205 Gti Rear Beam

[fangio 1]

I was wondering...does the independant trailing arm on a 205 have a rising rate?

[calvinhorse 870]

The trailing arm on a 205 is controlled by a torsion bar, torsion bars aren't usually progressive so won't have a rising rate between bottoming and topping out

[fangio 1]

Yes. I agree on that.

 

What I am getting at is the relationship between the amount of movement vertically by the wheel and the amount of arc described by the wheel. At different points corresponding to the horizontal plane this relationship doesn't remain a 1:1.

 

The thought came when I was setting the rear ride height and realised Peugeot set the height at a certain point with this in mind. So that as the rear arm reaches nearer the horizontal there is less vertical height required for an equivalent amount of arc, or twist of the TB.

 

Therefore, it must act as a 'rising rate'.

[dcc 855]

Long time no see Paul!

 

Can't help with your query as I think it depends a lot on the torsion bar & ARB composition, tempering and thickness.

 

Dan

 

(Peoples front of Judean)

[petert 585]

A torsion bar is just a straightened out spring. Unless the torsion bar changes diameter along it's length it's effective spring rate will be linear, within it's elastic limit.

[fangio 1]

Solidarity Dan!

 

I am not saying the spring isn't linear.

 

If you think of a compass face and the needle, (representing the trailing arm) pointing at 135 deg (i.e. 45deg. below the horizontal) for every 5 deg. the needle moves up, (i.e.the suspension comes under load), the vertical change at the pointy end, or wheel, in distance gets smaller.

 

The arm is describing an arc. Although we only think of the wheel going up and down, it is also moving backwards and forwards, as it follows the part of the circle.

 

The nearer to the horizontal increasingly less vertical distance is gained for the same degree of arc.

[Tom Fenton 1,541]

Yes you are correct but I honk the effect will be negligible in terms of spring rate.

 

We had a discussion some moons ago about the effect of the bending lengthways of the torsion bar whilst it travels through it's arc of torsion between the fixed end and the trailing arm end. The resulting increase in spring rate there is also negligible.

but I honk

Ha ha, THINK!!

[Tom Fenton 1,541]

 

I am not saying the spring isn't linear.

 

If you think of a compass face and the needle, (representing the trailing arm) pointing at 135 deg (i.e. 45deg. below the horizontal) for every 5 deg. the needle moves up, (i.e.the suspension comes under load), the vertical change at the pointy end, or wheel, in distance gets smaller.

 

The arm is describing an arc. Although we only think of the wheel going up and down, it is also moving backwards and forwards, as it follows the part of the circle.

 

The nearer to the horizontal increasingly less vertical distance is gained for the same degree of arc.

I understand what you are saying but I think (honk) you have it back to front. The input into the system is the bump of the wheel pushing the trailing arm. So the angular movement becomes less wrt the vertical bump load once you are past horizontal. Not the vertical distance becomes less for the same angular movement.

[jackherer 543]

Hello Paul

 

Although we only think of the wheel going up and down, it is also moving backwards and forwards, as it follows the part of the circle.

 

This is why Citroens with hydraulic suspension have the handbrake on the front wheels. If it was on the rear wheels the body of the car would move forward as the suspension lost pressure and sank.

[fangio 1]

I love it when a plan goes in totally the wrong direction!

 

While you've all posted I was looking at sine waves and realised that was what I was remembering from school 37 yrs ago - crikey.

 

Tom, you're spot on. It's a decreasing rate from 135 degs to 90. As you say not a lot in it;from sine tables 1:1 to 0.7:1. But lowering on std TBs you could lose 20% of available poundage.

 

The whole reason I gave it some thought was toying with the idea of dual rate coilovers with Eibach ERS on my ASTs and whether it would suit linear (or, decreasing rate ​ ,rears!)