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Old 09-15-2018, 12:49 PM   #1
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Join Date: Mar 2015
Location: Calgary
Posts: 479
Caster in steering: How Toads & crosswinds use for good & bad

In another post, Installing a Steering Stabilizer and Trim Unit
I explain how I installed a steering stabilizer on my Berk with a Freightliner chassis. In that post, I promised to discuss steering caster (or castor) and how it motivates the need for a steering stabilizer with trim adjustment. I will also explain something that perplexes many people: how are we able to steer a Toad in four-down mode. It’s because of steering caster in the Toad.

Think of a shopping cart with caster wheels. You can simply aim the cart in a new direction and the caster wheels turn to allow you to smoothly take the new direction. The caster effect happens because the contact patch of the wheel with the ground is behind the (vertical) axis upon which the wheel turns when steering. When you take a new direction with the shopping cart, the wheel assembly rotates around the steering axis to the new desired direction because the wheel’s friction with the ground is behind the steering axis.

Bicycles and motorcycles use a caster angle to allow the cycle to follow in the direction of the cyclist’s leaning and steering. In this case, the steering head of the cycle is no longer vertical, but it still rotates around an axis that touches the ground ahead of the contact patch of the tire with the ground. Without steering caster, a cycle would be extremely unstable–the front wheel could move in random directions without any natural force to make it point ahead. To confirm the importance of having the steering axis ahead of the contact patch, try the reverse. Hold a bicycle by its seat and don’t touch the handlebars. Then, walk backwards and notice the erratic and floppy turns of the bike.

Motor vehicles also have a caster angle built into their steering, but it is hidden behind the body work. A wheel alignment shop can adjust the caster angle to manufacturer’s specifications. If you have a MacPerson Strut suspension, the steering axis is the axis of the strut and the caster angle is the rearward angle of the strut from the vertical. This means that the turning axis hits the ground ahead of the car’s contact patch. If you have a double wishbone suspension, the steering axis is defined by the upper and lower ball joints, and it has a caster angle for steering.

If you have a solid front axle, as in a truck or most of our motorhomes, the steering axis is created by the King Pin, and the King Pin is angled backwards to give the caster angle for the steering.

The caster effect on steering in a vehicle does several things:
• It gives the return-to-centre force when you are finished turning a corner. When you turn a corner, there is centrifugal force pushing the vehicle outward. Since the tire contact patch is behind the contact point of the steering axis, this turns the front wheels towards the outward direction, following the centrifugal force. If the driver lets go of the steering wheel in a corner, the vehicle starts to go in a straight line because of this.
• When turning a corner with a toad being towed behind with all four wheels down, the towing force pulls the front of the toad in the direction of the turn. The steering caster of the toad dutifully turns the toad front wheels in that desired direction. This is the same effect as turning a shopping cart.
• A crosswind pushes a vehicle in a lateral direction. The caster steering effect causes the vehicle turn in that lateral direction, forcing the driver to counteract with the steering wheel.

It is this latter issue that motivates the use of a steering stabilizer that can be trimmed to give a steering angle that offsets a crosswind. A motorhome presents a very large sail to a crosswind, and that lateral force results in a much stronger effect than on a car. It also creates a stronger steering effect on a motorhome than on a semi-trailer truck, fifth wheel or travel trailer. This is because the crosswind only affects the caster steering of the the tow vehicle. The lateral force on the trailer doesn’t translate back up to the steering caster. So, we motorhome drivers have to fight crosswinds harder than do truck drivers and RV trailer owners. Of course, a lightly loaded semi has a problem that we don’t have: the wind can tip the trailer over. Our lower centre of gravity helps us from tipping in a crosswind.

RV travel trailers and fifth wheel rigs are affected by the wind, and they get a wobble when passing a truck. But, that effect is on the trailer and doesn’t come forward to affect the caster steering of the tow vehicle. Indeed, in a strong crosswind, these vehicles are almost balanced and don’t require the strong steering effort of a motorhome. Similarly, a front-engine gasser has a big overhang on the rear, which tends to balance it in crosswinds. So, the gassers aren’t affected as much by crosswinds as a diesel pusher.
–Gordon
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Gordon Sick, Calgary
2015 Berkshire 34QS
2005 Acura EL (aka Honda Civic)
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