Heavy Truck Lateral Acceleration Dynamic Load Transfer 2

[sbozy25]

Perhaps this is a dumb question, but my fellow engineers and I are drawing blanks on a potential issue...

First off... We are working with a fire truck that has a water tank attached to the top of the vehicle, which throws the CG way out of whack... So, for ease of calculations, we are assuming the tank is full, and the water can not move.

For some reason we can not seem to remember what the rule of thumb is for lateral acceleration causing roll.

I know in the automotive world roughly 1.4g is the limit for lateral acceleration. However, I can't find any of my reference material on heavy truck, and google is not cooperating with me.

I suppose I can turn on Mathcad and run the dynamic load transfer equations to get an exact number, but perhaps one of you can quickly give me a number that I agree with and run. I seem to want to think .45g was a good rule of thumb to work with...

Thoughts?

 

[imcjoek]

Roll, as in body roll?

Or roll as in "er, we seem to be upside down"?

the latter is easier as it can be approximated with a statics free body diagram.

the former you need to know the intimate details of the suspension geometries, and roll stiffness.

 

[sbozy25]

Dynamic body roll which results in "son of a bleep..." Were gonna wreck.....

I can run the equations, but we seem to think there is a rule of thumb such as automotive.

Small car / light truck rule of thumb is 1.4g lateral acceleration....

We think .45g is the heavy truck rule.... but can't find it written any where....

 

[BrianPetersen]

No rule of thumb, but half the track width divided by the center of gravity height should be close. Use the midpoint between the two wheels on each side for figuring the track width.

 

[Tmoose]

"The driver ... pulled over for safety, but the truck slid off the road and rolled."
Doesn't look like the shoulder is anywhere near 45 degrees (1 g).

http://wilco278.files.wordpress.com/2008/12/050-tank-truck-roll-over-at-alpine.jpg

Image that may be useful for the start of a graphic analysis here -

http://aadezyn.com/blog/wp-content/uploads/2009/07/Tank-Truck.jpg

A friend bought a newish Fiat 128 in the mid 70s and fitted it with oversized Michelin tires. He was a very aggressive driver and shortly thereafter succeeded in rolling it during some cornering maneuver on a public road. As testimony to his love of corners, Perhaps it should be noted he is the only person I have ever met who wore the sides/edges of his motorcycles' tires at the same rate as the center.

About that same time, before computerized effects in cinema, a European stunt driver was interviewed about what it took for him to drive some car up on 2 wheels for extended distances in some movie. I believe he rather proudly said he did not use ramps to initiate the stunt by tossing one side upward , and that he could perform that stunt with virtually any car without using ramps.

Even though a Porsche Suv-ish makes it through Road and Tracks slalom at a higher speed than a 1984 VW GTI, I think road cars should fit under my arm.

http://cache2.asset-cache.net/xc/50...6399091E831860380CC697622294C191F19EEEB4B00E7

 

[GregLocock]

I think 0.45 g is way optimistic, looking at a fire truck. Brian is on the right track (sorry).

As a very necessary refinement consider the lateral motion of the cg as the body rolls and the tires squash, etc.

I also suggest you contact your tire manufacturer and start to find out your tire properties.

You need to consider both tripped and untripped roll, and as you might expect, dynamic roll is different to static roll.

Check out the NHTSA Fishhook maneuver for some insight into that.

Finally tank sloshing will make it worse. Full tank is not worst case.

Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[sbozy25]

Thanks everyone....

Brian, thanks a bunch to you. Over the weekend, I enjoyed the nice weather with my lap top out side and ran the numbers. It took a few hours, but when it was done the value I calculated was within .02 of the value I get using the simple calculation you stated.

oh... and wouldn't you know it... as I was typing this, our service engineer brought me a technical document written by SAE that has a graphical analysis of roll over threshold of heavy trucks. The good news, is for this type of vehicle... The numbers are within the graph range.....

 

[FeX32]

Interestingly, from simple statics you get exactly what Brian gets.
I suspect dynamic roll situations are of a lesser value.


Fe