Thread: Front suspension

Originally Posted by isuzutoo-eh

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Springs are not used in Land Rovers as a handling component but more for load carrying and height adjustment.

Which shows just how important shock damping is.



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LRA do some nice things, but that statement is completely misleading.

Originally Posted by Dougal

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I run 180 lb/in springs that are 430mm long in my rangie, front axle weight is around 1150kg.
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With nothing firm to base it on I reckon the original Isuzu County springs were around 180lb/in.

All I can recall is that the last 110 County I rode in was a hell of a lot softer in the front end than my 220lb/in Defender is, and I'll allow that some of that is how I've valved the dampers in mine.

Originally Posted by isuzutoo-eh

Steve,
Front wire dia is 17.5 but the Dobinson rep said that'll be 0.5mm of powder coating so 17mm. Rears are 19.5 again with 0.5 of powder coating.
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Just measured mine and they are the same -
Front 17mm and 9 turns (C51-015 AFAIK).
Rears are 19mm and 7 turns (C51-019), not fitted yet.
As I mentioned earlier, with still about 100kg of things to refit on the front, I've got 120mm to the (isuzu extended) bump stops. The standard length shocks that are fitted are OME N45F, and I only have about 30mm of extension left in the shock.

In case you haven't already seen it, there were rates posted in this link for some of the Dobinsons springs (and also Kings and OME):
Lifting the Colonels's 130

A couple of people have posted the found those Dobinson fronts quite a good rate on an Isuzu - but are raised (CHT here: Another spring question?, and RAR110 earlier in this thread).
I know stuff all about suspension, but wondering if your hardness issue is just the shocks being valved too hard.

Question for you experienced suspension guys - apart from substituting different shocks, how do you tell if you've got a spring rate problem or shock valving. Is a short straight test drive without the shocks fitted any help to see if the springs feel OK over bumps etc by themselves?

Steve

Originally Posted by steveG

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Is a short straight test drive without the shocks fitted any help to see if the springs feel OK over bumps etc by themselves?

Steve

Oooh, never ever do that on a coiler, it'd be frightening
A multi-leaf pack has a limited form of damping from interleaf friction but a coiler or torsion bar has none.
The vehicle would be bouncing all over the place.

It's a toughy.

Most all drivers, including experienced race drivers aren't very good at analysing what's going on underneath them.

Some sort of travel indicator, eg a cable tie on a shock shaft can help in ascertaining if your spring rate is too hard.

Shockies/dampers don't limit travel, so a travel indicator will give you an idea if you are running too hard a spring, but in racing all we used to do was keep swapping spring rates till we found what worked at a given track, but we weren't looking to maximise travel, we were trying to maximise grip, and it's a bit impractical to do for the average punter in the real world.
Hell, it's something I've always wanted to do with the Defender and just yet couldn't justify the expense to myself.

To give you an idea, with the Tdi and roughly 105mm between the bump stops my pads on the diff housing are polished, and that's just from driving on the road. (220lb/in springs, 17" free length)
Even when the springs were new and the bump stop clearance was 117mm the bump stops were hitting, but it's never been a harsh whack, I only really know about it from the axle pads being clean.

Lots of low speed bump valving in a shock absorber can give a harsh, jiggly ride feeling (think Bilsteins with their HD valving) but a too hard spring will usually axle tramp and skip on small bumps and corrugations, whereas with the stiff shock valving you'll sense/feel the jiggle but the wheels wont lose contact with the road.

Originally Posted by rick130

With nothing firm to base it on I reckon the original Isuzu County springs were around 180lb/in.

All I can recall is that the last 110 County I rode in was a hell of a lot softer in the front end than my 220lb/in Defender is, and I'll allow that some of that is how I've valved the dampers in mine.

The best way to standardise spring rates is natural frequency, what that comes down to is simply spring rate vs the suspended weight. Many years ago now (2004) I unbolted my front shocks, bounced the vehicle and video'd it so I could check.

I'm not posting the video, mainly because it looks really stupid, but the result is a 0.8 second cycle, roughly 1.25 Hz. 1Hz is apparently ballpark for a road vehicle and you want the rear axle to be a slightly higher frequency than the front to stop it porpoising in time.

The upshot of this is there's probably no point in going firmer than 180 lb/in unless you've got a front axle weight approaching 1.5 ton.

Thanks Ben

Originally Posted by Dougal

The best way to standardise spring rates is natural frequency, what that comes down to is simply spring rate vs the suspended weight. Many years ago now (2004) I unbolted my front shocks, bounced the vehicle and video'd it so I could check.

I'm not posting the video, mainly because it looks really stupid, but the result is a 0.8 second cycle, roughly 1.25 Hz. 1Hz is apparently ballpark for a road vehicle and you want the rear axle to be a slightly higher frequency than the front to stop it porpoising in time.

The upshot of this is there's probably no point in going firmer than 180 lb/in unless you've got a front axle weight approaching 1.5 ton.

There's a lot of other variables to consider too, it's a very rough rule of thumb.

What is scary is when the springs natural frequency matches the tyres natural frequency and you have slightly underdamped shocks.

Been there, done that, have the drycleaning bill from when it bounced me off the road at 180km/h through one of the scariest corners in the country
The driver behind said that the tyres were bouncing clean off the road.

Originally Posted by rick130

There's a lot of other variables to consider too, it's a very rough rule of thumb.

What is scary is when the springs natural frequency matches the tyres natural frequency and you have slightly underdamped shocks.

Been there, done that, have the drycleaning bill from when it bounced me off the road at 180km/h through one of the scariest corners in the country
The driver behind said that the tyres were bouncing clean off the road.

Not so much a rule of thumb as designed to work with us finicky humans. We can handle up/down motion that's a similar frequency to us walking/running, which is around 1Hz. Stiffer (higher frequency) makes us feel all jiggled around, softer (lower frequency) makes us sea-sick. Race car drivers have to put up with being jiggled around.

I thought tyres (unless seriously wrong sized or aired down) were at least a magnitude stiffer than suspension? You wouldn't be running seriously aired down tyres at 180km/h.

Then I recall a mates boat trailer, skinny old tyres which I wasn't game to put more than 40psi into which squished about 3 inches while the leaf springs above them didn't flinch. Bounce, bounce, bounce all the way to the lake and back.