23 replies to this topic

[Grkify]

So my best bet is to go for lightened regular subframe or titanium good to know

Edited by Grkify, 22 May 2013 - 08:03 AM.

[tiger99]

Actually titanium also has a finite fatigue life, so must be properly designed and tested, and is horrendously expensive, and needs very special welding and forming techniques. Size for size, it is not quite as strong as steel, but it wins overall by being very much lighter.

[sonikk4]

Having seen first hand what happens to an aluminium suspension arm on a VW Phaeton that had a gentle knock against a curb and how easily it deformed was quite an eye opener.

 

There are different aluminium types and the way they are formed / treated etc.This in turns determines the strength characteristics of the item being fabricated. as cooperman has already said Aluminium aircraft structure / components are tested to destruction on test beds over a long period of time to determine their fatigue life. Cyclic, compression, torsional, tension, shear loads are constantly applied 24 hrs a day 7 days a week for months at a time and in some cases like the Hercules wings for years. Then these items if they have not failed are intensively inspected using numerous NDT processes.

 

Now doing this with a mini subframe would be time consuming and incredibly expensive to do. The Force racing Titanium one would have been developed for racing purposes and i would assume not every day use.How long would it last i wonder.

 

I would add extra welds to your original subbie and then lighten it by drilling. You could add in some extra gussets for a bit more strength if required.

[Captain Mainwaring]

Well said, Cooperman. It really WILL fatigue, the important question is when! It is amazing how many people are unaware of the fatigue problem with all light alloys. I suppose that they don't teach such stuff in school, but that is easily remedied, by those who are interested. I would recommend a book, The Science of Strong Materials, by J.E. Gordon. It is a Penguin so not very expensive, and a very entertaining read.

Considering that several car manufacturers have had a fairly recent track record of fatigue cracking in steel structures (certain BMW rear suspension mounts and Audi wishbones come to mind), I do have to wonder how safe the latest and expensive alloy-structured cars actually are. It only needs the owner to run it for a higher mileage, or manage to subject it to higher loads, than the designer expected and there is serious danger. Classic car owners beware too, the Triumph 2000 and presumably the TR series had alloy rear radius arms, and they are known to fail.

Don't get me going on wheels, or calipers, or certain parts made for Minis....

The K series engine is clever because, amongst other things, the alloy block is held in compression by the steel through bolts so the fatigue effects are minimal.

Saying "it will fatigue" is about the same as saying "make it out of aluminium", there are loads of different grades, and they are as different as chalk and cheese.  EN8 will fatigue, give it enough duty cycles and it will crack. The key is knowing when.

There are plenty of aluminium alloys that would work very well - only one major problem.....they'd rot out the body they were connected to.

Edited by Captain Mainwaring, 23 May 2013 - 10:31 AM.

[Alex_B]

It wouldnt be hard to avoid any corrosion from ally/steel contact and that wouldnt be the main issue, but the design work required to achieve an alloy subframe would be enourmous, I have a CAD file for a nearly complete mini, I will see if I  can run it through ANSYS with the subframe from aluminium at UNI just to see how it compares but I wont have enough time to modify it to perform the same as steel due to assignments and exams I currently have

[Captain Mainwaring]

Thanks for the replies I am tempted to save money  and go the diy route rather than buy a full out titanium one.

What's the weight difference? how much extra power would you need to retain the same power/weight ratio?

[Captain Mainwaring]

It wouldnt be hard to avoid any corrosion from ally/steel contact and that wouldnt be the main issue, but the design work required to achieve an alloy subframe would be enourmous, I have a CAD file for a nearly complete mini, I will see if I  can run it through ANSYS with the subframe from aluminium at UNI just to see how it compares but I wont have enough time to modify it to perform the same as steel due to assignments and exams I currently have

Think about that again....just how easy would it be to ensure complete isolation of the subframe from the body - if it's only a track day car, then this may not be an issue.

If you're not going to mod the design then it's a bit of a dead end exercise...just casting a weather eye over it you can almost guess where it will fail - be interesting to see where you determine though.

Anyway, what grade of ally are you going to use? 

Edited by Captain Mainwaring, 23 May 2013 - 10:45 AM.

[Alex_B]

Well the subframe is only mounted on a few sections but you could use a coating on the subframe or similar? and I actually dont know yet, I dont have any access to CAD or testing softaware at home so I need to see how complete the mini model is and if its possible to change the materials of the subframes, I didnt model it so it may not be complete enough to test for anything other than interest (I wouldnt be seriously considering designing an ally subframe just curious)

[Captain Mainwaring]

Well the subframe is only mounted on a few sections but you could use a coating on the subframe or similar? and I actually dont know yet, I dont have any access to CAD or testing softaware at home so I need to see how complete the mini model is and if its possible to change the materials of the subframes, I didnt model it so it may not be complete enough to test for anything other than interest (I wouldnt be seriously considering designing an ally subframe just curious)

You could rubberize it I suppose, it's more contact points between bottom and top arms and the subframe and brake pipes that I was thinking about.

 

I wonder what the difference in weight is between an alloy cylinder head and the CI one? 

What would be interesting if you can calculate the volume of the front sub, what the weight difference would be?