Take a look at the picture linked in the OP. I’m not seeing where chocks are going to help this.
Could you bolt on a half inch of rubber on the bottoms of the jacks? That would help with slide. Walking’s a different issue, more bracing to keep the frame true? Is that 33 tons your legal maximum?
Ever seen a wall held up by side supports? This is what a wheel chock does. Doesn’t mean it has to be a wheel chock but the picturesI looked at show at least on axle on the ground.
Here’s an image with a truck on jacks using wheel chocks.
So if one axle is on the ground, wheel chocks. All wheels off the ground, frame chocks.
Agreed, wheel chocks would certainly help in the pictures you looked at.
There seems to be a failure to understand how wheel chocks work with regard to
F= mu N.
Wheel chocks stop the wheel turning ,and that is all they do.
The wheel chock has rubber presenting to the tyre, and rubber on rubber is meant to grip very well… a large mu… but this is only relevant when the tyre turns.
Imagine the tyre sliding without turning. What if there is no force downward on the chock ? That means very very tiny “N” in the F=mu N equation… So with F (force of friction ) insignificant, the wheel chock will continue to slide. When the tyre slides, the chock is pulled under the tyre, which means the weight on the tyre then applies to the chock, raising N… But it all rests on the tyre TURNING !.
So… with the OP being about the truck sliding, we can assume the tyres do not turn.
Besides, the truck may slide sideways ( and off the chocks !)
The outriggers take the weight in order to increase the N in F= mu N … very large N and mu is pretty good too. With all that force down on the small pad, oil and water are just squashed out of the way. Also, any pebbles or other rolling objects are squashed into nothing too…
The metal pads do not necessarily work well on metal, such as metal lids or road plate . If the metal surface is greasy, the steel to steel contact may have the tiny mu … below that 0.09
I don’t think you understand how chocks work. They form a triangle and when pressed upon by ANYTHING, they dig in. It’s not just a function of lateral adhesion. The same principle applies to a wall support and can be applied to a truck frame.
As for sliding sideways that’s a different problem. The purpose of jacks are to stabilize the truck. If it’s sitting at an angle then the jacks are adjusted so the truck is level. I posted pictures of this. It can be leveled front to back or side to side.
If the truck is truly sitting on ice or something slippery then the only reasonable solution is to stake it. In this case I’d have plates welded up to receive the jack base and then drill a hole in them for a 2" metal stake. One for each side. The trucks not going anywhere and it’s easy to patch blacktop or concrete.
Another solution is to back another truck up the fire truck bumper after it’s been jacked up and chock that truck. Now you have 4 wheels that are chocked.
Chocking the tires while the truck is set up is part of the “solution” that is being discussed. One side (those concerned with a sliding truck) are convinced that only a wheel chock will stop the truck from moving. Of course, that it’s never slid doesn’t enter the equation. Other trucks have slid, so ours must too.
The other side feel that the truck hasn’t slid yet, and won’t, given the way it is set up and designed. When the truck is on its jacks, the weight is to be off of the tires. You may have a tire touching the ground, but the suspension is “unsprung” above it. Looking at this photo from Magiver’s post above, the tires are on the ground, but you can see that the weight is off of them - look how much space there is between the top of the tire and the wheel wells. The other photo is from a different manufacturer, with different jack styles. H-style jacks often don’t remove the weight from the suspension, leaving tires in in strong ground contact.
The anti-chockers have concerns about the chocks themselves. When the truck is on the jacks, there is less of the tire in contact with the ground. As a SWAG example, you may have 20 degrees of the tire’s radius on the ground with the truck down, but only 5 degrees or less when the jacks are deployed. If you slide the chock under the tire while on the jacks, then lower the truck back down (since we’re very concerned about sliding, chock always!), the tires land on the chocks. One of two things is eventually going to happen. First, the chocks are going to be damaged. They are being loaded in a way that I don’t think they are meant to be loaded. The other issue is that we’re going to damage a tire. If I pop a tire, I’m not moving the truck. And it isn’t like we have a spare in the trunk (although we do have jacks, of course).
Chocking when we’ve never had a slide, versus the fairly high potential of damaging the truck. Those seem to be the options. Thus the strong internal debate.
Where i am… we chock after we’re done setting the ladder up. we have a 98 sutphen… not that dissimilar to yours. Here’s the procedure:
Fire run with no ladder and we’re not gonna need it: No chocks
Fire run where we need the personnel on the ladder, but haven’t set up the ladder: Chock em
Fire run where we need to set it up: Fully set up the ladder (including lowing the front wheels and locking the suspension), then chock afterwards. Never had an issue.
You just have to remember to unchock before you lower from jacks.
Some experimenting might be worthwhile.
Take the truck to some place where there’s a short but substantial slope that’s clear of obstructions, so if the truck slides no problems ensue. Set it up in various configurations and see whether or not it slides and/or walks. Try soaking the support areas with water to see what effect that has.
The only thing I have to add is that flexing in the truck body itself - such as by moving the ladder around with a big weight on the end of it - might change the contact angle between the ground and the truck.
By 25 degrees, though? No.
That steel to aluminum joint is scary, though. It’s a big piece of metal, just the kind of thing that might collect a thin layer of grease. What if oil or diesel leaking from the truck gets on it?
Even seeing the numbers, I understand your perspective. Up on a big ladder like that, even the thought of sliding is a pretty scary one.
While that’s a wonderful idea based on common sense, the powers that be do not want the chocks removed, period. You can’t be too careful, after all. Never know when the truck will move away on its own.
Ghostman: the Sutphens have the suspension lock. Pierce has a maxi brake on the front axle. E-One has none of those; jack the wheels off the ground and go (and I don’t have to pin jacks, either).
Not an issue. The relevant angle is the angle of the pavement with respect to gravity. As long as that angle is 5 degrees, you can tilt the truck to whatever angle you want - hell, roll it over on its side - and it won’t slide.
KCB615, can you contact the manufacturer, and ask what they recommend? Maybe they have documentation available.
I doubt you’ll get a physics-based result that will ever convince them, since they probably don’t understand physics that well, and won’t trust it. But if you can point to the manufacturers recommendation, that could hold enough weight to convince them.
This. IME If they dream up such a problem. One that has never existed before, they will hang on to their belief in it with every ounce of their being. No amount of logic, math, or physics will budge them.
Sometimes the word from an outside “expert” will change their mind(s). Note: This 'Expert" may be as fake as a three dollar bill, but once they believe him they are once again convinced. Good Luck, You are going to need it!
The “expert” was the start of the problem.
I totally get that I’m tilting at windmills, but gosh darnit, me and Sancho are going to put up a good fight.
Thanks for the help folks; both physics and otherwise.
So how are you supposed to drive away? 
Look, you just said the process is to raise on jacks and then chock. This is just reversing that process. When you are ready to lower from jacks, you remove the chocks and then lower.
Yes, I realize how ridiculous that sounds.
I’m not saying you need chocks. The realistic problem is that, whatever the nominal process, someone can screw it up. Sure, the process is to remove the chocks before lowering the jacks, but you know some rookie is going to get it wrong, or the guy who has been slugging all afternoon on the fire and is exhausted physically and mentally now has to pack up the ladder truck, and, well, a momentary lapse in concentration and the truck sits down on the chocks. Now you’ll never get them out.
Yea, I hear you! Seriously, Good luck to you and Sancho, 48.
So chocking isn’t just a bad idea: it’s dangerous.
Worst case, you hammer the chocks in, taking weight off the jacks and making the truck unstable.
next worst case, when the truck slips, it throws weight onto the chocks, making the truck unstable. Even if the chocks are placed only “near” the wheels, the truck slipping onto the chocks will make the jacks unstable.
Best case, the truck falls off the jacks onto the chocks instead of onto the road. So when it falls off the jacks, it falls into an unstable postion.
Check with the supplier. I
m thinking they’ll tell you that all warrenties are void if you chock the wheels.
I read the first post and then started looking for the bolded bit. Engineers are both busy and clever people and the problem of fire engines, cranes and what not sliding about on a very gentle slope is pretty much a non-issue these days - as outlined above its a very basic problem with very favourable numbers. If you ask the manufacturer how steep a slope the truck can be used on they are probably going to come back with a pretty impressive number.
Having them topple over when the crane, ladder, grabarm etc. is in operation is a much more challenging proposition and as far as I am aware one of the side-effects of trying to solve that (with jacks, outriggers, etc) is to over-solve the sliding problem by several orders of magnitude. Or to put it another way - if they are worried about the thing sliding on a 5 degree slope then they should be positively terrified of putting the ladder up.