27 replies to this topic

[pete l]

The torsional rigidity and strength of a Mini Body Shell is pretty high, off hand and from memory around 2500 lb / in. Given the length and weight of the cars, nothing further is needed.

What would a more modern car be ?

[tiger99]

One modern European car was a downright disgrace. BMW E36 Z3,  5,600 Nm/deg on a good day with no rust. About as stiff as a tea tray. That set the alleged high quality manufacturers off on a ridiculous quest to achieve needlessly high numbers, to outdo one another. But the real measure of a successful design is the torsional resonant frequency, because it takes into account the vehicle mass. A lightweight car like the Mini does not require a stupidly high torsional stiffness, but should achieve as high a resonant frequency as a heavyweight. It is likely that the Mini is much better in that respect than certain ladder-chassis SUVs.

 

This site http://blogs.youwhee...rehensive-list/

 is largely confined to cars available in the USA but gives some interesting numbers. Not many classic cars, and those ladder chassised abominations with "torsion boxes" (which were a frequent source of structural cracking) would have scored very low indeed.

 

This link http://www.minimania...Body_Dimensions (please bookmark that site for future reference)

 gives the numbers you are all waiting for, and as expected the saloon is ahead of the van due to the opening rear doors and lack of rear bulkhead. I have converted them for a direct comparison with modern stuff. That is 11,460 Nm/deg for the saloon and "only" 9,320 Nm/deg for the van. Very adequate for a lightweight car!

 

I don't have access to the torsional resonant frequencies and can't calculate them as they depend on mass distribution. But what i can say is that Alec knew exactly what he was doing!

 

If making a convertible, you will not get back to the original stiffness by adding box sections in the sills only, and you will have to be careful not to get problems where thick metal joins thin, which is not to sat that you should not do it to improve bean (bending) strength. A bit too low to add useful side impact protection.

 

The late LJK Setright, possibly the only truly competent motoring journalist of last century (and we seemingly don't have any now!), used to consider that for a road car 5000 to 10000 lb ft/deg was ample. So the Mini's 8,450 in real units was well within the upper range of what he considered to be adequate.

 

I can't find the figure for the old A35 right now, but it was up in silly supercar numbers, designed by ex-aircraft people, as was the 1800 Land Crab, designed by Alec.

 

But please be aware that torsional stiffness degrades quite badly when parts of a monocoque which are only carrying secondary loads are corroded, because they allow the primary structure to flex more, and bulge under compressive loads. And I don't doubt that oversills will send it way down. After a proper restoration, with welds located in the correct places, it will be as original, or maybe even better if you have seam welded where only spots were used originally.

 

I am very interested in stiffness figures for classic cars, so if you have any, this might be a good place to post them, so we can all be enlightened, and see just how good our Minis really are.

 

But to return to the original question, you certainly do not need to connect the front and rear subframes other than via the standard monocoque. You would be hard pressed to make much difference anyway unless you used a cruciform box section like old Rolls Royces, etc, or a true triangulated space frame. It is hard to improve good things.

[Ethel]

It's worth noting that, save for wind drag, all forces must go through the tyre contact patches via the wheels. Little wheels mean less leverage. Small really is beautiful when it comes to dynamics.

 

I'd guess these braced cars are also front engine real wheel drive. A good chunk of the Min's reaction to the driving torque is contained in the front subby between the suspension pickups and the engine mountings, little gets further than the bulkhead engine steady.

[tiger99]

The wheel size has very little to with torsional loads. That is down to suspension stiffness and the difference between instantaneous roll angles of front and rear suspension, which are only affected marginally, being slightly reduced by high profile section tyres. So the instantaneous torsional loads are higher on a Mini with 13 inch wheels because the tyres have a lower profile and attenuate road irregularities less. But a 10 inch low profile tyre would be exactly the same.

And ALL the torsion due to anything but short term transients is fed from the front subframe, through the shell to the rear subframe. When you jack up one wheel under the suspension, the full amount of torsional stress is propagated through the monocoque as steady state loads are not dissipated by the inertia of local mass.

The subframes were introduced only because the previous structure did not adequately spread the point load from each suspension mount into the monocoque. They did not change the actual torsional stress in the centre section, but solved the local fatigue cracking problems.

Edited by tiger99, 26 November 2016 - 02:09 PM.

[sledgehammer]

If making a convertible, you will not get back to the original stiffness by adding box sections in the sills only, and you will have to be careful not to get problems where thick metal joins thin, which is not to sat that you should not do it to improve bean (bending) strength. A bit too low to add useful side impact protection.

 

The cars I have made are not only reinforced in the sill area - that would almost promote twisting in the floor pan

 

they are tied in with upper & lower cross members all gusseted , to give support in the front upper seat belt area , which makes the side impact resistance very strong

 

but only as strong as the door bars can take

 

the sill box section is welded on the lower edge to a swagged 2mm thick  fillet down to the floor pan / inner sill area ,

 

as the inner sill is approx 30 degrees to the bottom of the box , so forming a gap ,

 

the upper (of the box) is welded to the door step / frame , inner sill join approx 1cm down - plenty to stitch to , then fill up the stitch (with weld) when cooled

 

the trick to all this is keeping localised heat to a minimum - it is so tempting to carry on with a weld seam when it is going well ,

 

but heat will build quickly & distort the local area

 

car must be dead square on the ground when any major welding is done - with roof on before any cutting is done

 

and the car isn't much heavier than standard - due to roof & glass missing , weight will increase quite a bit on a car with the roof still on

Edited by sledgehammer, 26 November 2016 - 03:11 PM.

[tiger99]

That is interesting. The 3 dimensional structure will indeed make it very much stiffer. However a long term problem of fatigue cracking exists where thick metal is welded to thin, often at the ends. You may have mitigated that by ending the thick parts in long tapers or multiple radiused finger etc but there are many other ways. Often something like 1.2mm elliptical plates are used as intermediaries between heavy metal and 0.9mm. The seat belt anchors in the sills are probably rectangular but they are elliptical on many cars, not for fatigue loading, because they are generally only loaded once, but to avoid tearing through at corners.

The only bits of antifatigue construction I know of in the Mini is the way the welds are positioned between the inner wing and it's local reinforcement, the doubler plate at the crossmember and damper mount, and the holes drilled in the bulkhead gussets where they overlap the crossmember. These are probably encountering the heaviest loads in the shell and so the line of demarcation between strong and not so strong is intentionally somewhat diffuse. So it might not be an improvement to MIG seam weld these areas but rather plug weld them in the same locations as the existing spots. Except for a short life, highly stressed competition car of course, where long term fatigue is unimportant.

I am very interested in seeing the torsional stiffness of various modified Minis, we're they to be measured. Some limos and most of the cowboy lightening jobs would likely be embarrassing. In my opinion a torsional stiffness test should be in the MOT as it is a good way of detecting bodged structural repairs. Don't know what they would do if yours came out rather high, as can be expected!

[HUBBA.HUBBA]

Solid mounts and a roll cage do it for me

[tiger99]

Actually many roll cages are not stiff in torsion. That is not what they are optimised for. And, you have the potential for fatigue where the heavy sections meet the light sheet of the monocoque. Ideal for competition use but no way would I want one on the road. A Mini does not roll easily, but adding mass high up, in the form of a cage makes rollover more likely. Plus they inconvenience and can severely injure the occupants, and the insurance companies just love them as justification for higher premiums.

But I am very interested in side impact bars, seam welded passenger compartment (increases buckling resistance and both beam and torsion stiffness for hardly measurable extra weight), subframe anti-intrusion strengthening on toeboard, foam filled box sections (torsional stiffness and buckling resistance), crumple zones to the extent practicable, and any other ideas that seem workable. So if my next project, once I have working space for it, has a cage it will be coming out. Someone on this forum will likely be able to make use of it in an appropriate context. But as many other safety features as possible, suitable for normal road use with up to 4 occupants, will be going in. Can't legally fit airbags, but can fit good belts and seats. I would like a rear-firing cannon for blasting idiots in SUVs before they ram the Mini up the back, but I doubt if that will be allowed....

[Ethel]

Not all loads are transmitted via the road springs. Shear loads on the tyres' contact patches will be perpendicular and are what tie rods, lower arms etc are for. It's true you can have bigger wheels with lower swivel joints, but little wheels generally mean lower axles and centre of gravity, which goes with a less stressed body.

[nicklouse]

But as the overall size of the 10-12-13 wheels is just about the same little changes. Unless the ride height is changed or the caster camber adjusted. Which can move the roll centres and axis of roll.

[pete l]

Actually many roll cages are not stiff in torsion. That is not what they are optimised for. And, you have the potential for fatigue where the heavy sections meet the light sheet of the monocoque. Ideal for competition use but no way would I want one on the road. A Mini does not roll easily, but adding mass high up, in the form of a cage makes rollover more likely. Plus they inconvenience and can severely injure the occupants, and the insurance companies just love them as justification for higher premiums.
But I am very interested in side impact bars, seam welded passenger compartment (increases buckling resistance and both beam and torsion stiffness for hardly measurable extra weight), subframe anti-intrusion strengthening on toeboard, foam filled box sections (torsional stiffness and buckling resistance), crumple zones to the extent practicable, and any other ideas that seem workable. So if my next project, once I have working space for it, has a cage it will be coming out. Someone on this forum will likely be able to make use of it in an appropriate context. But as many other safety features as possible, suitable for normal road use with up to 4 occupants, will be going in. Can't legally fit airbags, but can fit good belts and seats. I would like a rear-firing cannon for blasting idiots in SUVs before they ram the Mini up the back, but I doubt if that will be allowed....

I like your way of thinking. I want to make my mini better in case of an accident but don't know where to start.

[Ethel]

True, all Mini wheels are much of a muchness - I had general car design trends in mind. Wheel size was reducing for much of the car's evolution (improving roads and tyres?), but over the last 2 or so decades that trend seems to have reversed. I'm not sure why exactly, it does coincide with cars becoming more of a "design" discipline than an engineering one.

 

You're always going to come off worse in a Mini by virtue of having half the mass of what your likely to bump in to. It doesn't help that the potential point of impact has been getting higher above the Min's main structure (floorpan) with every new generation of models from the mainstream manufacturers.

 

When I was young and stupid(er) and had the odd minor bump, my Minis fared much better hitting solid objects buried in the ground with their bumpers than other cars

[tiger99]

Raise the Mini to the height of a SUV? Spacers between subframes and shell of course, not daft geometry. But there would be PLENTY of room for large wheels and a much wider track.

 

I imagine the average moron in a BMW X5 would not like being looked down on from a Mini on stilts!

 

Ok, well off topic, I will shut up now.

 

 

 

 

 

 

 

 

Please, don't actually take that suggestion seriously!