minister of speed

Damian

The new ppp for MY03... will it fit the MY02???

Thank you


and... Prodrive spring pack for MY02 - how much lower f & r ?

Cheers

carmad*1

Hi Damian,
Thank u again for taking time out to answer all the ques,
with reference to your answer ATD kit costing £xxxx, on a sti ver4 type r is the kit (guestimate) going to cost £2k-£5k or £5k+

regards

ed

Damian Harty

ATD Cost Q: We're definitely aiming at the low end of the £2-5k region, not £5k+. However, our one-off car cost us about £40k but that's mainly because of the engineering time (that doesn't need repeating).

stephen30

Damian,
I have been reading all this and found it all very informative, thank you. I know strictly speaking this is not really a suspension question and may be a little bit personal, but I'd be interested in how you got into the job you do? Just wondering cos this is the kind of job I'd love to do but don't have the skills knowledge etc and at 27 it's probabaly too late. Fair enough if you don't want to get into a discussion about it.

Steve

carmad*1

cheers Damian,
I'll have to keep watching for the release date and exact prices.

thank's again

ed

mutant_matt

Theo,

I presume the spoilers in question are the UK300 spoilers?

Matt

Jza

Question on bonnet air scoops...

Does increasing the height of the scoop increase the amount of air the intercooler recieves? I'm pretty sure that air flow over the bonnet would effect this? There seems to be a debate that at high speed the flow can actually stop going into the scoop - and this causes cooling problems that people think may contribute to engine damage? Any views on this?

I notice that the new car has a larger scoop... have prodrive found an "optimum" size?

Jza

Tone Loc

iTrader: (1)

Just read the latest Professional Engineer magazine..... interesting contribution Damian.

Tony.

Claudius

Rear Suspension Design Question (GC8 - 1997)

Hi Damian

I have just come back from my suspension specialist shop where my Aragosta (Proflex assembled in Belgium) 3 way adjustable dampers hydraulics were adapted to my liking (I found the hydraulics too hard). The car used to be "jumpy" in the back. It still is, but much less.

They told me that the design of the lower rear suspension arm relatively to the damper angle is the cause for this, and that it happens with every damper that has got two guide rings for the cartridge, ie every damper except for the Konis (which only have one ring and thus flex a little). Initial damper travel is difficult to achieve due to a "resistance" appearently due to this damper angle vs. arm issue.

I'd like to add that this is only happening when not going fast (with lots of weight transfer and important damper travel), but only when driving along slowly (ie with traffic).

They added that the angle of the damper relative to the lower suspension arm was diminished slightly in the new body Impreza (from 2000 on I think).

I'd appreciate hearing your comments on this subject and suggestions for improvement if needed / possible.

Damian Harty

I've been in America so haven't been following the thread, apologies for the delay in replying.

How Did I Get Here Q: I'm not really sure myself, to tell the truth. I studied Automotive Engineering at Loughborough University and it was always the dynamics side of things that interested me the most. I then went and worked for a software company for a while and that got me good exposure to car companies and their processes, plus how much data gets recorded and never really used. After that I went and worked for Ford for a while in their research & engineering centre and that taught me a great deal about the reality of chassis design. I was around at the time the old twistbeam escort was being ripped up to turn it into the "control blade" Focus. I got to learn a lot about twistbeams and why Golfs were so good and Escorts so poor, and helped redress that balance in the 95 model year escorts (the ones that went to blobby, upside-down looking rear lights) to make it the car it could have been all along. The Focus was hard work mainly because no-one in Ford had engineered such an expensive suspension into such a cheap car before, but history seems to suggest it was worth it. Then I went to a consultancy company and got stuck into a lot of body structure and system behaviour modelling work and that seemed to round out my education well enough for Prodrive to take me on in 1997. I've always been a suspension and dynamics specialist there and using some of my skill/judgement/luck (delete as apropriate) I seem to have been in the right place at the right time ever since. I think the guiding principle is to know where you want to get to, and make every decision on the basis of it moving you closer to or farther from that goal. I don't know what skills you have but 27 is nothing like too late. There are plenty of part-time Master's degree courses that will give you all the information you need, you just need to add some enthusiasm. I did one at the University of Hertfordshire that I finished in 1999 and it was great as a kick up the bum to stop me getting complacent. I'm currently working on a PhD at Coventry University, mainly because it's close to where I live.

Air Scoop Q: To a point, increasing the height increases the air the intercooler receives. Two things influence the flow through the intercooler, one is where the air comes from and the other is where it goes to. If you imagine that the exit air is somehow restricted then you can increase the inlet air (i.e. bonnet scoop) all you like but it won't get any more air out the exit - i.e. through the intercooler. This is a slightly simplified version of reality but close enough. On the rally car you'll note we put the intercooler right down at the front and exit the air before it gets near the engine compartment, over the bonnet. Ironically enough it exits right into the now-blanked scoop. This has a lot to do with keeping air out from under the car (important for cutting lift) and a little to do with intercooler efficiency. We sacrifice some radiator area compared to the road car because we don't need to sit in a traffic queue climbing the hill out of Death Valley, which is just about the worst situation a road car is engineered for - low speed, high load, high ambient temperature. I'm sure the modified scoop on the new car is an incremental improvement although "optimum" suggests to me there's no possible further improvement, and I never really believe that...

Rear Suspension Design Q: Strut-type suspensions as fitted to the Impreza and many other cars have an inherent flaw in that they carry wheel loads by trying to bend the damper a little. For example, when cornering the wheel is trying to "tuck under", pivoting about the lower arm joints, and it's the stiffness of the strut itself that stops it doing so. When travelling normally it's true that the location and angle of the strut modifies things. However what matters is the lateral distance between the top of the strut and the middle of the wheel - difficult to set to zero in any practical strut suspension. That offset produces forces that try to bend the strut so that the wheel would assume more negative camber when travelling straight ahead. To some extent these forces can be compensated for by "twisting" the suspension spring - the outboard edge of the lower platform is higher than the inboard edge - and offsetting it from the axis of the strut somewhat. Both of these lower what we call the "breakout friction" of the strut and remove the sensation of it being "lively" or "restless" over very small road features. I'm sure the newer Imprezas have some different geometry to modify this behaviour - it's an area of ongoing optimisation. The problem is that what works in one situation doesn't in another and so in truth the strut is always compromised. The Koni solution sacrifices camber stiffness - in other words, under cornering the wheel is deflects a little more to give more positive camber and that's why we stick with the dual guide ring solution and then work with the spring design to try and cancel out the forces at ride height. Attention to detail on the damper top mount also helps reduce this behaviour - it can be used to insert some compliance while keeping the dual guide ring.

Hope that helped, sorry again for the delay in replying.

stephen30

Thanks very much for that Damian. Economics degree and Business A levels don't really lend themselves well to this sort of area. Best do some investigation into courses.

Steve

Claudius

Would reducing the rate of the helper spring contribute to better low speed absorption? Or is the helper spring fully compressed when the damper is installed / car rolling? I'm a little confused, the main spring rate in the rear is only 30 N/mm.

What exactly would you suggest I do to the top mounts?

Thanks again for your time

JIM THEO

Damian I am going to fit the OZ Superturismo Golds with 48mm offset (produced only for Subarus) to my new WRX "03 and have heard a lot of rumours about how bad is their offset and is better to stay with factory wheels.
As I understand 5mm lower offset from OEM wheels isn't so critical (in worst case it promotes some tramling) but can you tell us what are the offset limits, both, upper and lower, for an otherwise standard WRX (all years)?
Thanks
JIM

JIM THEO

Just to add in the above question, do you plan to produce springs or complete suspension kit for the new 03 WRX?
JIM

superstring

Damian

First let me say that all this makes fascinating reading! Thank you!

I own a MY98 Impreza 2.5RS (2 DR coupe/Type R bodyshell) with a Prodrive "Performance" (fast road) suspension kit, with which I'm very pleased.

However there's one thing about the kit that's always puzzled me. The rear of the car sits slightly lower than the front; in other words, the car has a slight amount of negative rake. (I'm not just talking about tyre to fender distance) The dealer who sold me the kit assured me that this was normal and that Prodrive had developed the kit "with particular attention to the front/rear roll centre relationship" and that "Imprezas handle better when they're a little squat in the rear".

Can you comment on these statements? I'm particularly interested in knowing what sort of roll centre relationship Prodrive was trying to achieve and the technical reasons for it. Also, once this relationship has been established, how much room (if any) is there to fiddle with it before you mess up what was intended - i.e. if I were to somehow raise the rear of my car by say 1/2 inch, would that change the handling balance in any significant way?

Thanks very much.

John Caviness
Canada

[Edited by superstring - 3/17/2003 12:53:34 AM]

[Edited by superstring - 3/17/2003 9:10:39 PM]

rex11

I live in the states, adn the roads/weather conditions don't always cooperate with a lowered car.

I've heard people say that lowering the car actually hurts the handeling in a WRX. This seems to go against all engineering logic of center of gravity. Any thoughts?

My real question is this. I want to put in a suspension that will be complacent enough to handel the harsh roads and conditions I live in, as well as giving good handeling. When choosing spring rates and roll-bar stiffness, which one is better to favor. It seems like the two both achieve the act of stiffening the suspension. If you use a lower spring rate and a stifffer roll-bar, will you still get the small road imperfection adaptability while maintaining the stiffened handeling. Or does the stiff roll-bar just overcome the softness of the spring to lead to juttering around corners.

It's late so hopefully this makes some sense. Thanks for the time.

Hoppy

Canada John, I have the same suspension as you on my UK00 saloon. It was so low at the rear that I couldn't clear the speed bumps in MacDonald's drive-thru.

Prodrive replaced the rear spings but within a week it had settled to the same level. Then Mike Wood sent me a set of rear springs for a wagon which are 1cm longer.

Problem solved.

Richard.

superstring

Richard

Thanks for the info! Could I ask a favour? Could you tell me what the measurements are, front and rear, from wheel centre to fender lip on your car? Also, looking underneath the car from the rear on a level surface, are the lateral links parallel with the road or do they angle up from the wheels to their attachment points on the cross member?

I imagine that not only are the wagon springs longer, but stiffer too, correct? I'm wondering how the car rides, especially if it has any strange pitching movements?

Thanks Richard!

John

Hoppy

John, front 335mm, rear 320mm. UK cars have plastic rust protector around the wheel arch but if yours does not, you should add 3-4mm to those figures.

Lateral links? Not sure which bits you mean.

Richard.

Phil Harrison

Damian,

Very interested in your "Rear Suspension design" response to Claudius, especially As you may be aware, I'm the most active and vocal complainer on SN about the "NSR Suspension rattle" on STi (7) Type UK, of which three of the characteristics are a) it happens principally at low speed, b) is more (tho not exclusively) likely to happen under lateral load, and c) is caused mainly by small flaws in road surface. And, I'm right am I not, STi7UK is the first UKDM car to have inverted struts??? You can see why your post has caught my attention

Car goes back to W Bromwich on Tuesday for its fifth warranty rectification, and I know for certain that there's a substantial number of SN owners who have the same problem on the 7UKs. Appreciating the sensitivity of a Prodrive employees commenting on S-UK issues (at least, on public BBS's) have you a view on likely causes and - most importantly, potential cures. Repeatedly changing the dampers and topmounts hasn't done the trick!!

Thanks

Phil Harrison
edited twice for control-character

[Edited by Phil Harrison - 3/16/2003 5:07:15 PM]

[Edited by Phil Harrison - 3/16/2003 5:08:57 PM]

mrciave

Very interesting. Thanks Damian.

[Edited by mrciave - 4/19/2003 11:04:00 AM]

[Edited by mrciave - 4/19/2003 11:15:03 AM]

Tone Loc

iTrader: (1)

Phil.... the inverted strut question... the P1 was the first Uk car to get inverted struts i think. All Sti's from version V onwards have these. I have them too and they are as quiet as a mouse but again mine is a 'classic' version. Be interesting if Damian had a theory on the version 7 noise tho.

Tony.

Phil Harrison

Interestingly, Tone, I know of a STi6 JDM which has it in small measure......

Cheers,

Phil

superstring

Message for Hoppy:

Thanks for the numbers, Richard. Just in case you're interested, my car measures 350mm front and 330mm rear.

The "lateral links" are the two control arms at the bottom of the rear suspension that run perpendicular to the wheel (as opposed to the "trailing arm"). Mine are basically parallel to the ground; I imagine your's are too.

Cheers, John

carl

Another question for Damian.

Assuming that power isn't an issue what, vehicle dynamics-wise, would you change on an Impreza that you intended to drive at 200mph?

David_Wallis

carl

carl

It's a serious question -- he may have some useful thoughts on the matter.
PS: I was thinking today, you're better off making the front lower than the back, lest any change in pitch causes a take off

Damian Harty

Another group of answers:

Helper Spring Q: Not sure exactly which form of helper spring you have but I'm presuming you're referring to a fairly standard type of rebound spring. At ride height, a rebound spring is usually fully compressed and only comes into play once the wheel goes appreciably into droop. Since the problem we're talking about is around ride height, the helper springs don't make a difference there, no.

Top Mount Q: This one defies an easy answer, but sometimes a jiggly ride caused by strut friction can be improved by softening the vertical stiffness of the top mounts. However, sometimes the mounts are already too soft (unlikely in a Scooby) in which case firming them up makes the dampers move and 'break out' of their friction lock earlier - it's a question of matching dampers to topmounts. Softening them too much leads to too much motion before the dampers move, though, and also it's difficult to soften the vertical stiffness without changing the lateral stiffness, which can hurt handling. Only refinement suffers if you make them harder. Without a top-mount supplier moulding prototypes for you, it's difficult to do anything constructive aftermarket!

'03 WRX Kit Question: No idea! Mike Wood will know the answer to that.

Roll Centre Q: When a car is near the lateral grip limit, the relationship between front and rear roll centre is very important in determining the way breaks away in response to small imperfections in the surface. If the roll centre is too high then the breakway becomes more aggressive. In general we'd prefer the front of the car to break away before the rear since it doesn't lead to a spin. Our circuit cars adhere to this principle, too. Therefore when we're near the lateral grip limit we prefer a slightly lower roll centre at the rear than the front. This can produce slightly odd behaviour during turn-in and so we use the dampers to compensate for this a little and "hold up" the rear of the car during turn-in - I think I talked about the different phases of turn-in in an earlier post on the subject. So basically, your man is right - ride height is a cheap way of adjusting roll centre height in a strut-based car and as long as we pay attention not to compromise travel and on-centre behaviour by re-valving the dampers, then we end up with an improvement in limit handling breakaway. If you raise the car by 1/2 inch then you'll notice the initial turn-in might well feel a little more secure, however, you may find the breakaway behaviour at the limit a little less benign. By all means try it somewhere where you have space not to damage anything. When experimenting with heights we sometimes place a "packing" ring under the spring seat - just a nylon or rubber ring of the appropriate height. You don't want to go much above 20 mm of height increase with one though - they can get a little unstable and "ping" out if they're much bigger.

Lower CG Worsens Handling Q: Lowering the CG is definitely better for grip but sometimes hurts handling. It's not a straightforward thing but is a real problem. Basically the suspension travel on the car is laid out to deal with the biggest obstacle you want the car to accomodate. If you lower the car too much then the suspension can run out of compression travel; when it does this the tyre becomes overloaded very suddenly and this can lead to very discontinuous and disconcerting behaviour of the car. One possible solution to this is to stiffen the springs and damping but then normal size bumps produce a bigger load variation at the tyre, which also hurts both handling and grip. So in general, for a given road environment - i.e. a a given size of obstacle to accomodate - lowering avehicle usually degrades handling because it usually reduces bump travel. If you take the vehicle out of one environment (e.g. road) and place it in another (e.g. track) then you can adjust the travel required to match the new environment. Going from road to track usually means less travel is required and so a car can be lowered to increase grip without hurting the handling. Note this works the other way too - check out the difference in ride height between our Tarmac-spec WRC and our Gravel-spec WRC.

Suspension Rattle Q: It sounds to me like there is a problem with the shim stack chattering rather than a topmount problem. Over small inputs the shims can't decide whether to be open or closed and so they sometimes chatter a little. Usually this isn't a problem but some vehicles amplify the noise very effectively into the cabin. The rear strut top mounts are a problematic area for all production car designers in terms of noise inputs into the car. The change in behaviour under lateral loading is quite a big clue. When driving straight ahead, the top mounts have what we describe as a "prop-out" load laterally - the strut wants to move inboard and the top mount restrains it. Under cornering this load steadily reduces to zero, then increases again in the other direction - as the wheel tries to roll in under the car, the strut wants to move outboard and is restrained by the top mount. The top mount stiffness changes with load and is lowest at zero lateral load - they transmit noise very differently under these different loading conditions. Vehicle bodyshells are surprisingly variable in their construction and one shell might shrug off this variation in noise transmission easily where another is very sensitive to it, despite them both being within specification. I think the problem may be with your bodyshell rather than the struts and top mounts - changing them repeatedly has clearly not cured the problem. There are a couple of possible solutions. One is to add some stiffness - like a strut brace or something - to make your shell less sensitive. Another is to try solid rear topmounts (nylon or similar) to try and get the damper moving earlier and make the shims make their mind up. It's definitely worth getting your dealer to remove the trim and check the shell really carefully to make sure there are no missing spotwelds, cracks, or anything else untoward in the whole rear shock tower area. The reason for it being a nearside problem is probably nothing more elaborate than the road being more bumpy nearside than offside (which it usually is unless it's a very new piece of road). Not sure if that helps but it maybe gives a little more information.

200 mph Question: Once speeds get up to those levels there are two big issues. You're right that it's a perfectly reasonable question - for me as an engineer most things are possible, just not very affordable. ;-) The first big issue is aerodynamics. 200 mph is the sort of speed most aeroplanes are off the ground by, and given that cars are flat underneath and rounded on top, that makes them look kinda like wings. That in turn means if we aren't careful we will be in a really bad aeroplane and not a really good car. Just reducing lift won't be enough, we'll need real downforce to be sure of stability at those speeds. We also need a lack of sensitivity to pitch for those forces - it's no good having it work perfectly in a wind tunnel but then taking off when it crosses a bump. And there are a lot of bumps at 200 mph - the bumps get bigger the faster you go, so it's usually a bad idea to lower the car too much. This would probably mean a fairly purposeful splitter and a fairly ugly rear wing. Some side strakes up the screen pillar might be an idea too to get the bonnet to work like a splitter for the whole car. These aren't usually practical for road cars unless they're made of rubber, since otherwise they become "pedestrian slicers". Imagine the rubber bonnet fences that used to appear on Opel Manta 400s and the like, but stuck on the A-Pillars. The next big issue is what we call yaw damping - vehicle stability, if you like. This falls off with speed and is the reason most sensible people don't swerve much at 100 mph - the car swings back and forth in a fairly sickening manner that it just doesn't do at low speeds. You can get back yaw damping with a combination of static toe and bump steer - make the front wheels go out and the rear wheels go in. It varies from car to car but usually raising or lowering the rack by surprisingly small amounts will achieve quite a lot. I'm presuming that the tyres are rated for 200 mph, but that's just a purchasing thing rather than a technical problem.

Damian Harty

A couple of other thoughts on the 200 mph Q too: Above about 160 mph, most cars have a problem where the roll behaviour of the car starts to interact with the directional behaviour. Cars need to be really quite stiff in roll and it might well prove impractically stiff for such a short wheelbase car as the Impreza, so some sort of "roll damper" might well be desirable. Imagine a motorcycle style rear shock with the spring removed, worked by a little linkage. It would probably be best to ditch the standard Anti-roll bars and arrange the linkage to use a spring in parallel with the damper. It's one of several good reasons why F1 cars and the like run "three spring" suspensions. The other thing I forgot is that the brakes might need a little work. ;-)

carl

Thanks Damian! I'll point David at this thread (he's building a 200mph Scoob )