Suspension stuff - Murena 1.6

[Roly]

I would like to stiffen up the front end on my car.  It has new dampers and doesn't handle badly but the body does tend to roll about a little.

Apparently the 2.2 anti-roll bar can be fitted but does it make a big difference?

Any other ways to give a firmer ride?

I'm also considering changing the rear struts for the same reason.  Anybody used 'sports' dampers or springs?

What options do I have for lowering the car?  Not sure how that would work at the front.

I appreciate this might duplicate an earlier thread but it was a while back now...

Thanks,
Roly

[Lennart Sorth]

I would like to stiffen up the front end on my car.  

I think you should be very caucious, - Matra worked hard at getting it right, and ended up with a relatively soft front end, to counter the effect of having 60% of the weight at the back.

Firmer ride will often be accompanied by less grip, and if there is anything you DON'T want with the Murena, then it's less grip in the front.

What options do I have for lowering the car?  Not sure how that would work at the front.

well , the front is easy, as it uses torsion bars, which are adjustable for height.

The rear would require new springs, or (my preference) height-adjustable shocks. Koni ans Spax makes a set, Polytechniqe and others list them in their catalogs.
 
But,- IMO you should only lower the car to get the wheels centered in the wheel-arc, - my 1.6 is (as most late 1.6's) almost an inch too high at the rear, which is why I keep playing with the idea of height-adjustable shocks

[Anders Dinsen]

What options do I have for lowering the car?  Not sure how that would work at the front.

well , the front is easy, as it uses torsion bars, which are adjustable for height.

I don't think it will be a good idea to lower the torsion bars since that will make it softer at the front. It will have the opposite effect as intended.

Torsion bars in the front of the 2.2 are different from the 1.6, but I don't know how.

There are various suspension kits available but as always with such things the ride is not necessarily improved.

[Lennart Sorth]

I don't think it will be a good idea to lower the torsion bars since that will make it softer at the front.

Not sure I understand you right, - but as far as I know, you won't change the springconstant by any measurable amount by adjusting the ride height by way of the torsionbar ??

there is the same weight suspended on the torsionbars, and the diameter stays the same, - thus the springconstant for the suspension must remain the same.

Aren't you confusing it with the antiroll-bar function ?

[Anders Dinsen]

I don't think it will be a good idea to lower the torsion bars since that will make it softer at the front.

Not sure I understand you right, - but as far as I know, you won't change the springconstant by any measurable amount by adjusting the ride height by way of the torsionbar ??

there is the same weight suspended on the torsionbars, and the diameter stays the same, - thus the springconstant for the suspension must remain the same.

Aren't you confusing it with the antiroll-bar function ?

Force produced by the spring is given by F = x * k where k is the spring constant and x is the compression of the spring. So changing x does actually change the 'softness' of the spring measured as the force needed to compress it by a given amount.

If you lower the ride height, the wheel will have less free play, which will mean that it takes less force to fully load the wheel than it did with the higher ride height.

So if you lower the car, you need stiffer suspension to keep the force to fully compress the wheels constant.

- Anders

[Lennart Sorth]

Force produced by the spring is given by F = x * k where k is the spring constant and x is the compression of the spring. So changing x does actually change the 'softness' of the spring measured as the force needed to compress it by a given amount.

no. - or yes, your spring formula (Hookes law) is correct, but x is in fact the same in this scenario.  - as you still have the same weight to counter with the torsionbar.

You turn the torsionbar a tad, and the rideheight changes accordingly. The car's weight still twist the torsionbar excactly the same amount.

If you want to change x, then you should lighten the front end of the car, - take out the sparewheel or fit helium baloons to the front :-)

Changing ride height by adjusting the torsionbar does *not* change the stiffness of the suspension.

However, any change in ride height (lowering), will quite obviously leave less free play, and normally you'd want stiffer suspension to counter that.

But I at least would NOT recommend stiffer front on a Murena. I have driven several, - some with agressive shockabsorbers, others  with coil-over-shock conversions, and to be honest, none of them felt better on the road, - most of them were noticably worse than a standard car. - and ALL of them had severely worse ride comfort.


coil-over-shock in the front is IMO a really bad idea, as the chassis was never built to take the load out there, - but rather used the ingeniuity of the torsionbar, to shift the anchorpoint to a central place in the chassis.


I believe the grip could be improved by fitting "better" shockabsorbers  - but I have no idea what parameters that would accomplish this on a Murena.
I know of several aftermarket absorbers, - often gas types, - but there doesn't seem to be any concensus about which parameters to change, or in which direction.

So I think replacing shocks with new ones with original or very similar specs. would be my suggestion, rather than making arbitrary changes to the basics of the suspension.

[Anders Dinsen]

Force produced by the spring is given by F = x * k where k is the spring constant and x is the compression of the spring. So changing x does actually change the 'softness' of the spring measured as the force needed to compress it by a given amount.

no. - or yes, your spring formula (Hookes law) is correct, but x is in fact the same in this scenario.  - as you still have the same weight to counter with the torsionbar.

You turn the torsionbar a tad, and the rideheight changes accordingly. The car's weight still twist the torsionbar excactly the same amount.

If you want to change x, then you should lighten the front end of the car, - take out the sparewheel or fit helium baloons to the front :-)

Changing ride height by adjusting the torsionbar does *not* change the stiffness of the suspension.

True, but less free play means it needs less force to compress it fully. The effect is that it will feel softer and will probably start hitting something when driving.

[Lennart Sorth]

The effect is that it will feel softer and will probably start hitting something when driving.

no.

The stiffness of a torsionbar purely relies on:
1: the material
2: the diameter
3: the length

Within geometric reason, any change in rideheight by rotating the anchorpoint of the torsionbar (i.e. adjust the rideheight) will NOT change the stiffness or apparent stiffness.

Wether a lower car will bottom out in potholes is a completely different matter.

[Anders Dinsen]

You are scientifically correct, Lennart!

However, if you turn the torsion bar so that the ride height of the vehicle is reduced from e.g. 150 mm to 100 mm, the force needed to compress the car to 0 (hitting the ground) would be 33% less than with the original ride height.

We have different views on what "softness" is.

Regarding Roly's original question: I checked the parts manual and there's no problem fitting the 2.2 anti-roll bar and torsion bars to the 1.6, but the bushings for the 2.2 bar are larger than those for the 1.6 since the bar is thicker, so they have to be replaced too.