MatraSport Forum

The murena already has rear wheel steering, if you don't belive me disconnect your rear spring and move the suspension through its full travel and watch the wheels turn in and out. This happens on almost all independant rear suspensions but the designers have kept quiet about it because it was considered an adverse side effect. Now however the clever engineers have learned to use it to advantage to increase dynamic stability.

You are right, Gizmo.

But I think the dynamic stability increase on the Murena rear suspension is much more due to the camber control whereby the tyre contact can be maximised. The steering effect (toe-in) will be different on either wheel - during cornering, the outside wheel (admittedly carrying the most load) will be steering inwards thereby adding to an under steer effect, but the inside wheel will do the opposite.

So I don't think the effect is particularly useful.

That said, however, I think the Murena rear suspension is brilliant in all its simplicity. The Matra engineers really knew their tools there!

- Anders

 Thanks for your reply Anders but I'm afraid I must disagree a little but yes the Matra engineers did do a good job.

The camber change on the rear of the murena is not ideal and the only reason the car has static negative camber is to prevent excesive positive camber during the suspension movement and to stabalise the rear suspension as it 'steers' in and out.

When the rear suspension is totally uncompessed (no weight on the wheels) the wheels have positive camber and they toe in, as the suspension is compressed the camber changes proportionally to negative but the wheels turn out (toe out) upto the mid point then they turn in again (toe in). This mid point however is not at the normal ride height of the car but when the suspension is raised by around 2cm.
So imagine we are taking a left hand bend at speed, the car leans to the right loading up the right hand rear wheel and compressing the suspension by say 2cm, the wheel will adopt increased negative camber (a little under 1°) and the wheel will toe in (ie it turns into the bend), the left hand rear wheel will be lightly loaded and the suspension will droop allowing the wheel to adopt a positive camber (1°) but the wheel will also 'toe out' (ie it turns into the bend). Assuming we have not exceeded the grip of the tyres all four wheels of the car are now turned in the same direction, into the bend. This is currently considered as the optimum statagy for a car which has a rear weight bias. WELL DONE MATRA. The downside ( there had to be one) is that lifting off the accelarator during a corner reverses all this and makes the rear wheels steer out of the bend causing at best a tail slide or at worst a spin. The morale of all this is that when you 'overcook it' you have more chance of getting round the corner by keeping the power on rather than 'lifting off'.

I would be interested in hearing from anyone that has lowered the suspension on a murena as this is a whole new ball game.

I don't think Anders needs reminding of this effect...

Oh dear!  I didn't mean to offend anyone.

I don't think you have offended anybody - but you put your finger on a (for some owners) very well known effect, Lets put it that way :-)

I follow your line of thoughts, but refuse to believe Matra's engineers got it that wrong. The rear suspension is very ingenious, and has deliberately been setup to do exactly that - I think they were possibly after something else than steering, - maybe grip ? 

An interesting note is than many 1.6's are for some reason standing about those 2cm taller at the rear, - maybe that plays a role in the reports from owners of both models, that the 1.6 is more driveable at the limit ?

Just guessing.

- the normal argument has been something with weight balance.

/Lennart

Maybe for simplicity? 
I find that Matra has constructed the murena in a rater Un-French way, and many places has gone for some remarkable straight forward technical solutions.
The issue of how to make the electric window assembly fit, to name one.   

I will NEVER forget that day when on a very wet track I lost my rear end because I was a bit too fast and lifted my foot from the throttle! It was NOT funny. Okay, maybe a little :D

And no, I am not offended at all.

Roy was with me as instructor (so I can always blame him), and he actually spun the car just before I did. In addition to the water on the track, I'm afraid we had a somewhat serious oil leak from the rocker cover gasket, so we might have deposited a reasonable amount of oil on the track, and as you all know - oil is very slippery! I think it just had to happen.

Besides, it taught me a lesson, and I think I'm driving my Murena much more safely having actually tried that, than if I hadn't.

Back to the discussion:

I've got Warren J. Rowley "An Introduction to Race Car Engineering" here (despite the title it's very complete) ;) Camber as a way to control "lateral acceleration is pretty well established" and while the explanation is "still up to a considerable amount of debate, even amongst the tire engineers", Rowley goes on to explain how the tire's thread handles centrifugal forces as a function of camber.

This was known in the 70's so camber was a very desired effect, but I must agree with you that the toe-change can also assist balancing in a corner. Thank you for pointing it out. I don't agree, however, that that is the reason why I lost my rear end - don't forget that friction between the road depends linearly on load, so (in the above situation), the moment I lifted the throttle, friction decreased. I would have lost her even without the camber/toe-effects.

Interestingly, only the rear wheels have negative camber on the Murena, whereas the front wheels have zero camber (unloaded). I've been told that this is quite common on cars that are weight balanced towards the rear end, but I think it is designed that way to give added directional stability at the cost of some understeering tendency. Having much shorter A-arms on the top, the front wheels by the way has the same negative camber increase with load.

Michal has lowered suspension, I think. He loves it :)

I love my Yokohama tyres and plan to stay standard for exactly the reason you point out - it will be a different ball game changing ride height. That said, however - it may work?

- Anders

I don't think Matra got it wrong, I just think they only got it half right. After all the test drivers on the original murena program would not have been exploring what happens when the throttle is lifted but how it reacts on the limit (I don't really think test drivers know how to 'lift off').


I don't cite this as the only reason for loss of rear end grip but it is a contributing factor. I assume by load and friction we are talking about the weight shift effect and not the tranmission of power. Also remember in the 70's high profile radial tyre were the norm and they needed the camber because of the side wall flexability. Now that we see lower profile tyres with stiff side walls camber change is less desireable. Of course you could say keep the high profile tyres but lower profile tyres have much greater control of slip angles, less tread distortion, less heat build up and lower energy absorbtion. That's a lot of plus points to ignore I think it will be better if we can find a way to use the new technology.


If the suspension is lowered I think the least you need to do is fit stiffer shock absorbers or the handling could be worse. Better still to experiment with raising the roll center height back to its original position then you really would have an improvement.

Hi Gizmo

This is an interesting discussion :)


I'm not sure I follow you here... you think the dynamic camber change of the rear suspension is "half right"?

Test drivers will certainly not get scared as easily as I did, but I don't see why the drivers testing the Murena wouldn't have experiemented with controlling the car on the throttle. Isn't that their job: To emulate the stupid things ordinary drivers like us might be doing with the car?


Ok :)


Exactly. Vertical load. 'Friction' is tire friction between the thread and the road. Frictional forces are proportional to the vertical load on the tyre thread. Power transmitted through the tyres add to the sum of forces excerted by the car to the wheels (the others being braking forces and centrifugal forces, and to a lesser extent gravity forces (on a banked road). You know this, of course, I'm just defining.


The understanding among race car engineers is that camber is not depenent on side wall height. What matters is the construction of the side wall. There is a lot of difference in the way race tyres react to camber changes and the way normal road tyres do. This is, a result of different side wall construction more than flexibility.


You are touching an interesting point there, but we are kind of stuck with the Murena as it is as there are zero adjustment possibilities on the rear end. Some time ago there was a swedish guy on the forum who wanted to build a race car from a Murena, and one of the improvements he considered was an adjustable semi-trailing arm suspension. Unfortunately he had to abandon his project, but my point is that adjustability was by him considered a very important property.

For a road legal car, however, we are stuck with the original design and geometries. I agree with you that it would be fun if we had a fully adjustable suspension, but I'm afraid the Murena will just never have that - except adjustable dampers (and spring seats) like the Gaz dampers Krede has or the Spax ones Michal has.

I'm absolutly certain that you will get better handling on your Murena by going towards wider, lower profile tyres. Even without changing the suspension setup. The problem is that you will also be ruining drive comfort, which from my point of view is only to a lesser extent acceptible.

Years ago, Roy Gillard of the UK club realised that by going to a 55 profle on the front, he could improve handling significantly. I have accepted his experience and use it myself, even on my winter tyres. I find that my car handles exceptionally well, though different tyres work very differently. On the Murena you can really feel what is happening. I have chosen Yokohamas for their soft compound, which I feel that the Murena really likes, but unfortunately I had to select different patterns front and rear to do so. Had I run with the standard profile on the front, I think I would have been able to pick the same.


"Experiment" is exactly where it gets difficult unless you have lots of time and access to a race track. Do you? :D

- Anders

Think you are onto something there Gizmo. :)
I have messed around a bit with the ride hight, and i have come to the conclusion that the murena is low enough as it is.
I started out lowering the rear a bit, while at the same time installing adjustable shocks.
At first I set the shocks to "half" stiffness , but couldnt tell much difference from the original setup.(then I set them to "kill" and that made a difference!!)
However.. I find that the camber of the rear quickly increases, witch might have some unexpected effects, and witch makes the car look rather .. well strange..
Also in my opinion the most important aspect of lowering the car, is to get the balance between the front and the rear just right.. the goal here being, to always have an adequate amount of weight on the front axle to get good grip for precise steering and good braking.       
Another important thing to remedy is the rear ends tendency to "sink" during acceleration, or hard cornering.Also the stiffer shocks I installed at the rear, made a huge improvement in my cars stabillity at high speeds (120km/h is high speed to me ;) )
 
I went to lower the front end as well.
But.. I find that you can only lower the front so much,before the wheels disappear into the wheel arches.. again looking silly..
So.. when i get around to it I will set up the ride hight exactly as matra prescribes, and then check the toe in/camber, and raise/lower the rear to get the latter to the original spec. If I'm satisfied with the result, Ill keep it there and never mess with it ever again.. :)   

Interesting. What exactly do you mean by stability? I have a bit of shaking from the steering at 110 and up and am going back to have my Yoko's rebalanced tomorrow, but otherwise the car is directionally and in other ways stable even at 140 km/h. I think the problem is balancing, but it may also be a vibration in the chassis which can be damped.

- Anders 8)

This is a very interesting discussion. To qualify the statement I made in a previous thread which sparked this lively discucssion off, the 4 wheel steer to which I refer is the electro-mechanical type with a separate rack for the rear wheels, like the Honda Prelude. I am all for passive rear steering like the Peugeot 306/Citroen ZX and Volvo 480 have, amongst others.

I used to always try and lower cars, but I realised one day, that one of the main reasons I like cars is handling. For me to then go and stick some cheap aftermarket shocks and short springs on was pointless.

Yes cars fell more planted etc, but the true test of a cars handling, for me, is the adjustability on the limit. The very best car in this regard is the Peugeot 306. Truly inspired handling and grip.

I used to get the feeling that the nose lifted when driving fast, or accelerating hard. Felt a bit like driving with a heavy trailer on the hook .., only of cause , my murena has none :)

Okay, well that's not something I have experienced. What tyre profile are you running on the front? 60's or 55's?

We (my son an I) were right behind you, and had no problems with oil.
With brand new tyres, we had no drama whatsoever, but were still cathing you up by half a lap over 3 laps. I'm sure your terrible tyres were to blame - and of course being on the absolute limit of their limited adhersion, lifting off - even in a straight line - is almost like pulling the handbrake a bit.

Does the larger capacity of the 2.2 make for significantly more engine braking than the 1.6 or even my 1.9 ?


/Lennart

Good evening chaps,

Is it possible to split this topic as I think that wheels/tyres and the various options deserve a thread of their own, I certainly have a few observations to make.

I suggest you post a new topic in the Murena section. We haven't discussed tyres much here.
Unless anyone objects I'll move this topic to the murena section too.

Yes they were very bad indeed, the tyres.
I was going carefully, though. It was only the second day I had the car, remember! :)

Difficult to say - engine management also has an influence here. E.g. the EFI system on your 1.9 will cut fuel when you coast and rpm is above a given limit (typically 2000 rpm) - that will probably give a good deal of engine braking. My carb will not do anything like that, it will revert to idle supply of fuel - i.e. still produce a bit of torque. That might make up for the difference in weight and size.

- Anders

Yes, I have noticed when watching murena's they do seem to exhibit a high degree of squat under power. Does anyone have a suggestion to lessen this effect?

I found my rear suspension very soft. 
When i make a long turn (example highway entry/exits) the following happens: When starting the turn the body start to roll. This roll is (in my opinion) excessive. This roll is effecting the course, the car want to make the turn tighter. Because of this tighter turn i have to correct the course by steering a little back. And because of the little steering back the bodyroll is decreasing. This decreasing of bodyroll is effecting the course, the car want to make the turn wider. Because of this wider turn i have to correct the course by steering a bit more. And because of the bit more steering the bodyroll is again increasing which again is effecting the course.

It feels like driving a citroen 2cv without dampers.
So start turn, bodyroll increase, turn gets tighter, steer back, roll decrease, turn gets wider, steer more, bodyroll increase, turn gets tighter, steer back, roll decrease, turn gets wider, steer more, bodyroll increase, turn gets tighter, steer back, roll decrease, turn gets wider, steer more, bodyroll increase, turn gets tighter, steer back, roll decrease, turn gets wider, steer more, bodyroll increase, turn gets tighter, steer back, roll decrease, turn gets wider, steer more.

I want to replace all the suspension rubber at the front, because that is the only explanation i can think off.

The front dampers are renewed (monroe), the back dampers are renewed (koni, set 75% hard) the original 1.6 springs are replaced by new 2.2 springs. the 1.6 anti-roll bar is replaced by a 2.2 anti-roll bar. But still i found the back suspension very soft.

Is this wobbly behaviour due to this 4 wheel steering? (more roll=tighter turn?)

Stiffer and shorter springs, and lowering the centre of gravity by lowering the suspension.

It's an effect of the trailing arm suspension. With a double a-arm suspension, the effect would have been smaller because the linkage to the chassis would be moved further back. Further, the effect could also have been reduced by tiliting the link points of the lower or both a-arms. But this is not relevant for us ;)

During braking, you may notice that the rear actually has a tendency to go lower. This is because of the rotational energy transmitted through the brake disc to the trailing arm. I would think this is a desirable effect, but not sure.

Bart, it sounds like what you are experiencing is oversteer, but it's difficult to say why.

Tyre pressure must be right, of course.

The rear anti roll bar won't help you: It's main purpose is actually to transfer load from rear to front wheels: When you fit a stiffer anti roll bar, you transfer load to the front of the car during cornering (remember, the chassis has much higher torsional rigidity and the CoG is roughly in the middle between the wheels) - thereby increasing the grip on the front.

If the weight of your engine is comparable to the 1.6 engine, I'd go back to the 1.6 roll bar and springs and start from there. After all, Matra did a lot of testing on the car to get it right.

- Anders

Except hes car is powered by an "infinite improbability drive"  - or at least something significantly more poweful than the original suspension is setup for.

Bart, wouldn't it be worth a try to fit a stiffer antirollbar, rather than (just) making the suspension stiffer ?

Maybe even one where you could adjust where it is connected to the trailingarm, so the effect can be adjusted ? - as the one on the MS-11 :

Try out harder dampers as a first measure.. made a huge difference to me

Sure, there's a difference there, but is his rotary really that much more powerful than a 2.2 for example?

- Anders

I wouldn't say so..
And remember that people run the 2.2 turbo on the same suspension.... and did they modify the suspension at all for the 16 valve prototype ?
If anything the rotary should be lighter, and easier for the suspension to cope with in corners .

The anti-roll bar is replaced by a 2.2 one, which is 1mm thicker than a 1.6 one. Didn't help a lot. 
Front dampers are replaced, by monroe ones. The original ones where completely worn. back dampers are new koni black ones set to 75% max. This adjustment is only on the out-move of the damper. The in-move is not adjustable.

The rotary engine plus gearbox weighs about 170 kg without carb, intake manifold and exhaust manifold.  Centre of gravity is (guess) 15-20 cm lower. What is the weight of a 1.6 engine?
(And the peugeot 1.9 is 165 kg with intake, without exhaust manifold)

When my car leans over, it makes the curve tighter. --> 4 wheel steering??????

My guess is that a 2.2 is about 140kg all included, so I guess the 1.6 is a little less.
170kg is A LOT for an engine!!

Krede, Bart is talking about weight of the whole drivetrain including gearbox

Bart, if you experience oversteer (i.e. tighter turn in a curve), then it's certainly not the "four wheel steering" kicking in as it will have a tendency to push the rear of the car in the same direction as the front wheels. That would lead to understeer, i.e. the car turns less tightly than the front wheels would dictate.

You have probably done a most of the following already ;), but here's what I can think of:

• Get tracking checked and corrected if necessary
• Tyre pressures must be right all way around. If the engine is as heavy as you say, you should probably go for the 2.2 numbers - 1.6 bar front, 2.5 bar rear.
• Since you have replaced the rear springs and anti-roll bar, you should also uprate the front end to 2.2 spec, especially the anti-roll bar. The anti-roll bar is a lod transfer device and it transfers loads between tyres during cornering between the left and right tyres, as well as the front and rears.
• Using an infra red thermometer, measure tyre temperature across the thread on all four wheels right after a good drive on winding roads or on a track. This will tell you a lot about the setup of the car and tyre pressure. Essentially, if you have uneven tyre temperature across the thread, then there is some kind of problem. How the temperatures a distributed tells us something about the problem.

If all of this doesn't help, remember that the Murena is so light and powerful (especially yours) that you can pull it off the road in any way you want. It depends on what you do!

It will understeer under acceleration out of a corner, oversteer if you release throttle early in a corner, jump sidewards on a banked corner during acceleration, and can do anything if tyres lock up during braking. It is a tricky little car, and while it's generally very comfortable and easy to drive, I guess we just can't help ourselves driving it hard. And that's where it becomes 'interesting' from a handling point of view.

There's an interesting article on skid pan testing in the July issue of Racecar Engineering. I haven't read it yet though.

- Anders 8)

1.6 / 2.5 bar? The manual says 1.6-1.8 front, 1.9-2.1 rear. I do 1.8 and 2.1.


What is "2.2 spec"? So a thicker anti-roll bar. What else?

The front end has to be quite soft to cope well with the lightness of that end, especially on the 2.2. Since your seem to have about the same weight distribution as the 2.2, you should go by those numbers. So I don't think you should follow the 1.6-numbers.


Torsion bars, i.e. the springs. I don't remember if the dampers are different too, but if they are, then uprate them too.

But start with the tyre pressure, I have received a few good lectures from Roy Gillard about this :) In one case, when his car was new, he almost lost the rear end in a roundabout (if I remember him correctly) because tyre pressure was set incorrectly by a garage. Getting it right (1.6/2.5 bars) cured the oversteer tendency for him.

- Anders

I don't know what is wrong with barts setup

ik have koni at 75% on my murena, with shortened 2.2 springs. i have a 1.6
in front i also had monroe. and original torsion bars. i have seen barts murena. and when i push the back down, it is as stiff as mine.

I had an excellent road holding with my murena, accept the front was leaning to much.

now i have koni's in the front, the roadholding is even better.

can't carjoy help you bart?

To the best of my knowledge.. thats the only difference .

See my comments on tyre pressures under the 'wheels and tyres' thread.