Importance Of Sprung To Unsprung Ratio

[Castorkid 11 1 Cars]

Dear Forum,

I have a fast road 1.9 205gti that I'm always looking to improve. I recently removed all the standard seats and carpets and fitted a bucket seat to see if the reduced weight made a significant difference to performance. As a result of these mods, I found that it was noticeably quicker in a straight line but much more unsettled on bumpy B roads (im running a Skip Brown phase 2 style suspension set up), so much so that I felt I could run far less corner speed through my favourite bends.

 

This got me thinking about whether I should be looking at the ratio of sprung to unsprung weight rather than just weight loss in general.

 

I'm no race engineer, but it seems to me that the limiting factor of the fast road performance of my 205 is the effect of the unsprung weight. I can see that on a smooth circuit, the sprung/unsprung ratio may matter less, but on a fast road car, where the car has to deal with more surface variations, am I right in thinking that this ratio is very important for the balance of the car?

 

Questions:

 

Is there an ideal ratio I should be aiming for?

 

How much reduction in unsprung weight can I realistically achieve with my 205?

 

Your thoughts are most appreciated.

 

Cheers

 

Nick

[B11 BOB 11]

 

r Forum,

I have a fast road 1.9 205gti that I'm always looking to improve. I recently removed all the standard seats and carpets and fitted a bucket seat to see if the reduced weight made a significant difference to performance. As a result of these mods, I found that it was noticeably quicker in a straight line but much more unsettled on bumpy B roads (im running a Skip Brown phase 2 style suspension set up), so much so that I felt I could run far less corner speed through my favourite bends.

 

This got me thinking about whether I should be looking at the ratio of sprung to unsprung weight rather than just weight loss in general.

 

I'm no race engineer, but it seems to me that the limiting factor of the fast road performance of my 205 is the effect of the unsprung weight. I can see that on a smooth circuit, the sprung/unsprung ratio may matter less, but on a fast road car, where the car has to deal with more surface variations, am I right in thinking that this ratio is very important for the balance of the car?

 

Questions:

 

Is there an ideal ratio I should be aiming for?

 

How much reduction in unsprung weight can I realistically achieve with my 205?

 

Your thoughts are most appreciated.

 

Cheers

 

Nick

Maybe you need to get it corner weighted ie making sure that it's equal weight all round with you in the car that's what the track racers do.

PS I

[kee 22]

unless your running a turreted and coilovered rear end, you wont be able to get it corner weighted.

 

unsprung weight is important, the less you have the better your suspension can work with the bumps etc. in the road. I don't think the ratio of sprung to unsprung is important, it's best, from a handling perspective, to go for the biggest reduction possibe for both.

 

If it's now bouncy then your springs are too stiff for the shocks or the shocks aren't as good as you thought.

Don't forget that when you remove weight from the car then to keep the same wheel rate you need lower spring rate

[Cameron 16]

What he said; reducing weight is always good, whether it be sprung or unsprung.

 

2 things (or a combination of both) could be happening: 1 - your dampers are no longer matched to the reduced sprung mass so the ride has become skittish, although just by removing some 30-40kg or seats this is pretty unlikely. 2 - your senses are fooling you, and because you have a much better "seat of the pants" feel of the car through the bucket seat you can tell what the car is doing beneath you a lot better; what's more the seating position may well be different which gives you a different impression of speed.

 

I think no. 2 is the more likely cause btw, it may just take some getting used to.

[allanallen 528 2 Cars]

unless your running a turreted and coilovered rear end, you wont be able to get it corner weighted.

 

Of course you can! You can adjust rear ride height on the torsion bars, you can also obviously adjust it at the front with adjustable platform struts, move weight around in the car. Another method (which I believe they used on the 306 touring cars) is to preload the rear anti roll bar with turnbuckles between the arb end plate and trailing arm.

[kee 22]

but how much adjustment does one spline give? not going to be anywhere near as accurate as coilovers. Anyway, if it was that far out then theres some serious issues with the shell I would think.

 

For a road car, you probably wouldn't notice difference, if you do then your driving too hard for a public road anyway.

[welshpug 1,655]

We don't do things by "spline" on here, we do it properly

 

Specifically, you would tend to ensure the rear is as equal as possible (you can get it damn close, enough not to be able to measure any difference) and adjust at the front.

[kee 22]

how else is the rear height adjusted then? without the need for adding pre-load to the ARB that is.

i'm still unfamiliar with the torsion beam being a ford man so far in life, coil springs make me happier

[allanallen 528 2 Cars]

how else is the rear height adjusted then? without the need for adding pre-load to the ARB that is.

i'm still unfamiliar with the torsion beam being a ford man so far in life, coil springs make me happier

Torsion bars have differing amounts of splines at either end so you get a 'vernier' effect allowing you to set the ride height very accurately at any desired height.

I'm not a fan of putting per-load on the ARB but apparently that's how some folk do it on the scales.

 

 

[Castorkid 11 1 Cars]

Thanks for all the input. I'm currently running the following set up:

Solid mounted rear beam with 306 negative camber radius arms
22mm torsion bars (soon to be replaced with 21mm PTS bars)
23mm rear arb
Group N top mount rubbers

309 front wishbones
Bilstein group N dampers front and rear:
Front O/S V36-0547

Front N/S V36-0548
Rear O/S and N/S B46-1038
Eibach prokit springs (30mm lower):
EW 7001 001 VA

Standard front arb

Group N engine mount kit
Yokohama Advan AD08's 195/55/15

 

Also looking into lighter wheels and brake setups in the summer. Need to put in some research time on the forum....

 

Would quite like to try a softer 30mm lowering spring on the front for comparison, does anyone have any suggestions?

 

Cheers

 

Nick

Edited December 28, 2012 by Castorkid

[Batfink 201]

I would not have said the Eibach springs are particularly stiff. I'd not go softer

[Castorkid 11 1 Cars]

Could just be me as Cameron said earlier, but I do find that the car 'bobs' a bit rather than tracks the road surface like a more modern car. I guess I just figured that as lightening the car made it feel a little worse, then then by reducing the unsprung weight and going softer might help me move in the right direction. To complicate matters further, I'm planning a Gti6 converion shortly also!

Nick

Edited December 28, 2012 by Castorkid

[allanallen 528 2 Cars]

I wouldn't go any softer on the front either. How come you're on 55 profile tyres?

[kee 22]

reducing unsprung weight will always help, lighter brake calipers and wheels being the obvious things.

 

how much will you be after for the 22mm torsion bars?

[Castorkid 11 1 Cars]

Allan, I'm on these tyres because I went to the Ring at quite short notice last year and in the time available they were the best on offer. Certainly much better than the previous (P6000s). Do you think this is an issue?

[GLPoomobile 958]

but I do find that the car 'bobs' a bit rather than tracks the road surface like a more modern car.

 

Dare I be the one to say that the problem possibly stems from you expecting too much of the 205 by comparing it to a modern car? Damping - for one thing - has come a long way since!

[welshpug 1,655]

mine on the same damper does not bob, maybe they are a bit tired.

[Batfink 201]

Could be too stiff rear arb for the roads you run on

[allanallen 528 2 Cars]

Dare I say the rear dampers aren't man enough for the bars!?

On paper it looks like a very nice setup to be honest, I think the 21mm bars will be an improvement. If its a mainly road biased car I'd be tempted to try the standard bars back in if you've got a pair?

What sort of ride height are you running at the rear?

As glpoo mentioned, do not try and compare your car with more modern stuff, it's never going to glide down a uneven pot holed road like a mk 5 golf etc, its what makes them fun

 

I've never run with 55 profile tyres so I couldn't tell you from experience if they'll have any detrimental effect on the car. I was just curious more than anything.

[Castorkid 11 1 Cars]

It could well be that I'm expecting too much from my 205. Thing is, I think it's great so I'm happy to plough on looking for solutions as I go. I've just been looking at the Harry Hockley website and his custom rear set ups - v.interesting. I still feel, the goal for my car is to reduce the unsprung weight as far as possible and then look at softer springs and higher quality dampers. At 80mph down my local bypass the bonnet bobs up and down in time with the wheels surely I can improve this.

[Castorkid 11 1 Cars]

Thanks Allan,

Thought about trying the standard rear torsion bars, but was concerned about terminal understeer unless I softened the front further. As for the ride height, the rear is too low in my opinion. The front has a 30mm drop from the Eibach springs and the rear tyres are approx 10mm below the arch trim. Was set up for me by a Rally specialist who does alot of 205 stuff. I find the set up too understeery so am going to have the rear raised when I get the 21mm bars installed.

[Tom Fenton 1,541]

I've used 55 profile tyres a lot on my white 205, to be honest there is hardly any noticeable difference, having said that they were Dunlop Direzzas that have a stiff sidewall, if you put a 55 tyre with a soft sidewall on I am sure there would be a difference.

[welshpug 1,655]

what is the "rake" on the sills? nose up will understeer, also too soft on the rear will understeer.

 

Correction to what I said earlier, I have GpA gravel rears, just fitted the standard diameter inverted fronts in place ofm the height adjustable inverted units.

[Batfink 201]

To be honest plenty of people have similar setups without a problem. I think either something like your dampers are failing or it's simply a mismatched setup. Try the softer torsion bars and see if its got more rear traction. If you have a softer arb then swap that. You will need to change one thing at a time so you work out what the issue is. How old are your dampers? It may be worth taking them off and checking for leaks and whether the shafts are working correctly. If they are leaking and low on fluid you will get a choppy ride but I would expect you to notice this on any bump not just when the car rolls into the corners

[welshpug 1,655]

I don’t think the ARB is the cause, 106's run stiffer as standard...