Calling the engineers...

[Tommo&Claire]

Inspired by Matt's experiences driving down through Africa and that horrendous road in Kenya, i was having a conversation the other night which illustrated that i still don't really understand suspension differences and why...

I know it's all to do with unsprung weight etc, but i still can't get my head around it.

Example we were discussing...


If you barrelling along in an 80 on that road and hit a bolder, would the weight and subsequent inertia of the much heavier live axle setup try and crash the bolder out of the way, causing damage, where as a 90, 100 or 120 with IFS would bounce over it with maybe lesser amount of damage because of the lower unsprung weight?

OR

Does the heavier duty setup of the live axle mean you are less likely to damage something?


The maths suggests the former probably is true, lower unsprung weight means less force is transferred when it hits the bolder. However common sense seems to suggest the heavy duty live axle is simply stronger and therefore less likely to be damaged?

Thoughts?

 

[Chas]

If you barrelling along in an 80 on that road and hit a bolder, would the weight and subsequent inertia of the much heavier live axle setup try and crash the bolder out of the way, causing damage, where as a 90, 100 or 120 with IFS would bounce over it with maybe lesser amount of damage because of the lower unsprung weight? Thoughts?

I did just that in Croatia in my 80 at about 50mph, clipped the corner on tarmac, but hit an unseen boulder in the grass verge, it threw the Landcruiser across the road into a very convienient side turning, result? no damage at all apart from a bent steering rod

 

[EvilEd]

if you hit the front of the axle, the live axle with coils would be pushed back against the control arms and would either, smash the object out of the way, or cause huge damage to the axle if it didn't break the object. if the wheel hit, on a live axle, again the backward pressure would be against the control arm and so the tire would not lift as easily. with IFS it would probably snap the wishbone and still leave you buggered, but the tire would lift easier if the object was slightly sloped.

On a rough road, the vertical nature of IFS should mean a smoother ride, but anything big will hurt it. With a live axle, the lift options are much better, and bigger tyres are possible (less chance of hitting, object would be lower, and you can armour the axle tubes and diff cover. you can also fit stronger control arms etc. but it's still like running around your garden with a rake and them dropping the rake into the ground with the handle at your belly. the rake is gonner stop and all that force is going up the handle to the load point.

This is all generalisation based on knowlege of Jeeps etc. not LC specific.

If you then add cart springs into the equasion, these are possible the best in the situation but the worst ride overall. with the cart springs... sorry, must call them leaf springs. they will allow the axle to twist a little in effect twisting the axle over the top. there will still be damage, but the u bolts that attach the axle to the springs would probably sheer and allow the axle to move back thus reducing the damage to the axle itself. your diff internals and driveshaft may be a different matter !

Far to many unknowns to answer fully

 

[Jon Wildsmith]

The live axle only really provides better protection from sticks and stones in tearing CV boots on IFS, you're more likely to end up with a bent solid axle casing than bent IFS suspension arms IMO. I'm assuming same size tyres and similar spring rates.

 

[Tommo&Claire]

If you barrelling along in an 80 on that road and hit a bolder, would the weight and subsequent inertia of the much heavier live axle setup try and crash the bolder out of the way, causing damage, where as a 90, 100 or 120 with IFS would bounce over it with maybe lesser amount of damage because of the lower unsprung weight? Thoughts?

I did just that in Croatia in my 80 at about 50mph, clipped the corner on tarmac, but hit an unseen boulder in the grass verge, it threw the Landcruiser across the road into a very convienient side turning, result? no damage at all apart from a bent steering rod

But what would have been the result if you had been driving a 100 say?

 

[Paul]

I think this whole argument will end up in what people prefer over what science suggests is the best design.
I've done rough terrain in both types and not sure what would survive a large impact best as all situations are different but with a solid axle i would think that general long term toughness for long periods on corrugations etc would be best but IFS would give a more comfy ride. this i have found leads to a false confidence in driving faster than you should though where as a solid axle would force you to slow down to stop shaking your teeth out so would may be better in an impact as you are going slower. I think solid axle is less affected with a loaded vehicle in this situation.
On the impact though, with IFS one of the principles is that one side can react to changes without affecting the other side and allow maximum tyre contact with the ground but with a solid axle when one side lifts it will reduce the contact patch on the other side maybe letting vehicle to jump side ways.
Struggling to find a conclusion but as i said, everyone will have an opinion. For 99% of the time these vehicles spend on the road and the majority that never see mud or sand, its no wonder that IFS is now the norm. The 105 seems to be the only real "crossover" and would be interesting to have a drive. Can't wait to see the 100 series that Greg is converting to a solid front axle.

Maybe i should take the left over parts and convert my 80 to IFS

 

[chriscolleman]

As to which design is tougher, I really couldn't say.
You can break anything and probably won't break anything when staying within design parameters.

As to hitting a bolder head on with a single wheel?
I expect the engineers who design these vehicles will sacrafice a part that takes the initial force out of the impact by bending.
Hence you can fix your vehicle and continu to drive it.

With the simplicity of the 80 or the 105 you can repair and align your vehicle with a minimum of tools and adjustments. And there are very few places dust and mud can get into the driveline.

With IFS it get's a little more involved. And when you count the number of seals there are more places sand, dust and mud can get into the driveline.

Luxury is always complicated.

 

[Tommo&Claire]

I have been watching series two of "4wd Take A Deep Breath" which some of you will have heard of, particularly the saffa's as it's made in SA.

This guy (Andrew St Pieere White) did an immense expedition, i think it was 40,000 k's or so, basically from Cape Town to the source of the Okavango River and back. He drives a 105 series but was accompanied by 70's, 100's and 150's for various sections and none of them had a single problem with any suspension related issues for the whole trip.

So i guess in the real world neither is superior to the other.