Math behind a WDH

[kaintucky]

I was just wondering does anyone know the actual calculations that go into determining how much weight is distributed across all axles of TV and TT? Is the ideal distribution 1/3 to each (front axle, rear axle, trailer axle(s))?

If so, would payload on the TV then be 2/3 of the tongue weight of the trailer? Perhaps I'm way off in my logic here as well. I'm not planning to go over vehicle capacities and I understand (I think) the general physics of WDH but wondering if there's published calculations on the amount of weight transferred to each vehicle.

 

[Gary RV_Wizard]

I've not seen anything like that and do not think there is any preferred percentage of distribution. The general theory is to restore the tow vehicle to the same stance it had before the trailer was hitched, which is pretty much level for most vehicle types. Don't want the rear end sagging or the front end either sagged or lifted (maintain the front end steering geometry at the factory settings).

 

[kaintucky]

Thanks @Gary RV_Wizard! I'm an engineer by trade so exact numbers always intrigue me but maybe I'll have to let this one go.

 

[Gary RV_Wizard]

The engineers that design WD hitches surely know what each hitch and spring bar combination can do, but the equation isn't of much help to a vehicle owner because the WD adjustment has to work in conjunction with the vehicle suspension. That's why it is basically a trail & error method where you shift weight until the desired tow vehicle suspension balance is achieved. A pick-up truck with a strong rear suspension (high cargo capacity) may not need any weight-shifting at all, while a soft-suspension passenger sedan could require a lot of help. There is no universal formula for all trailers and tow vehicles.

 

[kaintucky]

Right...I was thinking that if you had (which most do) payload, rear axle to tongue length, tongue to wdh connector length, tongue to TT center axle length, and tongue weight that you could calculate the force on each axle when distributed. I didn't think about how the upward force from the suspension (even as it ages) would affect the overall distribution of weight to level out the rig.

 

[Matt_C]

Kaintucky,

I like Gary's answer, but you seem to have missed is the the tongue weight is still the same. The big difference is that it is no longer on the back bumper. It still has to be counted as part of the TV payload. Think of a free-body diagram. All the weight is still down, but now some of the TV rear is moved to the front.

You can calculate this if you have all the information. I should know, I used to do this when testing trucks for an OE. But as long as the TV frame is strong enough to to take it, then it really does not matter. This is where you should go over a scale every so often to know how far you are from C/GVW.

I could tell you about some that did not make it. The most remarkable was a truck that bent so the box hit the cab. This was a very serious overload and the demonstration was done for litigation.

Matt

 

[kaintucky]

@Matt_C I understand the tongue weight is still the same and the weight of it must go somewhere. So maybe my understanding of a WDH is a little skewed. Does the WDH balance the tongue weight load to only the front and rear axle of the TV? Or does it balance it between TV front & rear axle AND trailer axle?

 

[IBTripping]

This video will likely answer your question.

 

[kaintucky]

@IBTripping thank you! I've been all over Weigh Safe's site and never saw this, very helpful. Thank you again!

 

[Lou Schneider]

A weight distributing hitch doesn't directly distribute weight between the axles. All it does is bring the trailer and tow vehicle back to a level posture instead of having the hitch weight make it sag in the middle with the front end rising and the rear springs sagging.

The amount of force transferred to each axle is determined by each of the spring rates. A spring that compresses 1000 lbs per inch will transmit half as much additional weight to the ground as one that deflects 2000 lbs per inch, assuming the equaling hitch is adjusted to compress each spring an equal amount, i.e. return the vehicle to it's unloaded posture.

 

[scottydl]

This is only one account, but a few years ago I weighed my prior tow vehicle (a 1995 Suburban C2500) in various configurations with/without towing and WDH engagement. This was on a commercial grain elevator scale. Axle weights:

• Suburban alone Front 3040# / Rear 3560#
• Suburban towing, no WDH engaged Front 2520# / Rear 4920#
• Suburban towing, WDH engaged Front 2760# / Rear 4620#

So putting my trailer's tongue weight onto the vehicle, it added 1360# to the rear but removed 520# from the front. The WDH then lightened the rear by 300# and added 240# back to the front... a little less than half of the initial tongue weight impact (on the front axle).

 

[Gary RV_Wizard]

Nice video - thanks IBTripping!

As it explains, the amount of weight to be relocated can be calculated for a given tow vehicle and trailer, but the goal is simply to bring the tow vehicle front suspension back to a net zero change, neither heavier nor lighter than before hitching. Nearly always the front axle incurs a negative weight change when the trailer is hitched, so weight needs to be restored to it. This is done by applying tension to the WD spring bars until the front suspension returns to its normal operating height. Some of the tongue weight ends up getting transferred back to the trailer axles, but that's academic. The key is that some also goes to the front axle and you need to apply spring bar tension until the front nets out to near-zero change.

The handiest tool for doing that is a tape measure rather than a scale or calculator. Measure the front end distance from the road surface before hitching, then hitch and apply spring tension until the front end comes down to that same distance again. Easy peasy! Measuring the distance from the top of the wheel well to the pavement is probably the simplest, but you can measure at any point above the front end suspension.

 

[Matt_C]

I like the measuring idea. I was spoiled. We had a scale in the floor of the build garage that we used to set up or confirm towing tests. Tape measures are much more available.

Matt

 

[CharlesinGA]

Most truck manufacturers are now saying to restore only about 50% of the lost front end weight, and I think that is reasonable. The key is keeping the front tires planted for effective steering and as noted, maintaining the steering geometry. It is possible to remove TOO much weight from the rear, and that is where being careful comes in.

Charles

 

[Arch Hoagland]

I used a weight distribution hitch to level my truck and travel trailer. I did a couple weighings with the WDH attached and then without it.

I found that approximately 200lb were taken away from my rear truck axle and transferred to the truck's front axle to make it level.

 

[scottydl]

Those stats are almost exactly what my scaled numbers ended up showing too. 520# taken away from the front axle when the trailer was hooked up, and then 240# put back with the WDH engaged. That was one opportunity I had to sit my rig on a scale and try some different configurations. Otherwise I typically used the height/tape measure method as Gary mentioned.

 

[Gary RV_Wizard]

Most truck manufacturers are now saying to restore only about 50% of the lost front end weight, and I think that is reasonable. The key is keeping the front tires planted for effective steering and as noted, maintaining the steering geometry.

Right. It's not necessary to get exact zero because the additional weight may lower both front and rear suspensions a bit. If it gets back to level without achieving zero impact up front, that's fine. Or the suspension may tolerate a bit of lift without getting squirrel-ly.

Trucks and big SUVs often can tolerate a bit of front end lift because they are designed with rear heavy cargo loads in mind; sedans and crossover SUVs not so much.

 

[mjf55]

I was just wondering does anyone know the actual calculations that go into determining how much weight is distributed across all axles of TV and TT? Is the ideal distribution 1/3 to each (front axle, rear axle, trailer axle(s))?

I know this thread is a little old, but I found this video that has the calculations requested.