Importance Of Sprung To Unsprung Ratio

[Castorkid 11 1 Cars]

Thanks Kev

Rear dampers are new! Have hit the bumpstops once, which I hope will be sorted once I've raised the rear a little.

[Cameron 16]

Everywhere I read it says raising the rear and lowering the front end will reduce turn in understeer.

This makes sense to me as surely by making these changes, it places more weight over the front wheels. The more weight there is placed over the springs, the more compressive force is available, and the harder it is for vertical wheel movement to overcome the inertia of the car and lose contact with the road.

 

As Allan mentioned, 'it's never going to glide down a uneven pot holed road like a mk 5 golf etc, its what makes them fun ;)'

 

Is that partly because a Mk5 golf weighs 1400kg and has a similar unsprung weight to a 205?

 

 

And therein lies the danger of internet research, as it only takes one person to say something misleading and it gets copied and pasted and misinterpreted for the rest of time. That assumption is based on vehicles that have rear suspension types with a roll centre height that can be adjusted (eg wishbone) as your roll centre axis inclination is something you can use to tune transient response. However, a 205 has trailing arms at the rear so the roll centre is at ground level at all times, so you get no such tuning of RCA inclination by adjusting rear ride height.

 

Another point you're getting confused with is that more mass on the tyres = more grip, but due to the non-linear relationship between grip and tyre vertical load this isn't exactly true. Also, as I said earlier the amount of longitudinal weight transfer available from small adjustments to ride height at either end is pretty minimal.. you're unlikely to change the weight distribution by any significant amount.

 

I'm not having a go by the way, I'm just trying to set you on the right path with your understanding and expectations. I do this stuff for a living you see.

Edited December 30, 2012 by Cameron

[Castorkid 11 1 Cars]

Cameron

I really appreciate your input. As you can tell I dont do this for a living, so thanks for bearing with me! Even if the relationship between grip and vertical load isnt linear there must still be a positive trend - a 911 is well known for being quick off the lights.

 

As for my 205, I guess I'm hoping that the small amount of weight transfer that occurs by raising the rear will be of benefit. I've only reduced the rear ARB from 25 to 23mm and lowered the rear a little so I'm probably not too far off. Before I made these changes, it was oversteery. I'd like to find more grip at the front end though rather than increasing the stiffness of the rear again as I prefer the car to be softer for road use.

 

I did think about disconnecting the front ARB to see how that felt. Any other thoughts/recommendations for softening the front whilst still keeping the 30mm ride height drop?

 

You still havent changed my mind on the importance of sprung to unsprung ratio though. But as I cant change all the dangly bits on my 205 to F1 style unobtanium I just wander if I added some carefully positioned ballast and another few bhp whether this would bring any positive benefits.

Edited December 30, 2012 by Castorkid

[Cameron 16]

That's true.. but 911's (especially the classic ones) are also known for being a little edgy at the limit!

 

Try raising the rear on your current setup and see if it makes a difference, it's an easy job and will cost you nothing. If you're trying to tune the balance it's important to only change one thing at a time (unless you absolutely know what you're doing) otherwise you can get confused about which change had what effect.

 

Bear in mind that any reduction in front roll stiffness (by softening spring / removing ARB) will increase the oversteer tendency on the limit, so if you don't like that then you should be careful. You'll notice that all the quick FWD cars have very stiff rear ends compared to the front as oversteer is preferable and very controllable once you get used to it.

[Castorkid 11 1 Cars]

So just come back from a quick test in the rain. I removed the droplinks from the 17mm front arb and there was a massive reduction in understeer tendency. Front set up is bilstein challenge dampers and eibach 30mm lowering springs and the rear is 22mm bars with a 23mm arb and 600/300 dampers. Unsurprisingly though, the front felt quite vague and a bit wobbly at higher speeds.

 

So which way to go next. I've got some 21mm torsion bars to fit in January which going on today's test is only going to make things worse. My gut instinct tells me to either:

 

a. Fit the 21mm torsion bars, raise the overall ride height of the car, fit softer front springs and dampers and keep the 17mm front arb or

b. Fit the 21mm torsion bars, keep the 30mm drop on the ride height and go for stiffer springs and coilovers and ditch the front arb.

 

Which way would you all go next?

 

Nick

[allanallen 528 2 Cars]

I'd firstly fit the 21mm bars and raise the rear ride height then go from there. The front springs are already fairly soft so you'll struggle going softer.

I'd be curious to know what front spring rate the skip brown phase2 kit runs, I'm sure the rear is 21tb/23arb so it'd be a good starting point for you.

[Anthony 1,000]

Rear on SBC Phase 2 is generally 20mm torsion bars and 23mm anti-roll bar. I believe some of the late cars may have had 21mm torsion bars.

 

I've got a set of SBC front springs in the garage that I can measure, but broadly they feel fairly similar to the Eibach's

[Castorkid 11 1 Cars]

Do you think raising the rear ride height will reduce understeer? Should I aim for the sill to be parallel to the ground or for the rear to be slightly higher than the front?

[allanallen 528 2 Cars]

Do you think raising the rear ride height will reduce understeer? Should I aim for the sill to be parallel to the ground or for the rear to be slightly higher than the front?

Your car looks low to me on the rear and I'm guessing not far off the bumpstops. I know that theory says it won't help your understeer problem but from experience I rekon it can only help. IMO you want a 10mm rake along the sill(higher at the back). If its hitting the stops on the rougher roads it will most certainly help your 'bobbing' problem and make it feel much safer.

 

Anthony, I had a ride in whitepugs car at curbrough a while back which I believe is basically a sbc2 setup but with billy coilovers, it must of been a later setup as that was on 21s. For the modesty of the set up it I was astonished how good it was, I'll definitely be going this way when I (eventually) put my mi back on the road.

Edited December 31, 2012 by allanallen

[Cameron 16]

If the rear ride height were the problem, why would it only become apparent when the seats were removed?

 

Seems counter-intuitive to me.

[allanallen 528 2 Cars]

If the rear ride height were the problem, why would it only become apparent when the seats were removed?

 

Seems counter-intuitive to me.

I personally don't think any issues have arisen from removing the rear seats, I think it's slightly in the OPs head (no offence mate). Plus I'm not talking about a couple of mm ride height change, more like 25mm+ at the rear

[Tom Fenton 1,526]

Maybe jumping back quite a bit here, but the bit that doesn't make much sense to me is that the car is running really quite a soft spring at the front, and yet the owner reports that the "bounce" test seems to suggest hardly any front suspension movement whatsoever. Now this doesn't sound quite right to me.

 

Also the rear dampers, I have something similar fitted to the back of my red car, now they make the thing electric on turning in to a corner, but the side effect is that they also make it VERY jiggly when travelling along even a very smooth road. However I also have 23mm rear bars and so as Al quite rightly says earlier in this thread, they need a damper man enough to keep them in check.

[allanallen 528 2 Cars]

Maybe jumping back quite a bit here, but the bit that doesn't make much sense to me is that the car is running really quite a soft spring at the front, and yet the owner reports that the "bounce" test seems to suggest hardly any front suspension movement whatsoever. Now this doesn't sound quite right to me.

 

I was putting this down to the (relatively) 'stiff' front dampers. In reality aren't they valved for around 180-200lb springs rather than the 130ish that the OPs running?! They're bloody firm on mine I can tell you that Surrey!

[Castorkid 11 1 Cars]

Thanks everyone for reading my post all your comments are apreciated. I'm sure some of it is in my head as it's all a fairly new experience for me as my previous set up experience is all with trackday motorbikes. However, I'll fit the 21mm torsion bars and raise the rear as planned and then reassess the situation as Allan suggested.

 

Last nights trip out without the front arb attached has given me food for thought and makes me think the front end is still too stiff (either too stiffly sprung or damped or both), I know that the forum says it should be quite a soft set up but when I bounce up and down on the car the front end moves maybe 10mm or so (guesstimate). I know the challenge dampers are 260/120.

 

I could be barking up the wrong tree but, I'm enjoying the process and keen to do some further experimentation. I quite fancy trying coilovers on the front with soft (say 150lb) springs with the front 17mm arb connected. What do you all think?

 

Thanks again

 

Nick

Edited December 31, 2012 by Castorkid

[Cameron 16]

How much do you weigh?

 

Say you weigh 70kg (11st) your entire weight on the front end would compress the suspension by ~10mm.. assuming your springs are in the region of 200lbs and the dampers having absolutely no effect. Obviously if you're bouncing on the suspension the dampers will have quite a large effect on the travel.

 

From your first post I got the assumption that you removed the seats with no other change to setup and found the car felt unsettled, so correct me if I'm wrong, but to me this says that your ride heights have nothing to do with the change in behaviour and that can be eliminated as a cause.

Edited December 31, 2012 by Cameron

[Castorkid 11 1 Cars]

I was thinking removing the seats etc just made the car that bit lighter and exaggerated the effect of the stiff front set up. As for the ride height thing, my simpletons logic tells me that if I tip the car forwards say giving it a 10mm forward rake then it will load the front a little more, even if the amount is akin to sliding the seat forwards...every little helps!

 

I wonder if Allen has hit the nail on the head though. Could my challenge dampers (260/120) be too stiff for my front springs??

[welshpug 1,626]

but its not stiff at all on the front

[Castorkid 11 1 Cars]

but its not stiff at all on the front

Allan mentioned that the challenge dampers are valved for much stiffer springs than the eibachs

[Cameron 16]

The last thing you want is more weight over the front, do you know what the weight distribution of a 205 is?

 

Yes it could well be that your dampers are too stiff, it certainly follows with you removing weight and finding the grip level was poor over bumps. It's surprising that you noticed such a difference by just removing some 20-40kg of seats though, but I put that down to you fitting a much more supportive bucket seat; it could well have been that it was borderline before.

 

As an example for you, I had 350lb front springs with 24mm TB's and the roadholding was excellent, so your spring setup really isn't what I'd call stiff. (This was with GAZ Gold dampers)

Edited December 31, 2012 by Cameron

[Castorkid 11 1 Cars]

Cameron,

Were you using a front arb with your 350lb springs?

Do you think I need to go coilovers or can you recommend a softer damper to try with my eibach lowering springs.

Edited December 31, 2012 by Castorkid

[Castorkid 11 1 Cars]

Dear all, have been looking for ages for some definitive front damper ratings for OE 205 dampers, B4, B6, GpN. Do they exist, if so can someone please point me in the right direction. Many thanks.

Edited December 31, 2012 by Castorkid

[EdCherry 34 1 Cars]

Damper Ratings?

 

Its all about the values produced by the damper when put into specific situations, normally a Force vs Velocity situation which many refer to as a Damper Curve.

 

Im not to sure on how Bilsteins work they're numbering system out but I assume someone with a far more varied work with dampers (PhillipM) might be along to explain but I expect it refers to 260/160 as a peak force at say 10in/s. I would expect the numbers also to be in order of compression/rebound.

 

Anyone care to confirm or deny this?

[Cameron 16]

Yes that was with a front ARB, I tried without but it suffered in the twisty bits. Sadly I never got round to fitting my 23mm rear ARB as that would have made a definite improvement.

 

Anywho.. you're in the right ballpark for fast road with 21/22mm TB's but I don't know what your front spring rates are so it's difficult to say, but you want to be looking at 180-200lbs/in going by the experience of others on here. I don't have much knowledge of the damper rates you have, I've always had coilovers re-valved to suit my car's weight and springs, so it's probably worth seeing if you can do the same with yours.

[Rippthrough 98]

For billies, it's the damping force in newtons divided by 10, measured at 20inch/s (0.52m/s), rebound first.

 

So a 260/160 would be 2600 newtons of rebound damping at 0.52m/s

Edited December 31, 2012 by Rippthrough

[EdCherry 34 1 Cars]

Better get the calculator out I guess! Linear curves mostly Phil?