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

[kyepan 291]

So... i'm bored and stuck inside with a bad leg... and just read through a suspension geometry article or 4.. it is quite enlightening.

 

Part 1

Part 2

Part 3

Part 4

 

 

This has me thinking about why why my conservatively lowered 205 car is misbehaves so much under braking, acceleration, and especially down uneven or steeply crowned lanes and uneven roads. Granted it turns in well, and mid corner sticks like poo to a blanket, but even then it feels nervous. I'm no expert but what they are saying does make sense, and when you combine all the factors together it really does make me feel a bit silly. If you can't be arsed to read the whole post, in a nutshell, lowering a standard car without making changes to counter act the knock on effect upon it's geometry has essentially made it unstable, together with neutral toe and a number of other factors. Our mcphereson strut suspension and trailing arms seems very sensitive to lowering...and i think i've done too much, and without sounding controversial... if what i've done is too much... then the generally accepted common lowering ammount may mean the majority of lowered 205's are also not working to their optimum

 

 

Up until now my understanding was the the following:

 

Lower the center of gravity by lowering the front and rear of the car

Reduce the body roll by fitting stiffer anti roll bars

Reduce the body roll by fitting stiffer springs

Up the damping considerably to critically damp the springs for their length and stiffness

use negative camber front and rear to keep the tyre flat when the car and consequently wheel rolls over in the corner

Use neutral toe at the front to give a more direct steering feel, and reduce the dead zone.

Run reasonably low tyre pressures (32f/30r)

get the lightest wheels possible to reduce unsprung mass.

 

Of course lowering was within reason as I didn't want to run out of suspension travel. I'd also heard it was not ideal to run with level lower arms (ie not pointing down like standard cars) although mine are level. and that neutral toe can make it twitchy, which mine is, but it was the movement over bumps and crashy ride that is most worrying me.

 

Going through their article i've picked out some quotes to demonstrate why lowering could have caused my dodgy handling.

Understeer... and tyre pressure

Originally i thought lower pressures meant more grip.. however this does not seem to be the case. It would seem that the slip angle (direction tyre points vs direction it's actually going when cornering) can be reduced by increasing pressure.

How does a tuner manipulate tire loading and slip angle? By tweaking the spring rates, anti-roll bar rates, tire sizing and pressure, and to a lesser degree, the shock damping. The first option a tuner has is to increase the tire pressure. The harder a tire is inflated, within reason, the smaller slip angle it develops. In the case of a nose-heavy front-wheel-drive car, if you add several psi to the front tires and take some pressure out of the rear, the front tires will run a smaller slip angle while the rear tires' slip angle will increase. This alone can do quite a bit to reduce understeer.

Nose heavy front wheel drive car... like a 205 mi-16, perhaps increasing front pressures might have helped reduce understeer instead of changing for softer springs and stiffer rear torsion bars. also saw this just now, nice coincidence.

 

 

Understeer and spring rates... explained with slip angle rather than "because it's stiffer"

Changing the spring and anti-roll bar rates has a large impact on slip angle. Running a stiffer spring or anti-roll bar on one end will cause more weight to be transferred onto the outside tire as the car tries to roll in a corner. The softer end will compress and the more stiffly sprung end will resist compression, putting more weight into the tire and causing it to run at a bigger slip angle.

So more weight will be transferred (and possibly more quickly) to the stiffest bit of suspension, hence understeer if you run a stiff front end and soft standard rear end.

 

 

Various damping settings have been used on my car, but i've ended up running about 12-20/40 at the front and 14-30/40 at the rear to make it turn in properly, much more than is actually needed to critically damp the springs, it would seem this is not ideal.

Shocks can improve response and help balance the car right after the initiation of a turn; soft shocks get the car to a steady point of weight transfer faster. When stiff, they can delay weight transfer. Thus, shocks affect how the car feels at turn-in and also how it feels past mid-turn. A car with the shocks set fairly hard will turn in sharply. If the shocks are set too hard, the balance might change later in the turn in an unpredictable way as the heavy damping slows the body roll and weight transfer.

So if set way hard.. the shock will be delaying the real weight transfer happening, but have lightning fast turn in, which is what i originally found when checking the shocks out when first fitted. If you were to run them at 5.. they are just about critically damped but the turn in was squidgy, run them hard and it was on rails... but not necessarily better when it got to the middle of the corner.

 

 

However... just to be a bit contradictory. Lowering the center of gravity is good because it reduces weight transfer by lowering the center of gravity, stiffening the suspension to cope as previously mentioned increases the speed of any weight transfer so the car responds more quickly to inputs.. now...

Of course, stiffer springs have more rebound energy. To prevent your car from bouncing like a pogo stick, you need shocks with more damping. Shocks don't affect how much a car rolls, but they do affect how the suspension responds to bumps and steering input. More rebound damping keeps the car from bouncing and floating over bumps and undulations. More damping also makes the car more responsive to steering input. Too much rebound damping can prevent the suspension from returning once compressed, causing it to pack down and gradually bottom out.

now... mine does bottom out alot... so either i'm running too much damping and it's packing down, or the springs are too short... or both. I've got another two inches of adjustment on the coilovers.. and if i ran a two inch longer 10" slightly harder spring and raised the ride height half an inch plus slacked off the damping a fair smidge, i would not have either issue.. It's on gaz coilovers and 200lbs 8 inch springs.

 

Toe... now mine is set neutral...

Below are guidelines for setting toe and how it can affect feel and handling.

Front Toe-Out

Just Right

Reduced understeer at turn-in Improved steering response Counteracts natural tendency for front- and all-wheel-drive cars to toe-in under throttle load

 

Too Much

Instability during braking Straight-line instability, especially over single-wheel bumps or split-traction surfaces Unrecoverable understeer

hmm, this all sounds very familiar, perhaps the "neutral toe" is actually with everything else and the bump steer that might be occuring turning into lots of toe out...hence chicken oriental handling.

 

 

Talking about lowering again...specifically how the center of gravity relates to the car's roll center. Roll center is the center about which all the suspension rolls when looking front on, and crucially usually higher than the center of gravity. I didn't really understand exactly what a roll center is.. but their explanation makes sense.

The distance between the roll center and the center of gravity is called the roll couple. The roll couple is the lever arm that centrifugal force working on the CG uses to make a car lean over in a turn around the roll center.

The longer the roll couple, the more weight is transferred to the outside wheels during cornering and the more the car will want to roll in a turn. A longer roll couple makes cars slower to respond to steering input. The resulting weight transfer from a long roll couple also uses the inside tires less effectively during cornering, thereby reducing the available grip.

so when we lower the center of gravity a bit by compressing the suspension we lower the roll center a s*it load, which makes the car respond more slowly to steering inputs and more susceptible to roll, therefore needs overly stiff suspension... which does not actually suspend the car, as much as make it bounce about... yet more argument to run somewhere nearer standard ride height...

 

 

now... a bit on our type of front suspension and how it reacts to being set in a compressed state (lowered)

Moving the suspension through a wide range of travel can also result in another problem. Most factory vehicles have compromised suspension geometry and several problems can occur when a car heels way over in a turn. First, the suspension can gain positive camber. This is worse in cars with MacPherson strut suspensions. With strut-type suspensions, the car rolls, but the tires don't. This forces the tire to roll onto its outside edge and reduces its contact patch-clearly not the best way to use the tire.

 

Negative camber is good, and 309 bones help give this... but if the lower wishbone at rest points upwards to the wheel as the suspension compresses as in pic 2... the blue compressed state shows that you get positive camber... not good

So all that money on 309 bones is out the window as soon as the suspension compresses. Think about it, when the arm is flat it is at it's longest, it will only get shorter as it goes up.pulling the bottom of the wheel inwards, so ideally you want most -ve camber and flat arms when the suspension is in compression.

 

The other evil effect of roll is bump steer. Bump steer is caused when the steering linkage and the rest of the suspension travel in different arcs throughout the range of motion. As a result, the tires can give steering input even if the steering wheel isn't moved when the car heels over. This translates to the driver as a twitchy and unstable chassis. Combine dive and squat and all of these problems add up to a serious lack of control.

one would assume that the peugeot designers made sure that around normal amounts of suspension movement, bumpsteer was kept to a minimum.. however as the suspension pivot points for the track rods are not in the same arc as the suspension linkage, bump steer will occur.

 

This image shows the ideal McPherson strut set up..

If memory serves our inner track rod linkage is higher and further inboard, and the outer is also higher..

Thinking back, someone already mentioned raising the rack up to help lowered cars achieve better bump steer characteristics.. The pulsars used to mount the track rods upside-down, to the same effect..

 

 

 

 

 

In summary lowering the car which has.

 

Moved the suspension into a compressed state geometry out of ideal position - Inducing more bump steer

Made things worse with neutral toe - Magnifying any bump steer issues.

Moved the wishbone into a level (or possibly slight upward facing ) resting position - meaning any compression will cause a reduction in -ve camber and possibly positive camber!

Lenghened the roll couple by lowering the roll center lots in comparison to reduction in center of gravity - dulling turn in, making the car react more slowly to input and roll with more force.

Stiffened the damping to try and improve turn in - causing slow weight transfer and uneven mid corner handling.

Stiffened the spring rates to reduce roll - increasing the weight transfer to the front wheels and causing increased slip angle and understeer, decreased ride comfort

Stiffened the rear spring rates to counter act the front slip angle - his actually helped neutralise the understeer somewhat..

Stiffened the rear anti roll bar stiffness ALOT - this helped front end grip on corner exit, and turn in... i'm not sure there are any negative effects, except the rear does not travel over one wheel bumps as well.

increased rear -ve camber - this also helps when the car rolls to keep the rear in contact.

Decreased rear toe in - if there is any inherent toe out through roll and suspension compression, it will oversteer more... eek.

 

and this is without taking into account a few other major factors...

 

All in all it feels after reading this that the generally trodden path of lower stiffer is not necessarily going to make your car handle better.

Now... there are two options..if my understanding is somewhere near correct.

 

1 - try to go back to a more standard front geometry, increase spring rates slightly, and lenghten the ammount of travel, increase front pressures, put a bit of toe in to the mix. Live with the higher center of gravity, in favour of a shorter roll couple, less damping, more travel, better ride.

 

2 - keep it low and try to redress some of the bump steer issues by moving either the rack, or the track rod ends position, lower the bottom arm outer joint to raise the roll center nearer the new center of gravity, increase spring rates considerably at the front to stop it bottoming out, reduce front damping to just enough to cover the damping of the springs.

 

 

Thoughts?

 

Cheers

 

Justin

Edited May 24, 2010 by kyepan

[Batfink 201]

Interesting stuff. Certainly the car could be raised a little. The trick is going to be to make small changes at a time and evaluate whether its helping..

[RossD 44]

Christ Surgie, the man researches and posts a really comprehensive topic regarding suspension setup - Something this forum is a valuable tool for in my opinion. Lets keep the comments constructive and pertinent to the discussion shall we?

 

It's probably worth reading this topic along with the 'Suspension and how it works' topic from a few years ago, which covers some 205\309 specific features, i.e talking about roll centres, iirc from the previous topic the 205 has a rear roll centre at ground level and the front slightly higher which is far from optimum etc.

[BusEngineer 7]

Very interesting read

 

I shall look forward to your developments along the lines of this article, if there are going to be any?

 

Its certainly something to consider as i slowly build mine up

 

I shall be trying the tyre pressures theory first as it really intrigues me to be honest

 

And finally i feel your pain on the stuck at home bored thing, im on my second week off work with an unexplained back problem and im bored rigid and very frustrated about not being able to do anything practical

Edited May 25, 2010 by BusEngineer

[brumster 135 1 Cars]

I think most people know deep in their heart that just straight lowering their car makes it worse - although granted maybe there's a few blinkered idiots who don't. Most of them in Saxo's I suspect. <cough>

 

Some lessons I've learnt over the years from practise, rather than any theory.

 

- Big heavy wheels. Look great, shag up acceleration and braking big-style. Witness GTM K3 fitted with 15" rims instead of the standard 13's. Oh dear, what was I thinking.

- Hydragas spring rates. They work weird. Dicking with ride height changes the intrinsic springing in the hydragas unit, result car is lower but handles like s*it. Witness as above. Thankfully no-one here needs worry about this because we don't use hydraspastic suspension.

 

More recently, on the rally car, there are lots of things you end up playing with to get the car handling the way you want for a given stage or venue.

 

I manage to tailor the handling to 90% of what I'm after, on the day, just by mucking with tyre compounds and pressures. Over time I'll hopefully build up a portfolio of damper settings for each given venue type but, as part of the learning curve, this might take a while. In terms of front to rear balance, I tend to achieve this pretty easily on the day mucking with the pressures/tyres themselves. Only if I need cater for a flat venue versus a jumpy, bumpy venue (eg. Epynt) do I start mucking with damper settings. I've taken the 309 ARB off the front and gone back to standard and, despite more obvious pitch in the corners, the car is umpteen times better for it.

[Pugnut 9 1 Cars]

i've hidden all bickering. this topic has good petential for future reference. Please keep on topic and constructive please.

[Rippthrough 98]

To add to that lot, you don't want to be anywhere near critical damping, even on a race car, about 0.7/0.8 will give faster responses and more grip.

However, it takes more skill to drive, as do soft springs vs hard springs.

You can generally spot the drivers who aren't as good with lines or anticipation at a track 'cause they're on million pound springs and rock hard dampers so they can force the car to go where they want it.

[Henry 1.9GTi 36]

how low is your car to get the wishbone geometry in the middle pic?? :/

 

Mine is somewhere like the left pic and is pretty damn low.

 

Ideally you want to know the optimum camber for the tyre you use, the manufacture should have this information. You then on a crude McPherson strut setup adjust the body roll and subsequent tyre roll against the static camber to achieve optimum around a corner, if only race tracks had one corner

 

Unfortunately at the rear of a 205 there is no camber compensation whatever body roll there is, is directly transferred to the rear tyre. So some good static camber would be required. A pic of my car in standard form around camp corner at castle combe shows the car practically running on the outside edge of the rear left tyre. Doesn’t look like it should still be under steering madly

 

Car setup is fascinating, great post.

[kyepan 291]

To add to that lot, you don't want to be anywhere near critical damping, even on a race car, about 0.7/0.8 will give faster responses and more grip.

Thought this might help, the center line is the car at rest, imagine pushing down on the front wing and releasing. Critical damping it won't bounce up above where it started, but won't take any longer to rise than it needs to.

 

0.7 is definitly much faster response if you look it's pretty much at rest in half the time to critical damping.

Edited May 25, 2010 by kyepan

[kyepan 291]

just read parts 5 and 6... on dampers.

 

As the suspension moves, oil is forced through a shock's orifices and valves at high speed. The oil pressure on the entry side of the valves metering orifice increases while it drops greatly on the exit side. The pressure will drop so much that localized boiling of the oil will occur, causing tiny gas bubbles to form. The localized boiling is called cavitation, which can become so severe that all of the oil inside the shock will start to foam from the bubbles. When the foamy oil passes through the damper's metering valves, the damping force becomes wildly inconsistent and the amount of damping force is greatly reduced. This phenomenon is called shock fade. Shock fade can easily happen any time a car is driven hard. Now you can smack me for using the word "shock."

 

another reason to run with the minimum damping possible, reducing the pressure drop across the valve to the minimum needed will reduce the likelihood of cavitation and dampers that behave inconsistently.

[swordfish210 20]

Great post Kyepan, you must have been bored It's nice to see all of that infomation in one post, um...sticky anyone

[GeorgeXS 32]

Yeah cracking post kyepan, makes good reading.

Now get back out and watch that rally!

 

Great post Kyepan, you must have been bored It's nice to see all of that infomation in one post, um...sticky anyone

Agreed!

[Rippthrough 98]

just read parts 5 and 6... on dampers.

 

 

 

another reason to run with the minimum damping possible, reducing the pressure drop across the valve to the minimum needed will reduce the likelihood of cavitation and dampers that behave inconsistently.

 

This is exactly what the 205 rally cars see with the rear dampers - small diameter pistons + crap damper/wheel travel ratio = high pressure differences across the piston to get the force needed.

 

This means when they hit a whoop or land a jump the damper cavitates, the huge pressure jump collapses the air chamber and turns the rear damper into a spring rather than damping - it slams through it's travel into the bumpstop, which drops the pressure suddenly across the piston, and the gas chamber then expands like a spring - forcing the piston back through the aereated oil on the rebound stroke - meaning lots of force and still very little damping - firing the back end of the car back into the air.

Especially if the damper oil is hot from the amount of force generated and the small size of the body!

 

I.E - This happens:

 

 

You can help prevent that by either moving to a larger piston, a better piston design which generates less cavitation through unstable flow, a cooler running damper, or with a set of compression valving in a remote canister set to balance the valving again the rise in oil pressure by raising the effective 'gas' pressure the oil sees.

 

A lovely set of Foxes would do it....

Edited May 25, 2010 by Rippthrough

[kyepan 291]

how did you manage to catch that!!!

[Rippthrough 98]

Not me

 

Most 205 rally cars do it unless they're running trick dampers/turrets, loads on youtube:

 

 

 

You've basically got the whole thing underdamped, the same as happens if you pop a pair of rear dampers (whoops):

 

 

 

Edited May 25, 2010 by Rippthrough

[Cameron 16]

Interesting stuff, I've always been interesting in handling and vehicle dynamics and those links make some pretty good reading. It's a bit loose on info here and there but it covers the basics pretty well.

 

Nice video too!

 

One thing I've been meaning to do for a while is to measure the suspension hardpoints and put them into Lotus Shark, and see how much difference lowering the suspension does to things like roll moments, weight transfer and bump steer. I have a feeling the effects of lowering won't be quite as severe as that article makes out though, especially on weight transfer. An important piece of info that they've left out is that there are two processes at work in weight transfer: one of them involving the GC force about the roll centre, and another being the resultant force at the roll centre about the ground. Look up rolling overturning moment and non-rolling overturning moment if you want to know more. Since you are moving the roll centre closer to the ground you are reducing the moment that this force creates and therefore reducing the weight transfer, so there will be a balance between the two effects which may mean weight transfer either doesn't increase as badly as predicted or is actually reduced.

 

Anyway, it's a great subject to get on to and I'm glad there's more people on here interested in this sort of thing.

[shalmaneser 26]

Very interesting article although I'm going to echo other's comments and say that the diagrams in question re. camber and wishbone angles represent a worst case scenario - my wishbones aren't at that angle with a 30mm drop, certainly.

 

damping rates thing is very thought provoking too, as is the lack of camber compensation due to body roll at the rear, not something i'd considered before which is definitely worth looking at.

 

I'm currently fiddling around with my damper settings at the moment, the change in feel is subtle but noticeable, I'd love to know what response rate I'm running.

[Rippthrough 98]

I'd love to know what response rate I'm running.

 

Depends how fast the damper is moving and how fast it's accelerating at the time, and how warm the oil is, and what the chassis is doing, and...

 

 

This is a good one:

 

Edited May 25, 2010 by Rippthrough

[fast_eddie 0]

Having been out of the PUG scene for a long time and coming from a time when my 309 ran 300lb front springs on uprated road dampers (bilstein) is there a trend now to move away from such high rate springs with the onset of 'decent' dampers.

I only ask as I will probably get the car back on the road again this year and I am sure it will feel ultra hard after not driving it for over 10 years

With the onset of the R26.r and the clio trophy running decent dampers, has the game moved on for fast road compliant dampers or is it still a case or running high spring rates for a nice handling road/trackday car?

 

Obviously, sticking a set of remote Ohlins on will prove a bonus but I was more looking for advice in relation to the OP's first post and its relevance to modern day or more recent damping progression without spending a fortune?

 

Thanks

[kyepan 291]

One thing I've been meaning to do for a while is to measure the suspension hardpoints and put them into Lotus Shark, and see how much difference lowering the suspension does to things like roll moments, weight transfer and bump steer. I have a feeling the effects of lowering won't be quite as severe as that article makes out though, especially on weight transfer.

how long would that take to do?

It would be very interesting to find out some hard facts.

 

I've been thinking up a simple method of measuring bump steer that i'd like to run past you guys.. although i'm sure there are better ways.

 

1)Jack up car and Remove wheel.

2)Mount a gauge or measure to the outer leading edge of the brake disc with a vernier from a fixed point on the chassis or similar non moving location, with the wheel in it's dead center position.

3)Mark the wheel at 30 deg angles.

4)Move wheel through each marked point on the wheel and measure deflection at the leading edge of the brake disc

5)Jack up the arm 1 inch and repeat 4 until there is no more suspension travel, noting the natural resting height of the suspension.

 

Plug all your values into excel and plot a 3d graph, it should look a bit like a fuel map show if there is a decrease or increase in bump steer as the wheel raises, both centered and when on left and right lock.

 

With a little bit of trig you should also be able to work out the angle of induced bump steer as a deviation from the resting height.

 

i bet there is a simpler way.. but it seemed logical

 

cheers

 

J

Edited May 26, 2010 by kyepan

[Cameron 16]

Not too long, I'm just waiting to get it on the flat floor at uni so I can measure it accurately. I'll also be finding out the GC height while I'm at it providing my car is below the 880kg limit of the corner weight scales.

 

I'm not sure you need to rotate the wheel while you're measuring bump steer (I think that's what you meant above). I think an easier way would be to use a variation on the "string box" tracking method, where you jack the car up so the wheels are just touching the floor and support it on stands. Then remove one suspension spring and place a jack under the wishbone. Then you can jack the wheel up from from full droop and measure from the string line to the front and rear edge of the wheel rim, subtracting the front from the rear to find the amount of toe. Jack the wheel up a certain amount and then measure again, then you'll have a simpler 2D line graph that shows your toe change through the whole suspension travel.

 

I'm not massively convinced that people will need to move to softer springs though, as nobody seems to be running what I'd call excessively high rates. 300-350lbin may seem quite high, but the high spring rate is doing 2 jobs: absorbing bumps and resisting roll. I think as long as your dampers are good enough to cope you'll benefit from rates that high through better body control keeping your camber angles more desirable.

[Rippthrough 98]

As above, 300-350lb isn't too excessive for a track only car (although probably edging to a bit uncomfortable on anything but smooth a-roads).

We run that sort of wheel rate on the rear of the buggy, which is comparable weight wise to the nose of a 205.

[kyepan 291]

Not too long, I'm just waiting to get it on the flat floor at uni so I can measure it accurately. I'll also be finding out the GC height while I'm at it providing my car is below the 880kg limit of the corner weight scales.

 

I'm not sure you need to rotate the wheel while you're measuring bump steer (I think that's what you meant above).

Well i'm in cast for another 5 weeks, so i won't be doing anything much to the car until then.

 

About the wheel rotation: mainly because sandy brown told me to on a previous post.

 

But when i thought about it carefully, bump steer is caused by the track rod pivot points moving in a different arc to the lower arm.

And when you apply lock, you move both the inner and outer pivot points of the track rod, which could induce bump steer relative to the actual amount of lock applied.

So to check bump steer properly it would need to be done at different heights and steering angles, and it would affect both wheels, so you might end up getting toe out on the outer wheel and toe in on the inner wheel or something like that.

 

Practically speaking, the bump steer suffered on the pulsar was woeful, but only present at 1/4-1/2 a turn of lock, there was very little if any up bump steer from straight line suspension compression. so what Sandy said was exactly what happened in reality.

 

cheers

 

J

Edited May 26, 2010 by kyepan

[Cameron 16]

Ah! I thought you meant the road wheel, not the steering wheel!

[DrSarty 90 1 Cars]

Yes: fascinating stuff J, and sorry to hear about your leg.

 

This I think is partly or even mainly why I had Colin Satchell and Sandy Brown do my car, as between them they seem to have a 'firm grip' on this massive, science filled topic.

 

Colin's held a hill climb record down south since 2004; although he did say they went up that many times that the car could've probably driven itself up in the end.

 

Their track cars are doing very well too, and each time I look at my car I see and learn of things which like your original post are perhaps contrary to what most people believed was the right way to go.

 

Ripp likewise seems to be extremely knowledgeable although at the same time I suspect just like with 4-stroke engine development and tuning, no-one really knows it all, and it is a constant trial, test and challenge for those involved.

 

Such are the joys of motorsport and performance car tuning.

 

I hope this does become a sticky.