

Ok, in this example each side has two different types of holes. This means that neither side's results are accurate because the stronger side will be displaying higher stresses than it would normally get, and the weaker side will be showing lower stresses than it would normally get.

However it IS possible to see the high stress concentrations in the side with many holes.

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#54 620Z

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Posted 05 November 2009 - 08:02 AM

I guess my question would be is why have holes in the bar in the first place. I know visually they look great but I don't see any other purpose. Weight saving in alloy would be minimal. But for every hole you are (per your test model) weakening the overall structure.
I am not trying to be negative but get us all thinking. As I would hate to see some produced, sold and break at some point.
From a personal point of view my car has 750nm of torque (I know a bit more than most) but you can in fact keep most of that on the ground in 14st gear given the right tyres and road surface.
I see on the Arizona Zed model it doesn't have holes all the way through but more indented areas which might be the way to go?
Just food for thought.  :)

#55 waxhead

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Posted 05 November 2009 - 08:06 AM

I understand what your thinking
But if i want to have it with no holes in it I may as well just have the stock one in there painted alloy colour
I was going for the look, the main thrust actually goes to the front mount
as this is the point furthest away from the centre of the movement
The next thing i am going to do is build a mount that stops the front coming up like the rt ones you can buy

#56 stevo_gj

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Posted 05 November 2009 - 09:29 AM

Mass of Steel bar: 6.25 kg  about 3 times the mass.

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#57 zedevan

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Posted 05 November 2009 - 11:32 AM

I guess my question would be is why have holes in the bar in the first place


the holes allow the bar to flex in areas which aren't undergoing the maximum load, and by doing so reduces the force at the maximum points

a part we were doing fea on at work was "stronger" when made from 3mm plate than 5mm plate when the designs were identical - but once holes are put in the correct locations in the 5mm plate to allowed it to flex and spread the load, it was ofcourse stronger

...and stevo turn down the deformation scale so it doesn't look so crazy :P lol

#58 stevo_gj

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Posted 05 November 2009 - 12:12 PM

It doesn't seem to allow me to do that, do you know where to find that feature in simulxpress?

#59 waxhead

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Posted 05 November 2009 - 12:32 PM

I will play with the drawing more tonight to see if i can get the figures higher
Im thinking maybe even cut it out of steel but make the material thiner as well as less metal , ie larger holes with an X in between to give it some strgh

#60 stevo_gj

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Posted 05 November 2009 - 02:26 PM

Use the FEA above where there are no holes in the design as a guide. The best places to put holes are where the local stresses are low, which are the blue areas.




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