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Subframe Lightening - Too Far?


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#1 davefitz

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Posted 28 May 2016 - 11:08 PM

Got a little carried away with some subframe lightening. Dont really know what I was thinking considering this is only a track day car and will be driven on the road especially given th etiny amount of weight saving. Anyway is this too far? holes might be a little on the large side? how do I know?

Any input would be greatly appreciated,

Dave

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#2 sledgehammer

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Posted 29 May 2016 - 12:03 AM

IMHO that is too much - would it pass an MOT ?

 

Normal subframes crack , with no holes drilled

 

even without crash damage

 

the driver is the easiest & safest thing to lighten in a car (not saying you are fat)


Edited by sledgehammer, 29 May 2016 - 12:04 AM.


#3 absx2

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Posted 29 May 2016 - 08:49 AM

The fact that you have asked the question means you already know its not structurally sound. Single bolt subframes are cheap so I would use another and not drill any holes in it.  Maybe you could seam weld the joints on the new one as per rally prep ? I take it you will be using alloy top and front subframe mounts which would make those braces redundant as the bulkhead will stop any movement.



#4 Ethel

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Posted 29 May 2016 - 09:14 AM

The gussets are worthwhile, the reaction forces from the tie rods and torsion from wheel to wheel are fed in to the subby. You could take your lead of the subby itself, if you really must lighten it, by leaving as much width metal on average as there is in the flanges.

 

Swaging the holes could add strength while losing weight.



#5 sledgehammer

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Posted 29 May 2016 - 09:32 AM

 

Swaging the holes could add strength while losing weight.

 

I think them holes are a bit big to swage - they would distort the cross member

 

smaller swaged holes definitely make up some of the strength lost with drilling

 

always wondered if a tube , across where the front cross member is , would be better for access / lighter

 

but it is always a compromise - too smaller tube won't handle the forces involved 


Edited by sledgehammer, 29 May 2016 - 09:36 AM.


#6 Moke Spider

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Posted 29 May 2016 - 10:08 AM

I don't see any holes there placed where they will cause a problem.

 

I'll just add that IMO, it's a fair bit of work for little gain, but none the less, a gain is a gain.



#7 limby2000

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Posted 29 May 2016 - 10:50 AM

It does look cool though, these subframes were designed before they thought up crumple zone,s. to be honest if you drive any sort of classic mini, the driver becomes the crumple zone.:(. P.s how long did that take you?.

#8 Italianjob

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Posted 29 May 2016 - 11:03 AM

I'd say it's weakened considerably, compare a piece of angled steel and some dexion(shelving stuff) at the same thickness, the dexion is just a wobbly mess, I personally wouldn't use it.

#9 vx220

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Posted 29 May 2016 - 11:41 AM

Wouldn't it have been stronger to use smaller holes, but more of them? To save having such large open spaces? I know it would take longer.

If you used a hole cutter, did you weigh what came off? How much did you save?

#10 Will16

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Posted 29 May 2016 - 12:55 PM

I'd say that's fine. I've got a lightened front subby, lightened more than that in fact! I haven't done many miles on it, but it was in a high power turbo car before mine and was fine! See this post:

http://www.theminifo...mini/?p=3356407

Wouldn't it have been stronger to use smaller holes, but more of them? To save having such large open spaces? I know it would take longer.
If you used a hole cutter, did you weigh what came off? How much did you save?


It saves about 2kg.

Edited by Will16, 29 May 2016 - 12:59 PM.


#11 Ethel

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Posted 29 May 2016 - 02:00 PM

Weighing before and after is the best way to weigh up what weight has gone away. :wacko:

 

The tensile strength is in the outer "skin" as that will be stretched most by a load distorting the component.

 

Mild steel is good for about 2.5 tonnes per square cm cross sectional area in tension, it's about twice as strong in compression, (actually hardness).

 

Compression amounts to much the same as in any practical application it's going to fail by buckling and so stretching the skin on the outside of the buckling to the point where it yields. 

 

Smaller holes have a tighter radius that could concentrate the force more at a specific point.



#12 tiger99

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Posted 29 May 2016 - 07:17 PM

That is excessive and unsafe. Fairly rapid fatigue cracking and complete failure are inevitable. It is also short of static strength.

Cutting holes with simple tools like a holesaw leaves a very rough bore with umpteen stress raisers which will initiate fatigue cracking. If you must have plain holes, and less of them, at least polish the bores with a flap wheel or grindstone.

I would suggest smaller, swaged holes next time. It is important that the method used for the swaging leaves the rim of the hole in compression, not tension, as this makes an enormous difference to the risk of fatigue cracking. A properly done subframe with safely swaged holes will be a little bit lighter, not very important in the overall scheme of things, but it will effectively introduce a crumple zone and reduce the very serious risk of trapped feet in a frontal impact.

What I can't yet tell you is where to get a set of swaging dies which will leave the edge in compression. I need some myself for something else. The best I could find were in the US and advertised as suitable for use on light aircraft, however as far as I could see they were leaving the edge in tension, which is seriously unsafe. I think you need a 2 stage process to do it correctly and get rid of any tension.

#13 sonikk4

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Posted 29 May 2016 - 07:40 PM

There is Dimple Dies on eBay who claim to make their dies in Sheffield and will make them to order. However nothing mentioned about metal thickness that it can be used on.

 

http://www.ebay.co.u...s-/301925019442



#14 Carlos W

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Posted 29 May 2016 - 07:47 PM

I'd be seam welding all the joins.

 

A couple of the areas from the horizontal to vertical look a bit thin



#15 davefitz

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Posted 29 May 2016 - 09:00 PM

Thanks all appreciate the feedback, and just to clarify the subframe has been seem welded in all the right areas with strengthening plates added etc. All the holes were finished out with a die grinder after the hole saw so are smooth. My main concern is with the front cross member, yes I should really have kept the holes a little smaller so as there was a bit more meat closer to the folded part of the cross member and I feel there is still considerable strength in tension and compression but wonder how much these subframes flex/twist during use. The auto test guys here are telling me its on the limit but should be ok. Has anyone actually seen a lightened subframe twist or buckle? Its hard to imagine what type of forces are actually transferred to this front cross member during cornering. Obviously they are considerable through the towers and rear section.

 

Swagging the holes is a nice idea and surely add strength, but hole are defo too big for it now. If I decide it is too far and the subframe is at risk of over flexing, buckling or twisting them I might just cut out that front section and weld in some tubular across in its place?






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