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How Not To Modify Your 205's Suspension...


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189 replies to this topic

#41
Cameron

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It's a fun game.
Multiply that 0.0025% across every component on the car...


Posted Image

#42
TT205

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Is that the Total Perspective Vortex?

#43
Batfink

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It's a fun game.
Multiply that 0.0025% across every component on the car...


and its then still 0.0025%.. :)

#44
Rippthrough

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Basically, we all know the square root of fook all :)

Anyway, swerving back a little more to the topic at hand, on balljoint extenders modifications - this is one of those that sounds great until you realise you generally can only have it a certain size due to getting the balljoint in there, loadings on the joint, etc, etc.
And that size might not be what you want, so then you have to go with what is nearest - which is probably more than you want - so you then end up with a higher roll centre at the front (although it doesn't make as much difference as you think, which is what Cameron was hinting at, the car doesn't actually roll around that, or even the instant centres, but it gives rough idea's of the cars behaviour), as well as more camber gain in roll.
But - you are now rolling less on the chassis....and tending to lift the car on the outside wheel instead - jacking the chassis up, which could well give slightly more lateral force on the tyre and slightly less vertically when the car is rolling to inputs - it can give that feel of the front end washing out on turn in in the wet/low grip situations, so you might now have a situation where you have great turn in and grip on a track with good tyres, yet the car feels horrible in the wet or on road tyres.
Even worse you now need to move either the rack or the track rods as they no longer follow a similar arc to the lower wishbone - the bump steer will likely be much more pronounced when a wheel drops into a pothole or similar - it'll toe in like buggery with the inside wheel on full extension - so now you have a car that's washes out a little on rough surfaces, but feels better at the front on high speed corners, yet never seems to want to turn in properly on tighter tracks....

This is all just theoretical without going out an getting measurements - but you can see what I'm getting at, one little change to the geometery can make such a huge difference to everything else, that you better be prepared to alter everything, or nothing.
Lowering, to an extent, isn't bad, because the suspension is designed to cope with that cycle in normal use, you might just want to shim the tie rods/rack slightly to bring the steering curve back to neutral at ride height if feeling adventurous, but you can soon find yourself chasing your tail in circles if you don't treat the geometery as a full package - and that includes considering the effect at the back when altering the front - not too much of a concern with trailing arms, but it does alter the dynamic balance of the car.

Edited by Rippthrough, 27 May 2010 - 12:02 PM.


#45
Cameron

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Amen.

There's too much going on to be able to determine exactly how it's going to behave dynamically from a static setup. You either need a really good simulation package like Lotus Shark, although that is still extremely limited, or a whoooole load of trial and error or experience. That's what you're paying for when you go to an expert who's already done all the testing and knows what works.

#46
B1ack_Mi16

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I'm actually working as an analysis engineer at this company now http://www.fedem.com..._...33&Itemid=5

And our software can handle tire models, and dynamic simulation of the complete car.

Only issue is that one would need all data on pickup points and such to really be able to make a realistic simulation model.

Can then run the car on bumpy surfaces and measure all the angles/responses to any component in the whole car.
Depends on how detailed the modelling is of course.

I could really like a FE model of the complete 205 Shell and front subfram, does anyone have this?
The rear beam is easy to make and most of the other parts, it's just the subframe has a bit complicated shape and that's even worse for the complete body..

I have been thinking of making a model of a 205 for a long time but have really never got any time for it.

#47
Cameron

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It crossed my mind every now and then, then I remember how much of a ballache it was modelling just the outer skin of the front of an Audi A4 and think yeah, f*** that! :)
Surfacing is very very time consuming!

#48
kyepan

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does anyone know the standard ride height of a 205 1.9? distance from floor to a point... or distance from the wheel to the arch at the front.

#49
welshpug

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I do have a standard 205 I can measure :)

I also have the minimum challenge/gp N / gp A ride heights somewhere too :)

#50
Cameron

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The height of the front inner wishbone point would be very nice if you could get it!

#51
B1ack_Mi16

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The height of the front inner wishbone point would be very nice if you could get it!


Or just measure the wishbone angle when car is sat level.

Heights will differ a little bit depending on what tires it's sat on.

#52
Cameron

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True.. but the wishbone angle is more likely to be inaccurate, it takes a very small fluctuation in angle to give a large fluctuation in height. Maybe measuring the inner wishbone point height and the tyre rolling radius would be better?

#53
kyepan

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even arch to wheel clearance would be a good relative indicator to standard.

#54
martyn180

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even arch to wheel clearance would be a good relative indicator to standard.


Checked mine yesterday funnily enough, as put new springs on.
Front 25"/Rear 24" from the floor, vertical through the centre of the wheel to the bottom of the wheel arch moulding.
on .1 9 gti Speedlines, with 185/55 15

#55
kyepan

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Checked mine yesterday funnily enough, as put new springs on.
Front 25"/Rear 24" from the floor, vertical through the centre of the wheel to the bottom of the wheel arch moulding.
on .1 9 gti Speedlines, with 185/55 15

thank ye very kindly!

#56
welshpug

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I took these wonderful pictures last night but forgot to measure the ride height :)



Posted Image

Posted Image


but I do have this from the rally build book;

Posted Image


I have no idea how they arrived at those figures, but they are lower than standard by about 25-30mm IIRC.

#57
engine killer

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Dear all,

After seeing Graham's post regarding sleeving the hub http://forum.205gtid...s...21487&st=10 , I am now thinking to fabricate an extension to the clamp (balljoint pickup) to reduce the roll couple.

The top side will be a solid round cylinder shape with diameter of 18mm (and boring the clamp to 18mm is necessary) and the bottom side will be same dimension/shape as the hub's clamp. The top side will be welded to the original clamp.

This will move the clampinng point roughly 25mm lower.

But it seems like when the car's center of gravity is lowered by say 25mm, the roll center will be lowered by much more than 25mm. I can hardly get access to a proper hoist to measure all the required measurement, so, how much shall be lowered?

Most importantly, do you guys think it is workable? Anything I have to take into consideration when fabricating the extension?

For the bump steer issue, I think I will do something similar to those aftermarket kits as they are already safe to run provened.

Any comment and suggestion are mostly welcomed.

Thanks

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#58
Cameron

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Personally, I wouldn't do it. Using rod ends in bending is bad enough, but using a spacer to lower the mounting point is going to put some very large bending moments into the spacer. If you then beef up the spacer it will be the hub that has to cope with the huge forces and you may well have problems with that ovalling.

Also you may be lowering the roll centre slightly, but that doesn't necessarily mean you'll see a matching reduction in load transfer, since that is made up of moments about both the roll centre and at ground level. You will reduce it somewhat but the results will more than likely be less than you'd expect, and could well mean it isn't worth the effort in the first place.

The largest benefit you'll get in my opinion would be greater camber compensation as you'll be returning the wishbone towards it's normal angle, but I reckon the gains in negative camber won't be sufficient to overcome the tendency towards positive camber change with roll that is inherent in the 205's McPherson strut design. Again you may find that the benefits are too small to notice, and / or aren't worth the hassle of designing your product to overcome my first point. You may find that you get more than adequate results by simply increasing the static camber, providing you don't go to any silly extremes.

Edited by Cameron, 06 July 2010 - 10:01 PM.


#59
engine killer

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Thanks for your valuable advice Cameron.

In the past few days I have been surfing the internet regarding the "bump steer kit" and "roll centre adjuster".

Both roll centre adjuster and bump steer kits from different brands are more or less the same.

For Whiteline, they are more on having a much taller balljoint instead of adding spacer.

For the bump steer kit, I may have a go and try. The bolt going through the rod-end will be much thicker since I have to bore the hole on the hub to same diameter, hope this will strenghten a bit.

And the roll centre..... since there aren't many 205 in Hong Kong and I surely won't spare my money on a new pair of hub (if the trial one failed), so unless I am lucky enough to get a cheap second hand hub, otherwise no go/try.

#60
kyepan

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but I do have this from the rally build book;
Posted Image

I have no idea how they arrived at those figures, but they are lower than standard by about 25-30mm IIRC.


hey welshpug,
i'm going to measure and reset the height of the coilovers at the front today, when you say arrived at those figures... which ones are standard in your eyes?
looking at other posts i've just answered my question yet again
332-325 = standard front height.

thanks again (and for the advice about the valve springs)

Justin.

Edited by kyepan, 11 July 2010 - 10:17 AM.