Saturday, May 03, 2014 at 21:08
WBS posted:
I'm mystified how an the airbag that fits inside the coil spring puts point load where it is not designed for?
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An air bag and a coil spring are both springs but they are not the same type. A coil spring is linear meaning it will keep on compressing in direct proportion to the load on it i.e double the load and you will double the amount it compresses.
An air bag is exponential and will keep on getting harder as it compresses. The more you compress it, the more it will resist further compression. They usually start getting hard when they are about 40% compressed and can be like a
rock at 60%.
Your standard bump rubbers are also exponential and do the same thing which is why they will bring the downward movement of the chassis to a very sudden stop if the coil, or leaf spring, bottoms out on severe bumps. When you put an air bag inside a coil, or between a leaf spring and the chassis, you are really installing an oversize bump rubber. If you have enough pressure in them, which is often the case when they are being used to lift an overloaded car, they can stop a falling chassis
well before it would normally reach the original bump rubbers. This can easily lead to chassis damage.
Few people seem to realise that when a falling chassis is suddenly stopped, whatever is behind the rear axle will want to keep falling. The forces generated increase by the square of the distance from the axle back to wherever the weight is or right back to the tow ball if you are towing something. That distance is like a lever sticking out the back of the car and we all know how big weights can be lifted easily with just a little effort on the end of a long lever. As the rear of the chassis slams down, the leverage on the end of the chassis rocks the car on the axle and tries to lift the front placing tremendous stress into the chassis around the axle or, as in the case with air bags, right on the bag mounting point.
The tow ball on these type of utes is usually around 1.2 metres behind the axle so a weight of 350kg on the ball, which is the
Ranger's maximum, can become 504 kg when the rear wheels drop suddenly into a washout or whatever on a track. The weight of whatever is in the tray plus the rear of the tray and even part of the chassis itself is also falling and will be adding to the downward force so the total amount will be
well above 504kg.
What is needed here is a WDH. It is a lever, not a spring, and will lift the rear of the chassis and transfer some of the weight back onto the front wheels. Air bags and heavier springs simply lift the car a little higher and make it look better from side on but they can not transfer weight. You still have all that weight behind the axle wacking the end of the chassis down on rough surfaces. That is why there have been so many broken chassis over the years and they have not all been in the bush. The last one that I saw was on a 2wd tradies ute earlier this year in the middle of
Canberra. It was the same old story i.e far too much weight too far behind the rear axle.
In the case of this
Ranger in question, and all other cars, the maximum tow ball weight will most like come with a few restrictions. The manufacturer may say a hitch must be used or it is for good sealed roads only and must be reduced as road surfaces get rougher. The only way to find out is to ask the manufacturer, not the dealer. They don't put specifications like this in the hand books because there are too many variables involved to try and cover them all.
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