Suspension calculation help please.

Hi All,
With thanks to all who offered help / advice (on and off list) re going to coilovers and I'd like to run some numbers by you to see if I
I'm going in the right direction please (and this is just for the basic static calcs so I'll worry about the Roll Center Height etc later on). ;-)
It's a Rickman Ranger kit car based on a 1978 MK2 Escort 1300 saloon. It's a 'jeep' style and I'd like soft / compliant rather than race rigid suspension and can't seem to get that via sock off_the_ shelf springs and bespoke are as expensive as coilovers so ..
Let's assume the thing weighs 1 tonne that's 2200 (good ole English) lbs.
Assume the weight is split 50:50 (front / back). I will put it on a weighbridge and weigh the front and back separately asap.
So that's 1100lb on the front, 550lb on each front corner.
So, if I start with a 110lb/in spring then to support that weight it will compress 5". So if the suspension has (say) 4" of travel and I wanted it to sit half way (wet kerbside weight etc) that would mean a preload on the spring of 3"? (do you get spring compressors to suit the 2-1/4" coilovers or is it just a matter of getting in there with the C spanner and winding it up please)?
Other considerations:
The spring sits inside the road wheel so I guess I'd get an increased load on the spring compared with the road wheel (lever effect).
I would subtract the un sprung weight (wheel, lower strut inc brake caliper assy, half the weight of the tca and anti roll bar etc)?
Could those two cancel each other out (roughly) so (in this case) I could end up with the straight 110lb spring?
I would also have to have a spring long enough to accept a full (4") suspension compression without the coils binding.
Looking at places like RallyDesign etc they show a 110lb 2-1/4" spring in only the 10.5" free length but the 100lb spring in lengths from 8" to 14".
FWIW (and according to the HBOL) the std Ford spring has a free length of ~13" but it's suggested was quite light at around 90 lb / in. As it's only got ~6 turns they are quite well spaced. I'm not sure what the details are with coilover types but will ring RD in while and see if they have any extra data etc (I just rang them and they didn't) ;-(
So, are my principles correct do you think, have I missed anything (before I waste any money on unsuitable bits, although I guess this isn't an exact science at this level).
All the best .. and thanks for your time
T i m
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The leverage works to your advantage in terms of coil length.

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On Wed, 23 May 2007 17:40:40 +0100, "Duncan Wood"

Ah, true, because it's inside the other point of the lever (the wheel).
Another 'consideration' (I have since learnt) is I believe the std strut has the lower spring cup off centre to try to offset the side loads on the damper rod?
But ignoring that, if the stock spring *was* 85lb/in than a 100lb/in coilover should be a direct functional replacement (ignoring all the angles etc) and the extra 15lb/in might offset the extra weight of the Rickman over the std saloon (and I still want a soft / compliant ride). The fact that the coilovers are adjustable covers the ride height.
And it started off such a simple solution ... ;-)
All the best ..
T i m
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wrote:

Can I add another equation into the pot, assuming the front end weighs the same (or almost the same) as a standard ford escort, surely the principle would be the same springs, but if the front end is lighter or you wanted a softer spring, it may be easier to get a set designed for an 1100 version as the engine is probably lighter so the spring will have a different compressed length,.......... and there is always the angle grinder if it's still a bit long.......( having watched a mate shorten the springs on his XR3) The Ranger IIRC is a jeep look alike so you will want the raised up look but not the off road stiff suspension.
Having a fair amount of experience with a MK2 escort rally car (read ...spent a lot of money) the rally springs tended to be longer and a hell of a lot stiffer!! mind you..thats 20 year old memory and an empty bank balance!
Having also built a couple of kit cars and getting the spring rate to my satisfaction was a bit of trial and error and visiting a few scrappies for spitfire,vitesse and herald springs and seeing what suited, (rear spring was a special with the 3rd leaf reversed)
remember KISS
Des
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wrote:

Please do ;-)

Indeed. In actual fact I believe the Ranger is quite a bit heavier than the standard saloon (hd steel ladder chassis and substantial fiberglass 'boxy' body).

Well there's the thing Des, in the Haynes manual (FWIW) it states the same front springs for 'All models' (so that's 75-80, 1100-1600) except van's and GT models. Those are often shorter and stiffer as well. ;-(
.......... and there is always the angle grinder if it's

Again I wish that was the case for me. The 'All models' spring is the longest (un sprung) and likely to sit the highest in use (I've tested that avenue I'm afraid). The other springs might start lower (more sporty etc) and be stiffer after that.

Quite right. The problem Des is that without extensive modifications to the suspension mounts etc the ride height is sorta dictated by the Escort setup (Xmember, leaf spring mounts / shackles etc) and the length of travel of the dampers etc. I am generally happy with the setup as long as I can get the ride right on the front and with it sitting in the middle of the damper movement range (so the road wheel can deal with a 2" bump and a 2" pothole etc).

Well if you fancied starting the sport up again Des I have a pair currently fitted in the Ranger you can have (but my bank balance wouldn't get you much either I'm afraid) ;-(

I've been through the stock replacement spring catalogues and there doesn't seem to be much else that was even close to the Escort setup. Similar era vehicles (Capri / Cortina etc) seemed to have completely differing numbers in that area (coil diameter, spring length, poundage etc). Also I'm not sue we have any 'local' breakers around here any more and if they are they seem to specialise in one make / model and ask prices not far off new (and are an unknown quantity etc). ;-(

Indeed. And that's sorta what I'm hoping the coilover conversion will allow me with a large rage of semi standard spring rates and lengths available and an 'adjustable' ride height (taking away the guesswork / time expensive trial_and_error) to some degree?
Al the best ..
T i m
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The Capri ought to be rather more than similar :-)

Umm Mk2 Escort springs are a lot cheaper than coilovers, you could easily buy a long one & then the right one for less than a coil over. You could try Peter Lloyd rallying.
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On Wed, 23 May 2007 23:44:06 +0100, "Duncan Wood"

Well you would have thought but I *think* I remember reading / hearing stuff like "if you want to lower it slightly you could always use Capri struts (or springs)" etc <shrug>

True, 26+Vat each from my local place Duncan.

Erm, every 'spring place' I've spoken to / emailed so far can either offer stuff like +1" but typically at 145lb/in (so probably what I have in there now) ;-(
A couple of places have offered to make me some 'std' rate springs +1" but they were 47 each so not far off the cost of the fully adjustable and easily swappable[1] coilovers? (trust me, if there was an cheap / easy solution I think I might have found it by now) ;-(
Even the tech guy at the Ranger club (a fountain of Escort knowledge) suggested coilovers (and we are talking less that 100 here) and as there is a fairly good chance the Ranger will out live the Rover and the Astra could be considered money well spent (and it's only nearly 30 years old now). ;-)

I think I did and have again just now, nothing that I can see.
The issue is I don't think there is much of a demand for long and soft? ;-)
All the best ..
T i m
[1] Thinking ahead and if I put the Pinto in there ...?
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Ah, old age is getting to me, I remember when they where 8 & came in all sorts of lengths......

Try asking Peter Lloyd, they used to fit long soft ones to rally escorts so they can probably give you a goood guestimate of what you need.
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On Thu, 24 May 2007 09:51:47 +0100, "Duncan Wood"

LOL (bless) ;-)

Just rang them .. if it's not an RS2000 strut they have nothing .. ;-(
Looks like coilovers it is then ...
All the best ..
T i m
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wrote: snip

snip
Sigh.., if it's not for a saxo or 206 you appear to be on a loser! what happened to SAH for triumph, all the tuning companies for mark 2 escorts I'll bet it would be a challenge to get a set of Wills rings for 998cc........<fx looks up, wraps blanket over knees and looks into the fire>
Des
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On Wed, 23 May 2007 17:11:43 +0100, "moray"

(thanks)
Yes, I had considered they one.

Ah, I hadn't considered that one. ;-)
Sooo, if a say 100lb/in spring was working perpendicularly to the vehicle / leg it would 'be' a 100lb solution. If the spring was working at say 20" off perpendicular (simplistically) then the effective rating would be n% less than 100lb?
I can't remember enough applied mechanics from college to know if that is a linear function (45 deg = 45% of max ect)?
In the Ranger I believe the angle is the same as per the donor Escort and that's not 'a lot' (but still something).
All the best and thanks ..
T i m
p.s. Because I don't have a known benchmark (the stock spring) I don't have a definitive value for what my starting point is spring pressure wise.
I *believe* the current (Rickman supplied) springs are 145lb and they are 'way' too stiff but what do you go down to? Would 100lb (the lowest offered by Rally design for example) still be too stiff?
The HBOL says for 'all saloons' circa 1978 as 520 lb ft mean load (what's that)?
In the older book for the 68-74 models it offers the 'Mean load' of 512lb (all models) but adds a 'Mean rate' of between 85lb/in for std saloons (hmm, and they have the same wire diameter as the basic 78 models <g>) to 135 lb/in for GT's and van etc?

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wrote:

Sin(angle between arm/coil over) x 100 = % of force applied.

Provided it's reasonably straight (ie near 90 degrees) under normal load, it won't make that much of a difference. It's only when you start getting quite a bit of line that these things become more critical.

I would of thought it would be a bit soft. You really need to figure out the corner weights, then do some calculations. It doesn't take that much difference in spring rates to seriously affect handling...

I have no idea, but given that I've just been looking at mountainbike rear shock springs, which are rated at their maximum weight, I would possibly hazard a guess it's the maximum loading for the springs, which would probably tie in with the other figures...
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On Wed, 23 May 2007 21:27:09 +0100, "moray"

Whoah, thanks. Now I know why I kept my Sine tables book for the last 35 years!

Ok and makes sense ta.

I did when we first built it had it weighed front and back separately (that confused the guy on the weighbridge, like asking KF to check the tracking on the rear of the Ranger (suspected bent axle casing)) but I can't find the figures again. It was 17 years ago now.

Thanks again. I'm not sure 'handling' is something that I'm too worried about with this particular vehicle <g>. However, previously it was at least fairly 'balanced', both ends felt like they were playing an equal part in the job. Now it feels like the front is dictating it all and it doesn't absorb speed ramps like it used to ... ;-(

Ok, and again that makes sense.
All the best and thanks again .. all helps to narrow the chance of error ...
T i m
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It's the sine of the angle to the vertical of the spring if the tca's horizontal

Pretty negligible in the assumptions list then :-)

For the same wire diameter a shorter spring (length of wire, not free length, hence less coils) will be stiffer.
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On Wed, 23 May 2007 23:13:37 +0100, "Duncan Wood"

So I've been told but repetition might help me remember (it didn't all those years ago so probably won't now either) ;-(

I think so Duncan ... all petty basic / simple under there (and per of the beauty of it).

Sooo, the spring I'll get could be pretty busy turns wise?
Then the next issue is making sure the 'metal length' of the spring (ie the fully compressed coil length) isn't longer than the space I have to compress it into? I assume I can work that out once I have the facts from a spring to work with. Like if that say it's 10 turns and 10mm dia wire that will be ~10 cm long fully compressed?
For rough numbers I said the coil would probably end up typically 5in compressed (550 lb on a 110 lb/in) so if the wire / gap spacing was 1:1 the spring would have to have at least a 10" free length?
That in turn would set the mounting point of the threaded tube on the strut to ideally give the correct ride height with a bit of room for adjustment either way. If I set the tube too low I would imaging I could turn a stepped steel (alloy?) 'collar' to make up the length but it would be difficult to resolve if I mount the tube too high.
Can you get spring compressors for this size spring do you know or do you just wind the preload on with the C spanner?
All the best ..
T i m
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Well, grinder, welder, new strut :-)

Most spring compressors fit, the cheap ones often have thinner heads than the medium priced ones.

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On Thu, 24 May 2007 14:21:16 +0100, "Duncan Wood"

Yeah, I suppose I could move the tube, if needbe .. they even sell them loose (no nut/cups etc). I just think if I fitted something I like to fit it once and have it stay there .. forever .. ;-)

Ok thanks. That was what I was thinking .. my existing set are quite 'chunky' and I remember were tricky to get between the coils of the std Escort spring when compressed, let alone some half the size with closer spacing?
All the best ..
T i m
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