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MG MGF Technical - Suspension Geometry Design
| I have finally got motivated to check the suspension geometry on my 1997 MGF which has given me a great deal of grief on more fronts than just handling problems including blown cylinder head gaskets which I now understand that it tends to be normal occurrance. I checked the wheels bump steer front and rear with a dial gauge and although I cannot claim to be 100% correct it is reasonable indication of what happens when the wheels go up and down. The readings have been repeatable which give me some confidence in the findings. Data: The vehicle has Koni adjustable shock absorbers front and rear which tend to lift the vehicle up somewhat. The trim height was 365mm front and 360mm rear. I lowered the vehicle and raised from its normal height in increments of 10mm REAR SUSPENSION: I checked the rear suspension with and without compliance washer. The results are: WITHOUT COMPLIANCE WASHERS Rebound Direction 10mm 2'toe out 20mm 3.5'toe out 30mm 5' toe out 40mm 7' toe out 50mm 9' toe out 60mm 11'toe out 70mm 13'toe out Bump Direction 10mm 2' toe in 40mm 7' toe in WITH COMPLIANCE WASHERS Rebound direction 10mm 2'toe out 20mm 2'toe out 40mm 2'toe in 60mm 4'toe in Bump Direction 30mm 2' toe in 40mm 2' toe in The rear suspension the bump steer is within acceptable limits particularly with the compliance washers fitted. I have attempted to test the rear wheel alignment with foward and backward movement of the vehicle with the handbrake on to simulate acceleration and braking loads. Without the compliance washers I have seen large toein toeout variation but could not take accurate readings but estimated movement in the order of 10' just by pushing the vehicle by hand which is only a fraction of the acceleration and braking loads. Note on compliance washers. I have used standard washer that you can find in a plumming shop. Go to the toilet section and ask for a toilet water cistern washers they are made of oil resistent rubber 55mm outside diameter 16mm Inside diameter and 5mm thick.The cost about $2 Australian currently worth two bits of stuff all in US Dollars. The only modification is that you need to chamfer the inside hole to allow it to slide on the existing bush when you assemble it. FRONT SUSPENSION Rebound Direction 10mm 7' toe in 20mm 13'toe in 30mm 19'toe in 40mm 25'toe in 50mm 29' toe in Bump Direction 10mm 7' toe out 20mm 13'toe out 30mm 17'toe out It appears that the wheel alignment is very sensitive to trim height hence as the hydragas settle with age or there is a change in ambient temperature the wheel alignment changes significantly. Looking at these figure you only have to sit in the car to put the wheel alignment out. No wonder handling and tyres problems has been such a topical subject. P.S. I also checked out an MX5 for comparison and from one extreme to the other of the wheel travel measured about 13' variation, whilst the MGF was 46'. The bloody thing has also camber and caster adjustments on all wheel. The MGF has none. This may explains: 1.Why the wheel alignment keeps on changing and the reason why I scrubbed a set front tyres within 15,000km. 2.The service department told me to do the wheel alignment every six months max 3. The car handling as an element of unpredicatability particularly with the Hydragas suspension setup which is linked front to rear. This forces large wheel movements to avoid pitch which in turn will give large toe in/out variation as the wheels go up and down. This coupled with the rears suspension without compliance washer which also alow toe in/out variation on acceleration and baking can make the vehicle unpredicable and on occasion scary to drive (I am speaking from experience) The use of polyurethane bushes does very little if anything to solve this problem as it is caused by the position of the steering rack in relation to the steering arm on the wheel hub. It is in the wrong place! In summary to fix this problem you either move the steering rack up by some 50mm guess! (but I will work out the details )which very is difficult if not impossible or bend the steering arms at the wheel hub end to achieve the same effect. I am sure that somebody out there has figured this out years ago! and I am interested to know if anybody out there has fixed the front end bump steer problems and if so how? John Without compliance washers |
| John Englaro |
| >>The use of polyurethane bushes does very little if anything to solve this problem as it is caused by the position of the steering rack in relation to the steering arm on the wheel hub. It is in the wrong place! << I totally agree. The front track control arms (the steering arms) are either too short or more likely, the inner pivot point is too high. Great work John- this is the kind of thing that I had in mind too, but free time being as sparse as it is at the moment I am very happy you've don't it! For my information really, but what were your baseline tracking values? I am presuming that the figures presented above are delta change values from the 365mm front trim height? As for a cure- short of engineering changes, then reducing the amount of jounce/bounce (wheel travel) movement will largely eliminate the enormous bounce steer you've recorded. The easiest way to achieve this is by lowering the car. Dropping the trim height to 320-330 mm will have the desired effect, with the additional benefit that the initial rates of roll are perceptably less. Unfortunately, the MGF has a low build height, which means that this is not necessarily the ideal solution for everyone... Rob |
| Rob Bell |
| OK petrolheads ... Does this mean its not really worth going for front poly bushes. I have the rears already. Should I try better shocks (on a similiar setting) to the standard ones. My main issue is there appears to be far too much roll at the moment. I don't want to lower the car 'cos of the irregular road surface in my area. Trim height is around 360. I have no tyre wear and never have had in the past. I do however still suffer with veering left under braking and I do have a little shuddering if braking hard while doing motorway speeds (Last two probably unrelated to this discussion). Garry |
| Gaz |
| Yes, but only if the suspension is lowered will you feel the full benefit, I suspect. As for the cost benefit ratio, well that's a difficult call. Rob |
| Rob Bell |
| John, this is great, thanks. The sort of bump steer you measured at the front has a reason: If you back off in a corner the front outer wheel will load up and go into toe out while the inner wheel will go to toe in which together results in stabilising understeer thus preventing lift off oversteer. The only thing I'm wondering about is why compliance washers at the back can change the kinematic behaviour as they normally can only change the elastokinematics. Strange. Nikolaj |
| Nikolaj |
| >>The only thing I'm wondering about is why compliance washers at the back can change the kinematic behaviour as they normally can only change the elastokinematics. Strange.<< That's a fair point Nikolaj; were the measurements performed with the hand brake off John (I'm sure they were). Now that Nikolaj has mentioned this, I'm a little confused too... Fore aft movement of the rear 'geometry' without compliance washers is significant- and hence the dynamic kinematic changes, but since even with hand movement of the car can alter toe angles, as you mentioned, I wonder if this is a confounding factor here? Rob |
| Rob Bell |
| Rob, 1. The front trim height of 365 was used as the datum point for all measurements. 2. The change in the rear toe in/out measurements must be associated with the fact that without the washers fitted the rear lower suspension arm can move relative to the tie rod.This can be 5 to 6mm forward and backward movement total. This is is easy to see just look at the gap between the 55mm steel washer and the lower suspension arm and lift and lower the vehicle. This probably explains the change in readings. 3. As a result of the test I did lower the vehicle to 355mm front trim height to improve the situation. The improvement was slight indicating that the rack must move a lot more that the 10mm that I achieved by dropping the vehicle height. This leads me to believe that we are looking for major surgery to achieve reasonable bump steer readings. Gary, I suggest that you forget changing the bushes on the basis that: 1. It will not give you the benefits that you are looking for. 2. It is a difficult task since you have to have to drop the front subframe to get to the lower suspension arm retaining bolts. ( PS This is unbelievable!) I have fitted Koni double acting shock absorbers front and rear. Use the Koni recommended shocks for the MGF and the lowest bump setting. The difference to the vehicle ride and handling is very very noticeable and the money is well spent and you can fit them yourself in a few hours. The cost I suspect is probably not much different to fitting PU bushes if you take the cost of labour for fitting them into account. This will reduce the wheel movement significantly and as a consequence the vehicle roll and pitch and inprove the handling, and reduce the handling unpredicatability which occasionally displayed. Well worth while investment With the toilet washers fitted to sandwich the rear lower suspension arm the roll steer that I was experincing by accelerating/decelerating around large radius corners seems to have been much reduced. Dropping the vehicle to 355 mm front trim heigh has also appeared to make some marginal improvement Nikolaj, Good try mate! I too occasionally try to find a scientific explanation for what usually has none. As I suggested in my previous experience working for BMC and later for BL it is aparent that the vehicle has been assembled from the Rover parts bin. In that case you tend to use what you can so that you reduce tooling up costs. Unfortunately this forces you to compromise hence the MGF problems experienced. Pity because with a bit more design, development and tooling the MGF could have been a good if not an excellent enthusiast car. |
| John Englaro |
| >Nikolaj, >Good try mate! I too occasionally try to find a scientific explanation for what usually has none. Errrr, nope! I do know what I'm talking about here. What I described is common practice and used on virtually every car you 'll find on the road today. It really does stabilise the car when backing off the throttle in a corner. Have a look at some professional literature and you'll have to agree. Nikolaj |
| Nikolaj |
| John, This has been one of the most interesting posts on this BBS in a long time ... I'm gonna look into getting new shocks first and maybe think about the front poly bushes after. Thanks. Gaz |
| Gaz |
| Hi, great measurements. Woun't comment details but only state one of the most important errors I was on for the last two years. Do some of you recall 'Mark the Tech' comments or the 1996 MG-bulletin ? I always thought that both the rear and the front gets toe OUT under bump conditions. Now obviously it isn't. 5' or 6' more out by each 10mm at the front. OK that was common and seams approved now. :)) 25' more toe OUT if the car gets lowered for 40mm without tracking adjustment. My godness. The poor tyres. Nearly no change at each 10mm change at the rear, or only a little more toe in. Hmm..... I need to redesign my tracking webpages. ;-) Rgds Dieter |
| Dieter Koennecke |
| >>I always thought that both the rear and the front gets toe OUT under bump conditions. Now obviously it isn't.<< I've been meaning to get some new web pages designed that explains the effects of the track control arms on toe angle with suspension compression and decompression. This is something that is important to me, as I like to know whether lowering suspension is as good an idea as people say it is... OK OK OK I'll do it soon!!! Rob |
| Rob Bell |
| One question, if you are changing the the rear toe angle and the front is measured relative to the rear, does this not make working out what is happening with an adjustment to the front toe angle even more complicated? If it had been designed properly the height of the car would make little difference to the required tracking. Also an adjustable camber would have been nice, to make up for the lousy quality control. Many Fs have an uneven out of specification camber on the front wheels. On my car the difference between right and left is 1 degree. Add to this the temperature sensitive front/rear connected Hydragas and it gets even more confusing and difficult to even measure properly. The bushes hold everything together much more solidly and prevent some of this play in the suspension. The compliance washers simply remove the excessive play in the suspension at the rear. This mainly prevents the tendency for the F to skip out at the back more sudenly than expected when cornering hard. Both of these modifications also make it more likely that the measurments are going to be accurate. I agree that decent shocks will no doubt also improve things, it is on my shopping list along with lowering knuckles (I want the car at 340mm). I think with the modifications, going any lower will not improve things to a great extent. As I'm stuck with the Hydragas I may as well get as much comfort as possible :-) Decent tyres are a must for anyone trying to get the most out of their F. The toe in setting at the front seems to have cured my tyre wear problem (touch wood) and makes me feel much more in control of the car than ever before. In combination with the bushes and compliance washers the stability in a straight line and round corners is vastly improved (even at 355mm height) with no splitter. Slight tendency to oversteer but that is mainly due to 195 tyres on the front and maybe the toe out setting at the rear. This tendency is perfect for me because I hate the "No Control" feeling of understeer, this was always my main problem with front wheel drive cars. I like the car to go where I point it and not somewhere else. Some may call me a control freak :-). Seperating the front hydragas from the rear is something I'm still not sure about, but I'm sure I'll warm to it once I fully understand the implications. It should make some things easier to measure at least, although initial setup is no doubt a bit fiddley. I assume this would make the hydragas unit act as a simple strut or a kind of large damper. The ideal situation would be for someone to come up with a redesigned suspension to replace the standard. I think this would be a very expensive solution though and I doubt if it would have the market. With a few modifications the existing system can be very good, if not so simple to get your head round. Remember, God did not come up with manufacturers recomended settings, they are not set in stone. All a manufacturer can do is aim at Mr. (or Mrs.) average, who does not really exist. Audi set the TT up for oversteer and a few of the customers got killed, no doubt after trading in their front wheel drive saloon cars. I assume Rover took the safe approach and went for an undresteer setup from the start. It was just a pitty that this had other effects on some of their customers, tyre wear for example. Dealers also seem to have had problems with being able to fully understand the suspension of the MGF. I think too many variables for many mainstream mechanics to comprehend, it gives me a headache, for sure. I had one dealer who's cure for everything was to pump it up to the sky. Anyway, this as an essay going towards my masters degree in MGF :-). |
| Tony Smith |
| >>One question, if you are changing the the rear toe angle and the front is measured relative to the rear, does this not make working out what is happening with an adjustment to the front toe angle even more complicated?<< No, because toe angle is always measured relative to the long axis of the vehicle. The laser dutum points are mounted on the wheels, but this is to estimate the long axis of the vehicle concerned. Is this right Dieter? >>If it had been designed properly the height of the car would make little difference to the required tracking.<< Again no, because most vehicles have some degree of bump steer built in to compensate for the influences of braking upon elastic kinematics for example (ie, toe-in may be built in to compensate for the toe-out induced by the braking movement about the steering axis, for example). Therefore, suspension height is something one play's with at one's peril- but is usually someting you can get away with simply because lowering usually reduces wheel travel, and therefore the propensity for huge toe-angle changes. The question of camber mods is an altogether different issue- and I agree, it would be very nice! But having said that, it is simply another adjustment that certain dealers could get wrong, so maybe it is a blessing in disguise!!! If we can all pass the MGF PhD then we should all go out and educate the unconverted mgf service teams!!! ;o) Rob |
| Rob Bell |
| Hi, on the topic of separating the Hydragas units: I have been thinking about that one for quite a while now and at the moment I think a valve between the units would be ideal. For normal road use you would have the valve open for maximum comfort, and for fast road or track use you could fine tune the flow between the units for minimum dive and squat and maximum control. Nikolaj |
| Nikolaj |
| I wonder how much movement there is in the front tracking under braking. I rather hope (having just picked my car up from having PolyBushes fitted!) that the rubber bushes at the front would give substantially more. I guess the same sort of test could be done locking the front wheel and "pushing" forwards to simulate hard braking and see what sort of affect that has on the toe... (Interstingly the guy who fitted the bushes had some RoverSport lower ball joints from a car he used to race - and they have oval holes to enable camber adjustment! They also had many other "special" bits that could be used to alter the geometry. See the dealer guide for how to get to him as "Meltune"....) Neil NNW49 - now with Pretty orange polybushes, GreenStuffs and new S02's - a very happy bunny, with a "broke" owner! |
| Neil |
| This is all very interesting.... And may even explain some of the ill handling traits that my car as been showing as of late. Lately my car has been feeling like the wind is blowing i around on the road - even if there is no or little wind. This really feels like the back end of the car is moving around very slightly, particually noticable at speed over 100 ~ 105 km/h. Also when turning into a high speed corner the steering seems to go vague for a split second when you go to turn into the corner. This feels like the front of the car has no weight on the front wheels (or the tyres are lifting off the road after comming over a hump in the road.) Just after this 'vagueness' goes away, the car feels like the rear end is going to let go (or it is moving around.. as in first point). I have 'played around' and have found that these feelings don't seem to have any effect on the Grip of the car, they are just unnerving. I found if you push through this feeling, the car just continues to grip. I have had the wheel alignment checked, and this is right, and I have just fitted a new set Bridgstone S02s to the car. The only difference between 'now' and 'before' is that the car height is set about 10~15mm higher than it was before. -Now sitting at about 355mm Prior to this, the card drove fantastically ! Do you think that the above handling traits could be due to the different ride height, and maybe suspension/wheel alignment due to worn or soft bushes and or worn shockies ? The car is now about 3 years old and has clocked up about 62,000km. Alot of it on roads that are far from smooth.. I have been scratching my head now for about 2 months trying to make sence of this.. My dealer has been of no help at all.. It have been a challange just to get some one to take the car for a test drive. ALSO... I have heard a few of you comment that the MG F uses a lot of parts shared with the MG Metro (which I also beleive borrowed parts from the Mini).. Is is possible to fit Urathane Bushes and shockies that were designed for the Metro or the Mini ? Thanks Scott (Sorry for the long message) |
| Scott Martin |
| Nikolaj, fantastic idea- I like the sound of that alot. Presumably it shouldn't be too difficult to locate an electrically operated hydraulic valve that will fit in-line with the displacer interconnecting pipe? I presume that one would only operate such a valve once the suspension has been set at equilibrium? Unfortunately, it'll be against Abingdon Trophy rules! :o/ Scott, given the road conditions, it may indeed be possible that the rubber components may not be quite as effective at containing suspension movement as they used to. Based on John's observations, the wayward rear suspension behaviour could be blamed on soft bushes. The light front end may have an aerodynamic explanation however- as there is quite substaintial front end lift. A solution to consider here is a front splitter whose benefit, paradoxically, is more palpable the higher the suspension trim height... HTH Rob |
| Rob Bell |
| Rob, >seperating the Hydragas Units. keep it simple :) No hydraulic valve required, but only two simple one way ball valve with tappet. Nikolaj had the original pipes of my old MGF wreck in his hands three weeks ago, when we 'pumped around' at his MGF. Back to the subject. I'm sorry but I have got not only language problems in reading and understanding all the above written stuff ;-) My only doubts relate to looking at the static behavior + measures only. John provided static measures. This could be no 'bible' IMO, cause the dynamic behavior under accelerating/decellerating/braking is also a strange behavior of the car. The sequence of all suspension parts with spring/damping behavior is the case I am looking foreward to. May be at any time any student at any University can make some more works ... Eeehy ! Nikolaj, how about the University of Aachen ? :-) They have a common Institute for car suspension terms ;-) Rgds Dieter |
| Dieter Koennecke |
| Anybody in favour of dumping the hydrogas for convential coil springs? Mike. |
| Mike Satur |
| Now, Mike your last point really got my attention... Coil over units like what have been done for the Mini would be very nice.. I suspect that there would have to be some strengthing of the mounting points done first though. - Not a major probem... The big problem would be getting insurance after this has been done.. - Mike, you maybe ablt to answer my questions about the interchangability of Bushes/Shockies etc between MGF, MGMetro, and possibly Mini.. (I live down under where the cost of purchasing parts in the UK, and getting them shipped down is almost prohibitive.. I'm looking for alternative options.) Scott |
| Scott Martin |
| Hey Mike.. How about adjustable suspension components ???.. So we can adjust out the manufacturing 'tolerances' that rover have built into the cars. Scott |
| Scott Martin |
| >Anybody in favour of dumping the hydrogas for convential coil springs? Me too, having seen the silly Moulton parts from inside. http://www.lame-delegation.de/mgf-net.de/cut_unit/index.htm But with the same doubts on MOT or TUV etc. May be we need to wait for at last 30 years until the MGF gets a 'historic car' ;-) and special approvals like at an MGA take place. Ahhhm, ...did I write 30 years ?? Teed, Paatrick !!, help!! :) >How about adjustable suspension components ???.. See the Cup cars setup. Adjustable wishbones with needle bearings instead of rubber. http://www.lame-delegation.de/mgf-net.de/mgf_treffen_2k/dk__3491.jpg http://www.lame-delegation.de/mgf-net.de/mgf_treffen_2k/dk__3492.jpg Rgds Dieter PS. please don't bother the new weblink. I moved the whole site last night. http://www.lame-delegation.de/mgf-net.de/ is equal to http:/www.mgf-net.de pages now. |
| Dieter Koennecke |
| Sorry, It's me again ;-) http://www.lame-delegation.de/mgf-net.de/mgf_treffen_2k/dk__3495.jpg There is another interesting modification at the front suspension of a Cup car. They got a stronger roll bar and adjustable assembly joints at the front wishbones. .. and while looking close at the dk__3492.jpg. See yourself this enlarged detail. The rear subframe front-mount and the tie bar to subframe mount look 'original' IMO. (230Kb JPG) http://www.lame-delegation.de/mgf-net.de/mgf_treffen_2k/dk_3492large.jpg Cheers Dieter PS. and I'll type also 2 slashes to the other link next time ;-) |
| Dieter Koennecke |
| Rob, I was under the impression that this was due to unequal lengths in the control arms (is that the right part name?). So as the car is lowered the toe angle changes to more toe out. I also understand that this is not the case with many car suspension systems. I may be totally wrong here, but I never had these problems with any other car.. This was what I was referring to along with the quite excessive movement in the suspension which is the real problem. And as for the camber adjustment, if the Camber was right in the first place adjustment would not be required :-) I have not trusted dealers to set my suspension for 12 months now, so it is a little unlikely that I would let them mess it up for me. Anyway, Rover hold no responsibility for my car now so I can have it how I like. The suspension and handling, depending on modification level, can range from absolutely lousy to excellent on an MGF. I think the main problem is not design but quality of components, it would just be nice if the tracking was not thrown so far out by trim height. Especially when the height can change a centimetre between summer and winter. Mike, I have to drive an example to say if I would pay out for it, the Hydragas is ok once you get a handle on it. |
| Tony Smith |
| Suspension gurus, You all seem to have a good tecnical grasp of MGF handling traits and may be able to help. Why does my 10000 mile, completely standard and mint condition vvc weave dramatically under high speed braking? This trait, I feel is a very big accident waiting to happen,as under heavy braking from 90+ mph the car has been known to veer across a whole lane of dual carriageway. Nothing appears to be loose and the tyres are fine and the right way round. I am sad to say my gut feeling suspects a major design flaw,(not unheard of in the products of our floundering motor industry), of the bodyshell flex variety. I suggest this because such manouveures are acompanied by loads of shudders and creaks arround the top of the screen/ hood and behind the dash. Has anyone tried a strut brace? Will I have to return to driving cars with roofs? |
| Jake Frith |
| As for the swerve to one side on braking, that used to happen to me, only for the first fraction of a second under braking in the wet, unnerving I can tell you. I think this was put down to brake disks being wet and not gripping evenly at first. This nolonger happens and seems to be fixed now (has been for a couple of years). Some Fs have a problem whereby the weld on the reinforcing strut joint behind the dash (like a scafold pole into a socket) comes lose. The noise you describe sounds like it could be this. I think Dirk also had this problem with his MGF, I have not had any personal experience of this though. Otherwise if it is second hand you never know what might have happend to the car previously. Is the history complete for the car? Has it been in an accident before you got it? It may not be a suspension problem at all. If it is a suspension problem, has a dealer looked for a solution to this? If so what did he say? |
| Tony Smith |
| Dieter, Great photographs! Do you or anybody else has a photograph of the front suspenion of the racing MGF showing the steering arm? The first thing that you usually do on a racing car is to get the bump steer as close as possible to zero. I would be very surprised if they have not resolved the bump steer problems by bending the wheel hub steering arms or fabricated a new ones. PS I understand why you would want to use a heavier antiroll bar at the front. This gets the front end to take a greater share of the cornering forces and induces understeer. The MGF has massive over steer characteristics. I speak from experience having spun the vehicle at track events numerous time atempting to keep up with stock standard MX5. However that was before I fitted the rubber washers on the lower rear suspension arm to stop the wheel moving backward and forward under load, Koni double acting shock absorbers and set the front wheels to toe in. I am looking forward taking the car to the next track event. John John John |
| John Englaro |
| Hi Tony >>This was what I was referring to along with the quite excessive movement in the suspension which is the real problem.<< The bump steer toe angle changes that John recorded are certainly alot more than I was expecting! Not too clever. Stiffening the bushes helps under dynamic conditions certainly- which includes cornering, braking and accelerating. >>I was under the impression that this was due to unequal lengths in the control arms (is that the right part name?). So as the car is lowered the toe angle changes to more toe out. I also understand that this is not the case with many car suspension systems. I may be totally wrong here, but I never had these problems with any other car..<< No, not from the wishbones themselves which should be neutral from a toe-angle perspective. Consider the dual wishbones as a single lever pivoted at the chassis. The track control arm/steering arm is the other lever, that is positioned behind the wishbone 'lever' (ie, further towards the rear of the car). The two levers are connected at the hub end by a link in our 'virtual' model. In our model, let's assume that that link is parallel to the long axis of the car. If both the wishbones and track control arm are pivoted in the same plane (vertically and horizontally, but with the control arm displaced rearwards), then both levers circumscribe the same arc, and thus, that link between the two levers remains parrallel to the long axis of the car. Now move the the pivot point of the track control arm closer to the wheel hub and away from the centre of the car, shortening the 'arm. Now the two levers circumscribe different arcs, the 'arm lever a much tighter circle, so as the wheel moves up and down, the track control arm 'pulls' the back of the link towards the centre of the car- effectively making the wheel toe-out at extremes of movement. The opposite occurs if the track control arm is made longer than the wishbone lever. Now, if you keep the levers the same length and parrallel in all planes, but move the pivot point upward. Again, the two levers will be circumscribing different arcs. At larger wheel drops, the track control arm will effectively be pulling the link towards the centre of the car- causing the wheel to toe-out. When the suspension is compressed, the wishbone lever will be effectively shorter than the control arm- pulling the front of the link towards the centre line of the car, so causing the wheel to toe-in. Drop the control arm below the wishbone pivot point, and the opposite occurs. This is really difficult to explain without the aid of diagrams, but I hope you get the picture? Hopefully, it is now possible to understand John's static measurements of bump steer, and how they impact upon chassis behaviour. It's a fascinating subject- and there's much more to dynamic suspension behaviour than meets the eye! Rob |
| Rob Bell |
| >>Why does my 10000 mile, completely standard and mint condition vvc weave dramatically under high speed braking? This trait, I feel is a very big accident waiting to happen,as under heavy braking from 90+ mph the car has been known to veer across a whole lane of dual carriageway.<< This is certainly not usual MGF braking behaviour, and is not a design fault of the car. Jake, it sounds as though you've made the sensible checks WRT tyre wear etc. I presume that the tyre pressures are okay? Standard Goodyear NCT-3 Touring tyres are very sensitive to pressure, and it is worth making sure that they are spot on. Next check I'd make is to ensure that the ride height and tracking are absolutely fine. The trim height (measured from wheel centre to wheel arch lip) should be no more than 370 mm (at this time of year IMO, because temp does have an effect on this variable). If too high, then get that seen to, as aerodynamic instability as well as high centre of gravity will do no favours to braking stability. Now get the tracking checked: take your car to a tyre specialist centre. Make sure that the settings are correct. Toe-in tracking at the front will improve braking stability, but I would not mess with this variable at this time. With standard tracking, you should be fine braking from high speed. >>I suggest this because such manouveures are acompanied by loads of shudders and creaks arround the top of the screen/ hood and behind the dash. Has anyone tried a strut brace? Will I have to return to driving cars with roofs? << Body flex is not a problem on MGFs. It is, famously, one of the stiffest roadsters you can buy. Therefore, the shudders and creaks are abnormal. Not sure what to suggest. Perhaps have a chat with your MG dealers. Personally, I'd get the braking stability sorted first. HTH Rob |
| Rob Bell |
Could also be worn bottom ball joints. I've just had them both replaced at 39k miles - both were somewhat loose. Mr Meltech who did it told me that the cup cars had 3 separate upper arms, and fully adjustable (as Dieter has shown) lower ones. The lower ball joints are even mounted with bolts though oval holes so that the camber can be adjusted. Neil. |
| Neil |
| >The lower ball joints are even mounted with bolts though oval holes so that the camber can be adjusted. Ahh, thats it. I wondered already how the got it managed at the front suspension. Camber was told to me at the Spa Cup Race with about somewhat at 5 or 7 degrees. Can't recall it exactly. Thanks Dieter |
| Dieter Koennecke |
| Rob, Ok, given that the suspension of the F is designed in such a way that height changes other important geometry. Do you feel this was wise to use this design in a suspension system implementing hydragas units where the height cannot be constant. Your description was excellent, I understood it and it makes sense, you seem to be bocoming quite an expert. I am just questioning the application. This is not a reason for the problem in it's self but makes precise setting impossible. But what do I know I failed engineering twice at college. Decent bushes are a simple thing that Rover could have used at very little cost to the customer. Given how much movement is in the suspension I think they would have been a very sensible thing to use. |
| Tony Smith |
| Height changes affect the geometry on every suspension design in the world - it's one of the classic compromises that suspension designers have to wrestle with. The F1 'solution' is to make the suspension pretty much solid and have all the movement in the tyre... Modern road cars have effects such as rear toe change deliberated designed in to fine tune the handling to the chosen level of safety/feel/fun etc (delete as appropriate). Mike |
| Mike Bees |
| Thanks for the compliment Tony- appreciate it! :o) I don't see a problem with hydragas per se- although as a system it has significant draw backs for the reasons you mention, and the relative lack of control of fore-aft pitching. The real problems occur when the ride height is set too high. I think there must be a trim-height at which there is maximal bump-steer- and I suspect that it is at this height where most people find inner edge tyre wear despite having entirely 'normal' static geometry. Almost any suspension movement would result in significant toe-angle change, which is your point Tony- and that's bad news. I'd suggest a lower ride height, that effectively reduces bump-steer at the front axle, and limits suspension movement. At 340 mm trim height (possible less now it's colder) I have no wear of my soft compound S-02s. I think that raising the suspension height, as many dealers are want to do is the wrong approach fostered by an inappropriate mis-understood doctrine... oops, appear to be ranting now! ;o) Decent bushes? Yes, definately a good idea. Better still is to irradicate the rubber bits entirely- and encouragingly, from a very recent Autocar article, that is exactly what the MG/Rover engineers have been up to! I await the new MGF 160 with baited breath! :o) Rob |
| Rob Bell |
This thread was discussed between 27/11/2000 and 07/12/2000
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