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Author Topic: Rear wheel steering on a murena!  (Read 15582 times)
Lennart Sorth
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« Reply #15 on: June 05, 2007, 07:16:31 pm »

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!

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

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Lennart.Sorth@matrasport.dk
Murena 1983 1.9i silver // Alfa Romeo Giulietta 2.0d 2012 white // Smart 4two cdi 2010 blue //
gizmo
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« Reply #16 on: June 05, 2007, 09:57:54 pm »


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.

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Anders Dinsen
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« Reply #17 on: June 05, 2007, 10:33:49 pm »


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.
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Anders Dinsen
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« Reply #18 on: June 05, 2007, 10:40:23 pm »

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!

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.

Yes they were very bad indeed, the tyres.
I was going carefully, though. It was only the second day I had the car, remember! Smiley
Quote
Does the larger capacity of the 2.2 make for significantly more engine braking than the 1.6 or even my 1.9 ?

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
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gizmo
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« Reply #19 on: June 06, 2007, 10:59:37 pm »


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 Smiley

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?

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Bart_Maztra
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« Reply #20 on: June 14, 2007, 09:40:07 pm »

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?)
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Anders Dinsen
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« Reply #21 on: June 15, 2007, 12:46:45 pm »

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?

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 Wink

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
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Lennart Sorth
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« Reply #22 on: June 23, 2007, 02:58:01 am »

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

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Lennart.Sorth@matrasport.dk
Murena 1983 1.9i silver // Alfa Romeo Giulietta 2.0d 2012 white // Smart 4two cdi 2010 blue //
krede
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« Reply #23 on: June 23, 2007, 07:42:14 am »

Quote
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?

Try out harder dampers as a first measure.. made a huge difference to me
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Anders Dinsen
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« Reply #24 on: June 23, 2007, 08:36:43 am »

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

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

- Anders
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« Reply #25 on: June 23, 2007, 08:48:07 am »

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Sure, there's a difference there, but is his rotary really that much more powerful than a 2.2 for example?
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 .
« Last Edit: June 23, 2007, 09:15:42 am by krede » Logged
Bart_Maztra
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« Reply #26 on: June 23, 2007, 07:34:15 pm »

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?Huh??
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krede
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« Reply #27 on: June 23, 2007, 09:05:28 pm »

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!!
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Anders Dinsen
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« Reply #28 on: June 24, 2007, 06:52:49 am »

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 Wink, 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 Cool
« Last Edit: June 24, 2007, 06:55:58 am by Anders Dinsen » Logged

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Bart_Maztra
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« Reply #29 on: June 24, 2007, 12:16:36 pm »

  • 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.

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.

  • 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.

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