Lightweight

[Guest ady]

I would not expect to build this car (knowing what i have been told) without adding additional strengthening where needed. The dif mounting will need strengthaning by the looks of these photos, all susspension mounts will need crush tubes in the box sections. As for side impact there is always the optio of a cage incoparating side impact bars, but this car keeps getting heavier.

 

I was also considering moving the coil overs inboard, to save on unsprung weight, improve airodynamics and remove some of the 'sharp' objects from the SVA mans 100mm ball. The the intended suspension mounts arn't up to the job as standard there is even more reasion to re-design things isn't there.

[E.Hunt]

pictures of diff assambly here

 

sorry they are so big trying to make them smaller

[Guest steve_m77]

 

 

This top shock mount will need modifying too, needs additional support.

[Guest red7]

Guys some of your comments worry me. As the owner of a stainless steel Monocoque Robin Hood S7 I have had to strengthen high stress areas of my car. If you consider stainless steel is many times stronger than aluminium and not as vunrable to corosion. Sorry to bring this up but your cars do look a bit under engineered.

 

The picture shows the stainless steel plate I have fitted to "beef up" my chassis in the area of the trailing arm attachment points used in the S7.

 

This was neccesary because the floor and back panel had started to bend. I guess due to potholes, speed bumps and maybe a days drag raceing!

[Guest Kieran]

Mmmmm, does look a bit flimsy if thats all thats holding the diff in place. I'd like to see perhaps 4 full height supports, either side of the drive shafts, reaching out to the tub or other structure to counter the inherent twisting here me thinks.

 

Also, the top shock mount doesn't look like the picture anymore, they use a piece of box section, mitred at each end, and mounted to the top of the chassis section.

 

Kieran

[Guest nikscott]

Keiran are you sure they have changed the front top damper mtg. I took delivery of mine 4 weeks ago and it is the same design as the pic above

[Guest Battery Bill]

Steve

Looking at your Strengthening plate, I feel that you could have trimmed a lot off it before final fitting.

The red line shows what I mean, you should round off corners and really using some sort of Jointing compound or sealant is better

I do agree that the LW looks a bit flimsy and requires a lot of strengthening

[Guest red7]

Thanks for that Bill, I'll run with what I've done for a bit and see how it goes. I could'nt face unbolting all those nylocks again when the weather is still OK for driving.

 

If a SS S7 bends what will a lightweight do I ask?

[peter_m7uk]

Can we just bring a bit of science to the debate?

 

In the attached picture, you can see that the fatigue strength of

aluminium decreases as it is stress-cycled eg vibration, absorption

of road shocks etc.

 

I think the problem is not so much the ultimate strength of aluminium,

but that it will always fatigue before steel. In fact, the lightweight

chassis is *guaranteed* to fail eventually, whereas a steel one will

not, unless rust attacks it. The number of cycles that we are looking

at could be in the order of billions, however, so I'd like to know how

that translates into years-of-road-use...

 

As for the diff, I would say my Pinto will generate something like 150Nm

peak torque and the diff mounting holes are around 20cm from the propshaft

axis. So, a quick calc suggests that each of the 6 holes will have a force of

(150/0.2)/6= 125Newtons acting on it, which is like a weight of 12.75kg.

I can't see any reason why each hole can't cope with less than 13kg?

Thinking about that though, the torque is multiplied by the gearbox in

the lower gears, isn't it?! What is the ratio in 1st gear for a Type9?

I suppose the other exception to that is if you stall the engine by putting it

straight into gear with the car stationary, so grinding the engine inertia

to a halt in a fraction of a second - That would send quite a forceful shock

through the diff! No one would drive that badly, though, would they?!!

 

Anyone care to argue or point out a mistake?

 

Pete

[Snapperpaul]

Ok just to put fuel on the fire, i can only look at your calculations and assume you are correst as i havn't got a clue. HOWEVER i did think about Torque multiplication in the lower gears that would load the bolt holes. I found many sites that explain this, here's one. http://www.mustangsandmore.com/ubb/DanJonesTorqueVsHP.html

 

Anyone care to argue or point out a mistake?

 

Type 9 1st gear 3.65 : 1

[Guest timswait]

Did you take account of the gear ratios? If not then for first gear then you need to multiply the engine torque by 3.65, so you've got a load of around 35kg on each hole. As you say, fatigue is the real issue, each time you apply and lift off the accelerator you are putting one cycle on it, so every gear change adds a fatigue cycle, I'd guess the number of gear changes you'd make in the life of the car is in the order 10000-100000, so (by the graph) you've got around about half the static strength available to use. Also there will be vibration from the engine transmitted through the diff. This will be of much lower amplitude, but will probably clock up 10^8 or more cycles. Fatigue analysis isn't straightforward.

The thing that most worries me about the lightweight chassis is the lack of a safety cell around it's occupants. From looking at the demonstrator on the RHE stand at Donny I couldn't see any rigid longitudinal members in the cockpit area that would stop it collapsing in the event of a crash. The design relies heavily on having the edges of sheets folded over to make a box section. The problem with this is that these sections aren't then closed by riveting/bonding the folded section back on itself. A closed section is at least an order of magnitude or two stiffer and stronger in bending and compression (the modes relevant in a crash) than an equivalent sized open one. As a result I could flex the edge of the cockpit with my hand, quite worrying when that appeared to be the main longitudinal member.

I also think that the chassis overall isn't stiff enough overall. There was a report somewhere on this board that the lighweight weaved over bumps. It was suggested that this was down to excessive unsprung mass, I suspect it was down to the chassis flexing. The torsional stiffness of a chassis should be at least ten times the roll stiffness of the suspension used. I very much doubt this is the case on the lightweight, and as far as I know RHE haven't done any tests on this.

Sorry to be such a pessimist, and I do think that a lightweight could be made into a decent car, but I think there's a lot of issues with the design, and the the builder will need to consider each bit of build carefully, and it may take more than bolting on the odd plate to properly solve.

[peter_m7uk]

Without appearing too flippant, the safety cell is not one of my concerns, just the mechanical integrity! I know that if I'm driving around in any sort of kit car, that

I'm taking a chance with my life in the event of a smash, but that's just a risk I'm

willing to take, as with a lot of Seven drivers, I expect.

 

As for the suggestion that the Lightweight weaves over bumps, no one could know that except the people at Robin Hood! No customer has yet finished a car and the

RH demonstrator was only recently SVA'd. I haven't heard of 1 person having a

test drive yet....

 

I think the chassis will be very stiff at the back and around the engine bay, but as

Tim says, the cockpit edge is not a closed box and will be required to cycle through tension and compression a lot, also at a riveted joint where the chassis

tapers toward the nose. Having said that, Richard Stewart told me he was "amazed" at how stiff the chassis was under testing (no one has seen the results!), so the jury is out....

 

Pete

[enforcer]

And all I'm worried about are the sharp edges on my repeater lenses!

[Guest ScotMac]

Pete, i don't want to rely on essentially sheet metal for my differential mount points. Even *if* theoretically it won't break, w/out reinforcement, it WILL flex more, and as i have said...flexing aluminum is (eventually) BREAKING aluminum.

 

Note, did you just use 5000 series aluminum as an example, or did you actually find out what grade aluminum the l/w is????

 

BTW, in terms of the front top coilover mount, switch to inboard shocks (didn't i mention it 2 or 3 pages of posts ago?? ;-). It also gets rid of some of that unsprung weight, that was recently complained about here.

 

Cheers, -sm

[Guest ady]

What we need to achieve here is stiffness not srength. using large flat sheets is not good for stiffness, even thick ones. I would think that ether ribs need to be added into this pannel or extra plates need to be added, bent into a U to provide the stiffness.