Can leaf springs really handle more weight than coils

Submitted: Thursday, Jul 23, 2009 at 14:34
ThreadID: 70908 Views:9283 Replies:12 FollowUps:4
This Thread has been Archived
Hi everyone, got a question.


The theory is that leaf springs can handle more weight than coil springs, but apart from that, coil springs are normally better, but I’ve been wondering if this is technically the case with most 4wd’s.

What gets me thinking, with nearly every 4wd, by the time that the car is that loaded up / towing that much that coil springs can’t handle the weight but leaf springs can, the car is way over its maximum load carrying and towing capacity and the rest of the car can’t handle it, like the brakes, engine, steering etc.

It’s like that with my 80 series turbo diesel, its maximum towing capacity is 2500 kg and when I’ve had about that weight, the engine and brakes and so forth can only just handle it, but the coil springs hardly sag enough to worry about and the inch more sagging than if it were leaf sprung isn’t noticeable when I’m driving, apart from that, I’ve got all the advantages of coil springs.

It’s the same with a 3 litre V6 petrol pathfinder I’ve got, when I’m towing a speedboat with it, it can only just do it and probably couldn’t do any more, but the springs don’t sag that much at all and it’s not noticeable when I’m behind the wheel, and that’s with poor quality suspension on it.

It’s like that with all your 4wd’s around in the 60’s, 70’s and 80’s, like 40 series cruisers. The weight they can handle with their engine and braking and so forth, coil springs would hardly notice, yet the manufacturers made them leaf springs worried that coils couldn’t handle the weight.

One thing to, if you’ve got coil springs, you can put those air bags in the springs, leaf springs you can’t.

I don’t know if this would be the case if you had a Hummer or an F350 or something, and it certainly wouldn’t be the case with trucks, but it makes me wonder about your ordinary 4wd. Technically, at the end of the day, does having leaf springs really mean that you can have more weight than if you had coils?

Is what I’m saying a good point or is it nonsense? Car engineers are meant to know a million times more about these things than me, is there something I don’t know?

Thanks everyone
Back Expand Un-Read 0 Moderator

Reply By: Skippype - Thursday, Jul 23, 2009 at 15:43

Thursday, Jul 23, 2009 at 15:43
Kingkennas
My Toyota 4.6 V8 turbo diesel ute has coils on the from and leaf springs on the back. I have added airbags in the back without any problems. You don't need coil springs for air bags....
Skip
AnswerID: 375834

Follow Up By: kingkennas - Thursday, Jul 23, 2009 at 15:45

Thursday, Jul 23, 2009 at 15:45
Thanks didn't know that, never researched them or used them
0
FollowupID: 643167

Reply By: tim_c - Thursday, Jul 23, 2009 at 15:52

Thursday, Jul 23, 2009 at 15:52
While not being a suspension expert, let me suggest the following:

The load carrying capacity of a spring (either leaf or coil) is more dependent on the rating of the spring rather than whether it is a coil or leaf. The rating will be determined by the type of material the spring is made from, the thickness of the material, and with leaf springs the number of leafs.

When towing, most of the load will be on the axle(s) of the trailer so the suspension on your car should not sag much. If it does, you need to fit a weight distributing hitch to keep the car level. The sag of your suspension will be more noticable if you load up the car to its rated GVM (gross vehicle mass).

Airbags (eg. polyairs etc.) can be fitted to leaf sprung vehicles, but they are still enclosed within a metal 'coil spring'. This additional coil does not affect the suspension characteristics but it acts to contain the airbag and prevent the sides from bulging out.

Finally, one important aspect of leaf springs is that a suspension system using these is far simpler to mount. A leaf spring can serve 3 functions by providing the spring (suspension), hold the axle in position and provides some damping (because of the friction of the leaves rubbing together). Meanwhile, a coil only provides the spring. Axles need to be held in position (ie. prevented from moving side to side or forwards and backwards) by a series of linkages - these complicate the set-up slightly, add further weight, and of course: cost in manufacturing. Finally, a coil spring provides no damping effect.
AnswerID: 375836

Reply By: Serendipity of Mandurah (WA) - Thursday, Jul 23, 2009 at 15:57

Thursday, Jul 23, 2009 at 15:57
Hi Kingkennas

Like you I am no engineer but I have a V8 vdj 79 series ute with coils on the front and leafs on the back and I have added air bag support just to carry my slide on camper. The thing with leaf springs is they spread their support over a greater distance with two attaching points per spring unlike coil springs that are pushing up in one place.

On my previous HJ ute I added coil helper springs on top of the leaf springs just in front of the axle. Worked a treat. Lift the back by about 50mm and when a load was put on the extra coils help carry it and stopped the whole tray sagging.

Another issue I could see with just coil springs on a load carrying vehicle like a ute is lateral movement is held by swing arms etc which is different to leaf springs. Not sure if this would be a big factor.

David

Lifetime Member
My Profile  My Blog  Send Message

AnswerID: 375839

Reply By: ben_gv3 - Thursday, Jul 23, 2009 at 16:36

Thursday, Jul 23, 2009 at 16:36
One easy way to compare is to look at the Patrol utes as they come in both coil sprung and leaf sprung flavours in the rear.

I believe the leaf sprung models had higher load capacities and the coil sprung versions had quite a big fault where the spring seat would fail and/or damage the chassis at the coil seat location.
AnswerID: 375843

Follow Up By: On Patrol & TONI - Thursday, Jul 23, 2009 at 18:46

Thursday, Jul 23, 2009 at 18:46
Proving the problem is not the spring rather the chassis builders design fault.

Any vehicle overloaded will retalliate in some way even leaf spring systems.
Cheers Colin.
0
FollowupID: 643193

Follow Up By: takenbyaliens (QLD member) - Thursday, Jul 23, 2009 at 19:18

Thursday, Jul 23, 2009 at 19:18
Yes re the patrol have a look at this months Overlander 4WD mag.
According to modern astronomers, space is finite..a very comforting thought particularly for people who can never remember where they left things

Lifetime Member
My Profile  My Blog  Send Message

0
FollowupID: 643198

Reply By: Geoff (Newcastle, NSW) - Thursday, Jul 23, 2009 at 16:44

Thursday, Jul 23, 2009 at 16:44
Hi,

At the risk of running to the ridiculous I can't imagine say a Daihatsu Charade with leaf springs being inherently more capable of carrying a load than an Army Unimog with its 4 coils just because it has leaf springs.

To me it's a design decision based on a few parameters. One of the major one's being cost.

The older 40 Series Landcruiser's you use in your example where built that way because of the simplicity of design, the cost and probably they'd always done it that way! I'd guess passenger car comfort wasn't very high in the design criteria either as most 4WD's in those days where owned by out and out enthusiasts or mining company's.

Generally speaking you'll get more wheel travel out of coil springs than leaf springs (assuming all other design parameters are basically the same) when they are both fitted to say a pair of live axles.

A coil spring is generally a more supple spring than a leaf hence the reason passenger cars now days use coils all round.

I'd say there are a whole host of design compromises that go into both types of spring.

To say one is inherently "better" than the other would in my opinion be overly simplistic.

Better is such a subjective term in engineering, it has no quantifiable unit.

Geoff

Geoff,
Landcruiser HDJ78,
Grey hair is hereditary, you get it from children. Baldness is caused by watching the Wallabies.

Lifetime Member
My Profile  My Blog  Send Message

AnswerID: 375844

Reply By: Member - Mfewster(SA) - Thursday, Jul 23, 2009 at 17:12

Thursday, Jul 23, 2009 at 17:12
I have a 60 series LC, leaf springs all round. I have Aussie super springs added to the rear and this has made a significant improvement to the handling. It now corners flat with or without a load onboard. Much cheaper than airbags.
I am of the opinion, and this may be incorrect, that there is much more articulation with leaf springs rather than coils
AnswerID: 375847

Follow Up By: viz - Thursday, Jul 23, 2009 at 20:50

Thursday, Jul 23, 2009 at 20:50
Disagree. Look at a Range Rover with its long travel coils (I am meaning pre IS era). Articulation is huge and one of the secrets of its legendary off road ability.

However those same coils mean that the RR rolls like a yacht in a storm around a corner, especially without swaybars...

viz
0
FollowupID: 643227

Reply By: On Patrol & TONI - Thursday, Jul 23, 2009 at 18:44

Thursday, Jul 23, 2009 at 18:44
kingkennas,
Ask your self this, do we need several thousand year old technology?
The answer is NO.

Even trains and very large trucks can do without them nowadays so why do we live in Roman times still re this antiquated horse & buggy system.

JMHO Colin.
AnswerID: 375855

Reply By: Robin Miller - Thursday, Jul 23, 2009 at 19:38

Thursday, Jul 23, 2009 at 19:38
In a practical application leaf springs can carry more load, for a basic reason.

This is that fundamentally the load is related to the cross sectional area of the steel used to make spring.

It is simply easier and more practical to have a large area with leaves than with coils.
Robin Miller

Member
My Profile  Send Message

AnswerID: 375871

Reply By: Member - ross m (WA) - Thursday, Jul 23, 2009 at 19:44

Thursday, Jul 23, 2009 at 19:44
How can you compare the 2 springs??
By weight a coil would have less metal in it
AnswerID: 375873

Reply By: ben_gv3 - Thursday, Jul 23, 2009 at 20:03

Thursday, Jul 23, 2009 at 20:03
From a suspension point of view, I would prefer a Defender with its 4 coils over a Troopcarrier with leaves in the rear.

Reliability is another matter...
AnswerID: 375882

Reply By: viz - Thursday, Jul 23, 2009 at 20:46

Thursday, Jul 23, 2009 at 20:46
For load carrying, leaf. Handling is compromised. May need helper springs or air bags - yes they do made airspring kits for leaf springs:

http://www.allair.com.au/mp/product.htm

For handling and comfort coils. For more load, bigger coils and/or long tavel coils, but empty they are not comfortable, and the height changes that occur light vs laden affect handling if not set up properly and comfort when light (jarring and pitching)

Compromise? I have air springs in place of the coils. Height remains the same, regardless of load and comfort lightly laden is not compromised. Heavy, it just gets more comfortable...

viz
AnswerID: 375899

Reply By: Eric Experience - Thursday, Jul 23, 2009 at 21:17

Thursday, Jul 23, 2009 at 21:17
Kingkennas
There are many advantages of leaf springs. The total weight of the vehicle is lower. The load is spread out over 2 points so the chassis can be lighter, There is a lot more room for tanks and exhaust parts, they are cheap and reliable, the rate of the spring and the unsprung weight are the determinants for comfort, these parameters can be the same with coils or leaves. The only advantage of coils apart from marketing considerations is lower road noise because the forces that transmit road noise can be separated with soft rubber used in the control linkages. Mercedes use carbon fibre leaf springs to reduce noise and get the other advantages. Eric
AnswerID: 375907

Sponsored Links