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# Roll stiffness ballpark valvue's for a 3 axle trailer

## Roll stiffness ballpark valvue's for a 3 axle trailer

(OP)
I would like to get some ballpark values for roll stiffness for a 3 axle trailer.

I'm not sure how to tackle this, hope some of you can pitch in

Do the airspring contribute to the roll stiffness?
I guess there is quite a difference for roll stability in long 'static' turns vs. in a fast steer actions
(for instance a lane change or something).
A disregard of the air spring influcence would seem to be ok, due to it being conservative, correct?
I'm not sure though.

Also i'm not sure on the weight I should use to determine the latheral force.
For instance can I 'simply' use the total weight of the trailer dived by 3 (because it is a 3 axle setup) or
should i use the rated loading of the axle itself instead and then basically see it as a single axle setup?

### RE: Roll stiffness ballpark valvue's for a 3 axle trailer

Of course the air springs contribute to the roll rate. Typically airsprings are quite linear and do get progressive at the end of their travel. In fact being constant in rate is one of their key characteristics. It is possible that the truck is equipped with "active" airsprings that will be filled with extra air to work against the load. This can reduce rollangle but will NOT change rate.

In order to have a ballpark number for roll stiffness of a truck you could start assuming that a truck can do 0.7g (on good tires) and go up to 7-8° of roll angle. Having solid axles the RCH will be around wheelcenter height meaning that the rollmoment will be approximately unsprung mass * 0,7g + (Height CoG - RCH)= ... Nm . This will have to be counteracted by the springs and anti-rollbar.

With respect to the axle load this one is a bit more tricky since a multi-axle truck is by definition an over-constrained system meaning that each contact patch load is dependend of the load of the other contact patches. The good news is that you are running air springs which basically means the loads will be tried as equal as possible. So I guess it would be fair to consider the trailer as a 2 axle vehicle with the front axle and a "virtual" rear axle that combines loads and intermediate wheelbase of the two rear axles.

Cheers,

dynatune, www.dynatune-xl.com

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