ex-webby

Hi All

As promised, Damian Harty is now here to answer your questions on the subject of vehicle dynamics and suspension tuning, etc.

IMPORTANT :

He will not be able to tell you which brand is "best" for a scooby, or even which settings are "best".

He can answer questions around the affects of different types of tuning, etc and general scooby related vehicle dynamics.

Please respect the time he has given us so that he and others might do the same in the future.

Leave your questions here and Damian will be along shortly to answer them

All the best

Simon

carl

OK, why is there so much bumpsteer dialled into a standard Scoob?

fivepint

ok here goes, firstly thx for taking the time to talk to us and thx to webby for organising it...

there are conflicting reports as to benefits of fitting strutbraces to "classic" scoobies, what is your opinion/experience on the matter?

Cheers

AndyC_772

iTrader: (2)

Here's one:

A recent puncture forced me to replace my rear tyres (Bridgestone S-02PP with 5mm tread) with a pair of new Toyos. These obviously have a different tread pattern and compound to the fronts.

I'd appreciate the benefit of an expert opinion as to the safety of having this mismatch between front and rear. Potential issues I can see are unpredictable behaviour during wet cornering, and strain on the centre differential.

For what it's worth, I chickened out and had the fronts replaced this morning anyway - but it seemed a shame to bin two perfectly good tyres.

A.

ex-webby

great questions so far chaps

Edcase

Thanks to all involved.

To clear up a good discussion on here recently, how much difference does reducing unsprung weight (IE lightweight calipers / wheels) make to the overall handling, performance and feel of the car.

P20SPD

iTrader: (1)

Hi Damian, thank you for your time.

Could you possibly explain, in laymens terms, how the fitting of an Anti Lift Kit, and Solid Drop Links, would benefit the handling of the Impreza, in particular the Classic shape.

Cheers

Steven

Edcase

Also, having uprated and stiffened my suspension, I'm getting more pronounced tramlining. Is this a direct result of the suspension mods or is it just highlighting the inadequacies of the OE tyres?

Damian Harty

Crikey, that was fast...

Bumpsteer Q: I'll have a look at the exact numbers and get back to you on that one. Toe-change in bump ('bumpsteer') is usually done to try and compensate for something else that isn't as desired, for example a high yaw inertia or nose-heavy weight distribution. As a rule of thumb, on the front axle it's done for steering response and on the rear for stability though (as ever) life isn't so well separated as that. Like I say I'll find the numbers and get back to you.

Strutbrace Q: There is an awful lot of mythology around this stuff that goes back to body-on-chassis beam-axled racers in the 20s - I'm not kidding. There are two effects that are important. Basically, the body structure is like a "fifth spring" between the front and rear suspension. If that spring is not stiff enough then the body relaxes out any redictribution of roll moment that the anti-roll bars were trying to make and so the car is unresponsive to handling balance tuning with springs and bars. However, once the body is "stiff enough" then making it stiffer offers no benefits. A good rule of thumb is that the torsional stiffness of the body (between suspension mount points) needs to be about ten times the roll stiffness of the stiffest suspension end (usually the front). If you chase the numbers through you end up with about 5-7 kNn/degree as the requirement for a body structure. Most modern monocoques comfortably exceed that and the Subaru with its bonded screens both ends does too. Somewhere I have a figure but can't recall it right now. Anything over 10 is good, 15-17 is current "state-of-the-art". So in that case, a strut brace does very little. The second effect is a bit more complicated. The body moves on the suspension but the wheel also moves on the tyre - the tyre is in many ways a "secondary" suspension system. That movement is partially controlled by the tyre and partially by the suspension damper and happens 10 to 15 times a second. The body is a flexible thing that has its own resonances and because of the shape of most cars at the front - they need a hole to put the engine in - it can get quite flexible in just about this frequency region. If the body goes flexible - goes into resonance - then the damper just moves with the body and can't contribute to the control of the tyre and in fact can do something bad called "mass loading" where the tyre is carrying not only itself but also part of the body _while it resonates_ (it's important to separate the static 'weight carrying' from dynamic things in your mind). In those cases, the strut brace can help because if it is a good design then it stiffens the front end usefully.

What all of the above means is that the effectiveness of a strut brace is strongly connected to the stiffness of your suspension. So for a Scoob that is fairly standard, the strut brace probably doesn't do a great deal, but as they get modified and stiffened the brace will contribute more to a well controlled feel over less-than-perfect surfaces.

The final point about strut braces relates to the emporor's new clothes - having bought a flash one, not many people will admit they can't really tell the difference.

Sorry for such a long answer but it isn't straightforward, especially when I'm denied my usual engineering shorthand of "Hz" and "modal damping" and so on...

Tyre Q: Unless you're mixing really crap tyres with really good ones then it's unlikely to be dangerous. And anyway, one man's dangerous is another man's amusing - see my anecdote about Mk1 Escorts... My advice would be to take some time and space to experiment with the car and make sure it doesn't do anything you don't expect, either wet or dry, and then put it out of your mind. If it does do something you don't like, try swapping the tyres end-to-end. That will at the very least confirm the tyres are the problem (if there's no change then something else is wrong and you've eliminated the tyres) and may well reverse the problem. If you really want a dangerous laugh, get some economy tyres and put them on the back. They are bad beyond belief and I am surprised they are on sale! Personally, I've always regretted "saving" money on tyres.

Tone Loc

iTrader: (1)

Thanks for organising this and thanks to Damian for participating .

My scooby centred question relates to the inherant understeer as standard. There are several companies that offer solutions to try to dial this out... just woundering what Prodrive would suggest. Im not looking for a suggestion of manufactures kit but a more of an over view of why it does it and what to do about it. For example increasing the rear roll bar, geometry settings etc.

Also on a broader dynamics base.... what do Prodrive use to evaluate the dynamics of the vehicle. This would probably depend on which side of Prodrive you work for... motorsport of road car development i guess. Is it initially done in the virtual environment with a full working model or are parts tested in the real world etc?

Cheers,

Tony.

SCOOBY TOWERS

iTrader: (7)

I have a totally standard UK MY99.
I am considering things like solid drop links, strut braces and geometry setting.However the funds are a little low at the moment.
Where would you start on achieving better grip / handling for a minimum outlay ?

fivepint

excellent, thank you very much...
...not long at all but very interesting

Cheers

ex-webby

Top reply from Damian! and more great questions.

Just spoken to Damian and he is very pleased with the questions "exactly what [he'd] hoped for" and will be replying as time allows. Please bare with him if replies are delayed from time to time.

All the best

Simon

CraigH

What is the "ideal" wheel/tyre combo for a Scoob - as in size. There must be a point where going for a certain size/width is detrimental to the car?

carl

Related to Tone Loc's question: the solutions that reduce understeer on the Scoob invariably lead to more snap oversteer on lift off. Is this a function of the solutions used (anti-bumpsteer geometry, more neg camber on the front wheels) or is this a general trade off (i.e. less understeer under power always equals more snap oversteer on trailing throttle)?

mutant_matt

Damian,

Can you explain the difference between "normal" (as fitted to most of the Impreza range) and "inverted/upside down" dampers (as fitted to the MY02/03 STi) and the reasons that Subaru may have switched from one to the other.

When the MY02 STi was announced, there was much discussion on this board about the matter with some people proffering that the upside down dampers were more rigid and some peolpe saying that they weaker. Others sited unsprung weight as the reason....?

Can you also explain how these may "feel", comparitively?

Cheers,

Matt

David_Wallis

Whats your favourite colour impreza?

Seriously though..

Do prodrive lower the engine and transmission position in the car to improve handling?

Do you fit uprated bushes to everything?

Can anything be done to stop my rear diff trying to enter the car through the boot..

Also what mods can be done to improve the std car with minimum outlay... (IE fabricated by myself )

Cheers..

PS Can you lend me a STI7 Gear Linkage

David

ex-webby

CraigH

Remember that terms like "better" cannot really be applied to subjects like this, as (almost without exception) everything is always a trade-off of some sort.

What may produce certain improved handling characteristics, may also reduce grip, etc. It all comes down to specifics of what you want to achieve.

All the best

Simon

Catalunya Pete

Damian, how much effect does the wheel & tyre weight have on ride quality. Would for instance a 17in wheel weighing 2 kg more than another 17in effect the damping enough to have a negative effect. If it does would stiffening up the dampers a bit compensate for this.

Pete

Edcase

My unsprung weight question SHOULD hopefully cover that one

jeremy

Damien,

I have driven several of the rally greats for the road, Integrale, Impreza, Evo and UrQuattro. It is easy to note that each have their qualities with respect to handling.

I have a question about the general handling balance of sporting cars set up for the road such as you did with the P1.

I've noted that the old Quattro, while it certainly does not have to reflex's, or agility of the Impreza, does have one unique party trick up its sleeve. It seems that almost no matter the coditions, nor how hard you push it (within reason) as it begins to lose grip a simple lift off the throttle and it very quickly snaps back into grip. While with the Impreza P1 you actually have more grip and can round corners even that much faster, but that when grip is breeched to get it back takes a deft hand.

Do you agree that there are several equally effective ways to set a sporting car up for fast yet safe road driving?

Last, how would you compare the self-centering ability of the Quattro to other more active handling requirments of more agile car such as the Impreza- such as driver controled oversteer, for making a press-on drive as safe as possible?

Claudius

Wow, nice one Simon and Damian!

Would you agree that "the suspension is the very basis of a fast car"?

What parts of an Impreza GT (classic shape) should I be concentrating on when seam welding it? I heard the floor pan is the weakest part?

Does fitting a bigger stiffer rear anti-roll bar "improve" the handling (take away some understeer) by diminishing the rear grip or by increasing front grip?

Thank you for your time

Damian Harty

OK, a bit more while I wait for someone else to come and thrash - er, test - our ATD Impreza.

Unsprung Weight Q: Reducing unsprung mass (weight) rarely does any harm. How much good it does depends a lot on the context and the key question is always "does the benefit justify the cost?" The less perfect the road, the more important unsprung mass is because over every little bump the wheel has to move up and down. I mentioned in a previous post that the tyre is like a secondary suspension for the wheel, and the key to using the tyres to their very best is not changing the loads on them. The bigger the unsprung mass the more load change is needed to make it follow a given road profile. Now, as for overall handling performance and feel, the truth in a road car is that you need a tyre, you need a wheel to hold it, you need a bearing to hold that and you need a brake rotor and caliper. Once you have all these things, there is only so much you can do to change the unsprung mass and it starts to get very exotic very quickly - costing lots and lots. Unless either you have a lot of money on your hands and want to spend it or are competing at a level where every little counts, it's probably more cost effective to buy better tyres, better performing brakes and a better performing driver to improve the car all round than to expend money simply on saving unsprung weight.

Tramlining Q: If you imagine a "perfect ride" suspension where the wheels went up and down with no load change and the body wafted magically above them, road features wouldn't influence tyre load at all and there would be no tramlining. If you go to the other extreme and imagine no suspension, then every little road feature would dramatically change tyre load. Because the tyres run a bit of toe and a bit of camber, this changing load results in a changing side load and the phenomenon of tramlining. So yes, in general, firmer suspension exacerbates tramlining. What you might find is that because the car rolls less you need less static camber; reducing camber a little might well restore the tramlining to its previous level.

Understeer Q: There are two types of understeer, one is a "technical" understeer and the other is a "perceived" understeer. They are related but aren't the same. The "perceived" understeer is all about how the car becomes more understeer the faster you go on a given corner. There also different "phases" to understeer, too. The first is as you turn the wheels, and static toe influences this a lot. Running less toe-in or some toe-out on the rear wheels helps suppress this and costs nothing, but be careful because it degrades high speed stability, especially over imperfect roads. The next is as the car settles into the turn and is greatly influenced by dampers. More rebound damping at the rear end is a great way to help suppress understeer here. The third phase is once the car is settled into the corner, and traditional learning about big rear anti-roll bars applies here. Anything that gets the rear bar working earlier is also a good thing, like stiffer ARB bushings and so on. The final phase is as you accelerate out of the corner. A "more locked" centre diff - i.e. thicker fluid in a visco or more clamp on a centre plate diff helps here - but sorry, I've no idea what is available as tuning parts on that front.

Dynamics Evaluation Q: We use a combination of mathematical modelling and physical testing. The key to success is to understand what "better" really means. On the motorsport side it's easy to define - a faster lap or stage time - but hard to achieve. For road cars the opposite is true - hard to define but more or less anything is possible; think of the difference between your Scooby and a Rolls Royce. Or worst of all, imagine you want to make a 1.8 Ford Focus - it has to be all things to all men, and be cheap too! In general, the more defined the target audience for the car, the easier it is to deliver what the engineers *think* that audience wants. I'm the first to admit we're not always right but we try really, really hard, honest! This is a big reason for my interest in this forum - it's a group of intelligent people who are really the target audience.

Better Grip/Handling for Minimum Outlay Q: Step 1 - Really good driver training. I'm not talking about our ProdriveLive "taster" days but rather some high quality one-to-one coaching with someone like Don Palmer (www.drivingdevelopment.co.uk). Don taught me, and if you think I'm a bad driver now you should have seen me before Don educated me. Step 2 - the best tyres you can afford. Step 3 - the best brakes you can afford. Step 4 - OK, now you're going quite well it's time to pay attention to little details like strut braces and geometry, solid ARB bushes and so on. With all these things there is a law of diminishing returns and to start with strut braces is to set yourself up for the "emporor's new clothes" syndrome I mentioned earlier. I guarantee that even if you are a really good driver you will still benefit from starting with your technique and it will be as good as much more expensive modifications to the car.

More answers to follow shortly...

Damian Harty

I've realised I missed a Q so I'll start this bunch of answers with that one.

Anti-Lift Kit/Solid Drop Links Q: Hopefully I've hinted at the answer to the solid links in the answer I gave about understeer generally. To be honest, you need to pay attention quite closely to notice the difference because unless the standard ARB links are very soft then they will be "nearly stiff enough". I would probably prioritise the body mounts for the bar over the links since they have higher forces on them and so making them stiff matters more. Ideally, do body mounts and links together since if you do only one, you "transfer" the effect to the other. Nothing bad happens to handling by doing this, only a tiny bit more noise. As far as I can tell, CD players and Jimmi Hendrix were invented to cure that particular problem. As for an "Anti-Lift Kit", I'm afraid I don't really know what it is - can you enlighten me?

Ideal Tyre Size Q: Obviously the wheels and tyres sold by Prodrive are the ideal... :-) Seriously, yes it is possible to go too far. The problem with defining ideal is that ideal for the road is compromised for the track and compromised for the rally stage and so on. You'll note that the gravel tyres on the WRC are a significantly different size to the tarmac ones. In general, bigger wheels & tyres mean more unsprung weight and in general that is a bad thing. What it does mean is that 19" slicks might be really rather good on a smooth track but that the 18s we sell are a great road/track compromise. Obviously FHI think 17s are best for the road...so sadly there is no clear answer. In general more width is better but this isn't always true, particularly when there is water around, so even for a given application there is no definite answer. And as the World Rally Championship proves time and again, even the experts don't always choose the right tyres. SO my advice is, buy tyres you like the behaviour of and buy wheels you like the look of. If in doubt, buy what Mike Wood sells because more than anyone else he gets to try a lot of different combinations and he chooses well, in my opinion. If you actually buy them from Mike, that's a bonus to us... :-)

Snap Oversteer Q: This is a great question and might well be rewarded with a drive in the ATD Impreza unless a better one comes along before the end of the weekend. In general yes, reducing understeer reduces stability. If you look at the answer I gave to the understeer question about the "phases", the same kind of logic can illuminate the lift-off behaviour. Usually the lift-off is made from an on-throttle situation, so I'll imagine that's the case. Any kind of drive force reduces the ability of the tyres to grip laterally and so the first thing that happens is the lateral forces "jump up" on the tyres. Side-force toe out on the front axle or side-force toe in on the rear axle helps stabilise the car here. These are achieved with things like compliant rack bushings on the front and a compliant inboard front lateral link mount on the rear. but they are small modifiers in the overall scheme of things. The next thing that happens is a bit more body roll and so suspension geometry that promotes understeer with roll stabilises that - however, we just went the other way to fix the understeer problem, so that's no good. During the roll, dampers modify things but once again, we just went the other way to fix the understeer. Once the car has rolled, traditional things like anti-roll bars apply and yet again we just went the other way to fix the understeer problem. So really, the best fix for snap oversteer on lift-off is not to have the understeer on power in the first place. That's why I came up with ATD and that's why the question deserves a ride in the car!

Inverted Damper Q: In principle, in a strut-type suspension you want biggest (i.e. outside) tube to be where the bending moment is highest. This is at the junction between upright and damper tube and so for a given amount of metal, inverted dampers are weaker/less stiff. Note this isn't the same as inverted motorcycle forks because the biggest bending moment is at the top where the fork tube emerges from the yoke. However, pushing the big bit to the body end is a useful reduction in unsprung weight for not much change in function. In truth with a bit of imagination the reduced strength/stiffness can be overcome with some good design so you're left with the unsprung weight benefit and if the design is properly good you improve stiffness and strength as well, although getting there can nearly undo the weight benefit. The reason they're not on everything is the sealing technology to stop the oil running down the damper leg costs more than just making the bottom tube into a bucket for the oil. In terms of feeling, both of them lead to an improved feeling of "security" or "steadiness" over imperfect surfaces and the stiffness in particular benefits "tautness" of the car during turn-in in a similar fashion to lower profile tyres. As with many things, once the dampers are 'stiff enough' then going further brings no benefit.

Impreza Colour Q: Blue. Though the ATD Impreza - arguably my favourite Impreza - is white. :-)

Engine Lowering Q: In the rally car we move everything as much as the regulations allow to improve weight distribution. As far as I know, yes, we move the engine & transmission down and back to that end. On a road car, some air is left around the engine to ensure the crush behaviour is correct during a crash. Since we have a gurt bug roll cage, four point harnesses and so on, we sacrifice that. Interestingly though, with a boxer engine there comes a point where lowering it too much means too much curvature on the exhaust headers and it costs us power.

Uprated Bushing Q: If you looked under our rally car you would see "industrial strength" metal bits everywhere. We throw away bushes and use ball joints everywhere. As well as the stiffness improvements they are necessary to survive the things those rough boys do with the cars. If only they were more gentle they'd be able to use rubber bushings and hence hear the stereo properly...

Rear Diff Q: Unfortunately this could be the rear diff really trying to enter, in which case nylon bushes will fix it (they make the driveline kinda jerky though) or it might sound like it's trying to get in, in which case softer bushes are needed to isolate the noise and thumps and bumps. A cheap and cheerful fix is to drill holes in the rubber to make bushes softer (they don't last as long though) and to poke steel rod into the holes to make the bushes harder. If you're interested it can be an amusing thing to experiment with.

Wheel & Tyre Weight Q: These things affect what we call "secondary ride". It's the response of the vehicle to 'small' features in the road and feels like a shuddering, quivering sensation from the relevant corner if you get it wrong. If you pull off the balance weights from a wheel you'll get an idea of the sensation, but of course then it is always there rather than only over road features so that's probably a bad idea as a learning exercise. 2kg is a relatively small proportion of the unsprung weight but I would say if you are paying attention you will probably notice it. Damping changes can improve matters but you need to target the damping change at the specific damper speed excited by the problem, otherwise you make the rest of the ride worse. Generally, so-called "adjustable" dampers don't have that much flexibility, you would be needing to rebuild a damper with an adjusted shim pack to match the damper to the new wheels. Unless you really really like the look of the heavier wheels then I wouldn't recommend going for them; if you do go for them I'd leave the dampers alone unless you really can't live with the ride degradation. To be honest I would have thought it will be pretty bearable if you turn up the stereo and think about how good the wheels look...

Damian Harty

Several Effective Ways Q: I absolutely agree that there are several effective ways to do things. One mans delight is another man's dishwater. There is only one exception to that rule - I can't believe anyone actually really likes Mitsubishi Evos... ;-)

General Handling Balance Q: Handling is a really hard thing to define. The definition I use is "that quality in a car that encourages you to use all the grip available". It is very definitely an emotional thing and not an engineering thing like "grip" or "acceleration". We use lots of different measures to quantify different aspects of handling. What I really want in a car is something that allows me to clown about with the power if I'm in the mood but otherwise goes where I want it to go with the steering wheel, more or less come what may. I haven't made that car yet, but technologies like ATD are a great step towards it. The innate ability of the Audi in that respect is a good thing and is one of the behaviours I've tried to engineer into the ATD car. Sorry if I sound like I'm in advertising mode - I'm just very pleased with the technology and am working on the next technology now.

Suspension Basis Q: I absolutely agree with your statement that suspension is the basis of a fast car. If you see my definition of handling in the previous answer, you'll see that without good handling I think one is unlikely to use all the available grip. And if you're not using all the available grip, you could be going faster. Something theoretically fast but scary doesn't get driven as fast as something 80% as fast but reassuring, in my opinion. Hands up anyone who's ever been overtaken by a Turbo 911 Porsche. No takers? Surprise.

Seam Welding Q: All the seams in the floor between the struts are important to make it work effectively as a shear panel. The same is true for the front bulkhead. If you are allowed a seam welded rear bulkhead (we aren't in WRC) then that is a great help. The other places to concentrate on are the everywhere a pillar joins somewhere else. In production these joints are designed to allow spotweld access and are a structurally a compromise for that reason. Last I heard, we spend 250 hours turning a standard shell into a WRC shell and we're very well practised at it. It's worth every minute. If you can, consider bolted or welded "closures" (bonnet, boot, doors) - again depending on rules and practicality. They are very effective shear panels if connected well to the body structure.

Big Rear Bar Q: If the car understeers at the limit then by definition it has less front grip than rear. The anti-roll bar robs Peter to pay Paul - it both improves front grip and reduces rear grip. In general, it improves the front more than it degrades the rear - a net gain - in an understeering car. "Optimum" is when both ends lose grip at exactly the same time but in practice this can prove surprisingly difficult to drive so a little bit of understeer is good for confidence.




Phew! I think I've caught up now. Thanks for all your interest, I hope some of what I've said makes sense and I'll answer more questions when I next have a chunk of time.

carl

Thanks, Damian

Paul N P

Damien,

Many thnks for your very informative answers above.

I've just had a superb EcuTeK remap on my STi7, the only problem now is that I'm getting loads (& I means loads) of front end wheel spin which I wasn't getting with standard ECU settings. My suspension and tyres are completely standard, when I apply the power the nose of the car points upwards literally.

What would you recommend to sort out this issue?

Hope you can find time to respond.

Cheers

Paul

Edcase

Damien, I'll second that, many thanks for the answers. All makes for great reading.

Having just changed my pants from a ride in Pauls car, I'll second what he says, and also use that to explain the anti-lift kit, which effectively tries to prevent the nose lifting under hard acceleration (frighteningly hard in the case of Pauls car!) by presumably stiffening the rear bushes / rebound, and thus giving better traction and control out of slower bends and under general acceleration.

To add to Pauls question, will this kind of kit have any negative affects under 'normal' hard driving IE other than when accelerating hard out of a bend?

Again, thanks for your input.

mutant_matt

On the ALK front chaps, there was quite some debate on here and on MRT Rally Forums here. I think, in the end, it was agreed that it isn't in fact an Anti-Lift Kit at all but just a castor increase with harder bushes (but read the thread).

It's still a top mod though

Matt

Phil Harrison

Hi Damian - Thanks for taking the time to advise.

You may be aware that a significant number of STi7(UK)s have a low-speed 'knock/rattle' from high up in the NSR suspension; and some of them have not been cured by as many as 5 substitutions of strut/topmount/springs in varying combinations.

1) Have any of Prodrive's 7s had this fault apparent on delivery
2) If so, how did you cure it, and
3) Can you think of any reason why it should be invariably be NSR?

TIA

Phil Harrison