Hi all

I wonder if anyone can shed some light on how to construct a De-Deon back axle assembly using a sierra diff or maybe even a Rover SD1 LSD. I have seen various post regarding this system but none that show a fool like me how to go about the construction and the materials one might require.

Thanks again in advance
Julian Brewer

Well i have had a think about this and sketched a fag packet drawing. Is this in essence a de deon system. I take that it will still require a pan hard rod at one side but would you think only one is required?

With reference to the de deon tube itself has anyone any idea on the material type and sizes.

Thanks again
Julian Brewer

Julian, that is a good representation of a DeDion system.....other than the fact (as you say) you need some some lateral loctation......panhard rod typically...

Without out any sideways location the axle can float left and right by using up the the spare travel on the splines (not good..)

Typically I've seen De Dion tubes around 2.5"/3" x 16g...in regular mild steel tube......maybe someone can measure one...?

HTH,

Alan

Check this out. The P6's De-dion setup.


http://forum.locostbuilders.co.uk/xmb/photos/cgi-bin/showpic.pl?p6_de_dion.jpg

I (used to) have some pictures of the setup used by MK Engineering somewhere. I can dig them out if you want.

ATB

Neil.

Hi,
Check out my website for pictures of my MK engineering based de-dion setup. I think the de-dion tube is 50mm with a 2.5mm wall as someone else asked me to measure it. I will check tonight for you. I also use a panhard rod bolted to a double shear bracket welded to the back of one of the de-dion "ears". I think it is an excellent setup and would be happy to provide any dimensions or photographs (I have a digi camera) or advice that anyone would like. Just ask and it shall be done!!

I think that more people should use de-dion as it is a very simple way to convert a live axle car to use Sierra rear running gear. All you need is a pipe bender and a few bits of metal cut from a pattern and welded to form the ears and hub carrier mounts. Much easier than IRS and with equally good handling qualities and better traction by all accounts.

You can also make it to suit any width of car by simply extending the de-dion tube which would make it very flexible for those using wider chassis'

Hope this helps,
Craig.

Thanks guys for the information and i would be very grateful if you could send me as many pics of the De deon that you all have to hand.

I have yet to take my v8 sd1 to bits and i don’t know if i can utilise the rear diff but i hope its possible. I don’t think its the same as the older p6 but i am hoping it is a lsd diff.


Thanks again for all the help
Julian Brewer
Email Julian@fsdv3.com

Does anybody have pictures/drawings of the de-dion system used by cat..ham? That would surely provide a useful starting point.

Although my build is mid engine, I fabricated a deDion systyem that may give you some ideas. I used 3" round 16ga with a single bend at the center to clear the diff. Check out the 'Engine Cradle' link in the Gallery section of my website.

I am in the process of converting from Panhard bar into a Watts Link. But the website still shows the Panhard bar.

Graber

This post should have a 'www' button at the bottom.

Can anyone help with drawings, pictures details on the de dion rear end. I'd like to get my head round it, but haven't really seen a decent picture or drawing.

Steve,
I'm curious, why go for the watts link instead of the ubiquitous panhard rod? I'm not trying to be funny or anything btw, I'm genuinely curious what the differences are?

One other thing, how do you use a watts link when the diff is fixed to the chassis and not to the live axle. I've never seen a watts link on a non-live axle car. Have I missed something?

Cheers,
Craig.

Ah cool, someone's asking the de-dion questions. I was going to get in touch with you soon craig, and pick your brains..

if you can send pictures, dimensions etc my way as well, that would be great.

my co-builder and I are working on a CAD model of a mcsorley 7+4 with dedion, cymtriks mods to take a rover v8, in consultation with jim mcsorley. now seems as good a time as any to start figuring out the dedion

rather than hijack this thread I will start another about diffs..

Crowley, I have tons of photos on my website. It functions like a live axle, but the diff is bolted to the frame.

Craig, Actually, I would have simply kept the Panhard had the dang thing not been going across the only place I could route the stock exhaust for my transverse mid-engine. It was either fabricate a completely custom exhaust header or use a watts link.

I am very, very close to finishing the Watts Link. I promise to post some photos of how it attaches to the DeDion and to the frame.
Rescued attachment WattsLinkBala.gif

There has been a lot of discussion about various rear axle locating on http://autos.groups.yahoo.com/group/Locost_Theory/
Have a look. There are some useful discussion and links.


Terry

Steve,
That makes sense now.
Of course you can attach the watts pivot to the de-dion tube itself... Doh!

Thanks for the diagram.
Craig.

The SD1 has a live axle located by a watts link and torque tube.The axle would be to wide and is also rather heavy, I had an SD1 and it did handle very well for the size of the car but i dont think any of the rear end could easily be used in a Locost.
Paul.

quote:

---

Originally posted by Julian B

I have yet to take my v8 sd1 to bits and i don’t know if i can utilise the rear diff but i hope its possible. I don’t think its the same as the older p6 but i am hoping it is a lsd diff.


Thanks again for all the help
Julian Brewer
Email Julian@fsdv3.com

---

I mentioned earlier in this thread that I would post a photo of the Watts link I am building. Well, it's finished. I have posted many more photos of it on my website. Go to the Diary and follow the link off the latest entry dated 7-7-03

Graber

You may note that my deDion tube is 3" 16ga.

[Edited on 7/8/03 by sgraber]
Rescued attachment Bala07-07-03 009.jpg

I would not advise using less than 2.5 inch diameter or less than 2mm/14 gauge wall thickness for the tube.

I have measured a Caterham tube OD and it is 2.5 inches.

For a Deon tube resistance to bending is much more important than axial stress so diameter is more important than the cross sectional area of metal

For a 20000inchlb bending torque the stresses are-

2.0-16g 109500psi
2.0-14g 89800psi
2.0-12g 74000psi
2.0-10g 61500psi
2.5-16g 68800psi
2.5-14g 56100psi
2.5-12g 46000psi
2.5-10g 37900psi
3.0-16g 47100psi
3.0-14g 38300psi
3.0-12g 31300psi
3.0-10g 25700psi

As poor steel yeilds at about 50000psi and decent steel at about 70000psi I'D advise chrome moly for the tube as oppose to any old stuff.

The 20000inchlb moment is a guess at what the maximum might be. It is down to you to determine if this suits your car as this is getting a bit like guess work. How big is a pothole or a bump?

I seriously doubt if most kit makers even think as far as the above calcs. Some kit parts I've seen look as if they were just made up on the day without any real thought at all. I would advise that you copy the competition with care!

Cymtriks,
Any chance of a diagram 'cause I'm having trouble visualising the force vectors and moments. My de-dion system designed by MK Eng uses 2" 2.5mm wall tubing and "intuitively" it looks sturdy enough for the application. I am wondering if stress depends on where your springs are attached and the distance between the axle centre and de-dion tube centre.

I'm not doubting your figures but I find it hard to visualise without further info.

Cheers,
Craig.

Cymtriks,

A few points/questions...

1 - Where did you get the 2000 inlb bending moment figure from? I don't dispute your stress figures...I'd just like to know why you chose 2000 inlb

2 - I agree about the stiffness (resistance to bending) being more important....so why suggest Chromoly? when it's stiffness is similar to mild steel......I can't imagine anyone (other than a racer) having a chromoly DeDion tube...

BTW, I'm just trying promote further discussion and understanding, not nit-picking or anything.....

Ooops sorry, I see you did explain the 2000 figure ......

I agree with Alan. The diference in stiffness between MS and CroMo wouldn't warrant the use of CroMo in this situation, never mind the additional hassles of cost/welding.
I've made deDion tubes from 57.2mm X 2mm ( 2 1/4" X .078" ) MS tube, for cars weighing around 1100kg, and believe me, they saw some severe abuse.
On a slightly different tack, I've also made perfectly good ARBs from plain old ERW tube too.
I don't dispute your figures either, I just have a good handle on what works and what doesn't.
Anyone care to chop up a Rover (or a more similarly weighted car to a sevenesque) deDion tube for a comparrison?

I don't have a good grasp of the forces involved in a dedion tube, but with the shocks mounted near the wheels it doesn't seem, intuitively, that it would have to resist a particularly great bending force. Just need to rotate the opposite wheel when one bounces up and down.

Feel free to enlighten me. I speak maths and physics, by the way.

All I can say is that my deDion tube feels very light (lite as air compared to a live axle eh?) and it is 3" 16ga.

Blueshift - I lived in St. Albans for 2 years as a young child. I still remember the St. Albans cathedral and the large lake where I once fell in...

Blueshift,
Yes that is exactly what I meant by "intuitively" seeming sturdy enough. I would love to see a force diagram showing where the bending forces come from and the relative magnitudes. I also have a great deal of respect for MK Eng. and if they designed my de-dion axle then I am pretty sure that it is up to the job!

C'mon Cymtriks, you've got us all worried now...
Cheers,
Craig.

The 20000inch lbs is a guess at what the worst bending load may be.

I have assumed that a lateral load in cornering may be up to 3g. This includes bumps before anyone says it looks a lot for cornering! By the way 3g is pretty close to what some colleagues of mine actually measured in a real car being given a spin round a fairly grotty area of tarmac. I suppose it could still be argued that sensors inside the car do not see what the suspension does but 3g as good as my real data gets.

I've also assumed that the corner weight is 385lbs. That's one quater of a total weight of 1540lbs (700Kg) composed of car plus two occupants. Another guess but not too far out for most sevens.

For a Deon beam 11.5 inches above the ground that gives 385 x 3 x 11.5 = 13282inchlbs.

I added a safety margin of 50% giving 20000 inchlbs. I know that 50% isn't that big for a safety margin but this seems to give reasonable results compared to actual Deon tubes.

For the tube sizes given by Rorty and Craig the stresses are-

2.0 12g (2.6mm) 74000psi
2.25 14g (2.0mm) 70000psi

The reason I suggested chrome moly is that it has a higher yield strength than cheaper steels. I've seen lots of figures quoted for steel and standard mild steel seems to be in the range 50000 to 70000psi depending on the exact steel in question. As the stresses I calculated are at the top of this range I suggested going to chrome moly as this usually lies above 70000psi for yield.

I am interested in what you all think of this approach and I am aware that it seems to err on the strong side. Does anyone actually know what the yield strength of typical chassis tube steel is?

Cymtriks,
Ah, now I know what you are talking about. (You have to excuse my slowness of uptake...)

You are concerned that the ends of the de-dion tube may bend and thus collapse due to excessive bending loads introduced by the "ears" of the axle from the leverage of the road on the wheel. (At least I hope I have understood you correctly...)

I'm not sure how you convert bending torque into pressure though. I'm not a mech engineer, only a mere elec engineer, could you please explain the maths if you get a moment?

Cheers,
Craig.

cymtriks, does your model assume that the bar is loaded at one point, on an end face as it were?

I wonder if it makes a difference that the hub carrier attaches to the dedion bar by two ears spaced some distance apart (around 30mm I think on the plans I have).

Intuitively, again, to me it seems that the further apart those ears are the less.. whatever. twist? on the bar. I'm not sure what the right word would be, torque, load in a specific area, or what. but it would be more spread out over length, if you see what I mean.

Guys,

Let's put this in perspective...

MK have made many cars with and without DeDion rears.....
Caterham also...
Myself and Rorty between us have actually built in real life many cars and raced many of them...

I am aware of no failures of a serious design related nature at all.......

With all respect to Mr Cymtriks when he shows us some real life examples of his work I'll take his warnings more seriously than the combined experience of the above.....

I'll state again...I do not dispute the figures only the assumptions and parameters on which they are based....especially when they are in conflict with real life experience.....

I don't say this as an old guy knocking new technology...I have a mech eng. degree and have used FEA etc. ......but, in the real world an ounce of practice is worth a ton of theory....I know this because I've done both....

Again, no offence intended to Cymtriks....but as an "old guy" I'll always go with what is known to work....

<flameproof suit on.......>...

So alan, what size / thickness tube do you think one could get away with for locost dedion?

we want to stick 200+lbft of torque through our diff if that makes any difference.

Personally, I'd go with 3" 16g...as Steve did......if you can't fit 3" in then I would consider thicker wall............Cymtriks list is a useful guide of their relative strengths in a bending application..the lower the stress figure the better.....

As I said earlier I'm not disputing the numbers....I'm just saying don't ignore what is already proven to work..

Guys,
I know my results indicate a tube built on the solid end of the scale but I'm genuinely interested in where the figures went wrong.

The tube stresses are right for the load given. The results I posted above were from basic text book beam equations. However I have made an f.e. model today and it gave virtually identical stresses. What the f.e. does show is that the maximum stress is distributed over a narrow strip going from the loaded end to where the lateral restraint is applied. My model resembles a Caterham design so this is at the centre of the tube. I hope to be able to post this as a picture soon.

The wrong part of the analysis that leads to big tubes may be sought out as follows-

11.5 inches distance from the road to the beam. This an assumed value but should be close for most applications.

3g lateral. This is a measured value but was measured inside a car. What the suspension sees may be different.

385lbs corner weight. This is an estimated value but should be close enough. Using the corner weight may not be appropriate for lateral forces.

plus 50% for luck. A complete guess but this is lower than typical safety factors. The lowness is my attempt to reduce the tank like answers.

50000psi yield strength for basic steel. This figure is published in plenty of texts but so are a whole range of yeild strengths from 45000 to 220000 for different types of steel.

If anyone has any thoughts on the above then this might help indicate the actual load on a suspension. With that my table of stresses could be used as a look up table for design; i.e. estimate the weight on the rear axle and look up the sizes of tube suitable in standard tube and for chrome moly.

No more guesswork and no more overweight designs just to be safe.

A final thought. Most of the experince indicates tubes with stresses about the same or lower than my suggestion.

I'm curious to know what model of a dedion bar you're using. I don't know how a caterham bar compares with the ones I have plans for.

I am CADing up my bar at the moment and could send you the model / pictures if they'd be of use, just to see if I'm on the same page as you.

Yeah I was thinking the same thing.
Does anyone have a picture of the Caterham design so we can see roughly how it goes together?

Cheers,
Craig.

quote:

---

Originally posted by Syd Bridgeif the corty and escort diffs worked in general use, and racing, then this is the size tube to use?

Just a thought, anyway.

Cheers,
Syd.

---

true but i was just pointing out that the general racing comment could be mistaken.
and the way ford recomended the upgrade.

BTW the capri 'BRSCC' cars had the same mods as the rally kit but then they were also running cossie developed 3.4 v6's

Hang on though, on a de-dion setup wouldn't the bending force be localised around the point where the "ears" attach to the outboard ends of the de-dion tube? If I understand you correctly here Tim you are meaning that the escort axle used to break near the diff? If so then I think this is different altogether. A de-dion system wouldn't place significant loads in the middle of the tube IMHO. Would it? Hmm?

Also, isn't the material used in a typical live axle inferior to CDS tubing?

Cheers,
Craig.

it didn't break as such the axle tubes used to bend so the half shafts didn't run concentric.

and the material used was very inferior to cds tubing. the loads on the de-deon would be at the greatest at the hub location points so very good welding would be required.

De Dion bar that Craig might recognise at
http://www.leetfighter.com/gallery/view_photo.php?set_albumName=solidworks&id=dedion

No measurements on that picture, but you can see where the trailing arms link to the ears, so that's from your chassis.. and the sierra hub carriers bolt on the outside, so that dimension is whatever it is.

the bar is 2" OD 2mm, carrier plates 1/2", carrier tubes 4" OD 2mm, everything else 3mm plate, all mild steel

don't read too much into the relative positions in that picture, they were just guess-fits.

No panhard rod / watts linkage on there yet either.

That the kind of thing you're modelling, cymtriks?

Blueshift, I just checked out your other pics....good stuff..

Picture 12 is erm..?..........interesting...

A slight photoshop job on a picture of my co-builder mackie there. Bit of an in-joke

The tube I've modeled is basically the Caterham design. I hope to post some actual pictures of this analysis soon. I need to get a picture off my works computer onto a disk and then take it home to post.

I modeled the Caterham as it has a simple straight tube with a central A frame and two upper trailing links making it a very "clean" design i.e. easy to model in a restricted time. It is also proven and I have actually measured one as having 2.5 inch OD.

In order to simplify things the ears are represented by a force distributor which transfers the lateral force from the tyre contact patch directly onto the tube ends. This saves having to include the tyres, wheels, hubs, etc... However it does mean that the ears are not included, just the tube itself.

The results for lateral cornering forces show that the compressive stresses are concentrated in a long thin strip along the side of the tube nearest the contact patch of the tyre. This is exactly what simple beam theory predicts but is interesting to see it visually.

This may be why the very high stress that I calculated in earlier posts does not seem to be a problem in practice. It may be high but it is acting on a small area so its effect is not as much as you'd think. I know from real compressive buckling tests that I have done on Aerospace parts that sometimes the maximum stress, if acting on a small area, can be less important than a smaller stress applied over a large area.

Another reason why the high stresses may not be problem is that I have assumed an over the top load in the first place as a lot of assumptions are included in its calculation. If anyone knows what loads actually apply or if anyone knows what the specification of off the shelf tube is likley to be then let me know.

An A-frame? sounds like the caterham tube is quite different to the plans I have. Did you see the picture I posted?

Timf,
Regarding the "very good welding" you mentioned. I agree 100% and that's why I have only tack welded my axle bits in position so far and intend to get them welded professionally.

That raises a slightly off topic but relevant question: What type of welding is strongest for an application such as this? Tig or Mig (or other?) I was guessing Tig but it might be easier to find someone who is a "good" mig welder which I guess will be yield a stronger weld than an "average" Tig welder?

Blueshift, yes that de-dion tube does ring a bell...
I think from what has been said I would build a de-dion tube using 2.5" 2mm wall if I did it all again but I'm certainly not worried too much about my existing axle design. As Syd said, I'm not planning to Rally it or jump off ramps so it should cope fine.

Cheers,
Craig.