Talk About Wheelbase, Overhang and Tires

[mstephens]

|----O<-----TV wb-------->O----@<-----------TT-----OO---|

Where O is a wheel, and @ is the ball, and TT is the length from ball to axles. And specifically with regard to the larger AS trailers - 25 to 34.

We always here about the overhang on the TV - the distance from rear axle to ball. Often expressed as a ratio of the overhang to the total wheelbase. I find two common instructions about this. First, make the overhang as short as possible. Second, make the TV wheelbase long relative to the overhang. e.g. overhang ratio. They mean different things. And, on top of that, I also hear another idea which is make the overhang short relative to the WEIGHT of the TT. Another totally different idea.

My C 300 has a 120" wb and 44" overhang, or a ratio of 2.72:1. My TT weight is going to be around 6000# to pick a round number. It's easy to understand that when the overhang is long, regardless of wheelbase, lateral force applied at the ball will be magnified proportional to this distance. Long is bad, short is good on an absolute basis. What's a little less intuitive is how important the wheelbase is in mitigating this issue. On one hand a long wb may reduce the effects of the lateral force, but on the other hand a long wb makes it harder to transfer weight forward with a WD hitch. And of course, we haven't even discussed the tires!

As to the weight of the TT, I have to assume the simplicity that any increase in weight increases the ease of putting lateral force on the ball. More weight, longer overhang is bad. Less weight, shorter overhang is good.

Tires. I think it is understood here that the idea is a very stiff sidewall. This is often accomplished by simply reducing the profile of the tire from something like 70 to something like 40. With stiff sidewalls and a short overhang, and a long wb, it will be hard for those lateral forces to affect the path of the car.

Questions:
Which is more crucial, the absolute value of overhang, or the ratio of overhang to WB?

What sort of overhang numbers are possible with the vehicles being used commonly? Yours?

Although we talk of TT tires often, I don't hear as much talk of TV tires. Can you put low profile tires on PUTs and SUVs?

What's the shortest WB that is feasible for the bigger AS trailers?

[StokerDoc]

Here is my attempt at answering your questions.

#1. The ratio of overhang to WB is more important. It's basic mathematics. In your case of 2.72:1, for ever pound of lateral force the trailer pushes on the ball, the front wheels have 2.72 pounds to counter that with. That can extend in the vertical direction as well, the larger the ratio, the less porpoising will result.

#2. My Tundra has a WB of 146", and my overhang is 60". That is 2.43:1. Andrew T said he could get me a shank that would shorten that by 5". That would bring the ratio to 2.65:1, not bad. I am trading the Tundra in on a new Mercedes GL 350 in the spring, I hope for at least similar, but better numbers. I will be stopping at Cam-Am next summer to get our set-up tuned with shortest overhang possible.

#3. For the Tundra we choose the "Limited" trim level which increases wheel size from 18" to 20" without changing overall tire diameter. Same for the Mercedes, getting the "Sport Package" goes from 20" to 21" wheels with lower profile tires. Less side to side play with shorter sidewalls. Not sure how stiff they are, but the shorter sidewall means less sideways deflection whether softer or stiffer. I would think soft(ish) sidewalls as Mercedes is known for a smooth comfortable ride.

#4. I don't think any discussion about pulling a trailer that involves wheelbase should occur without discussing overhang and the ratio of the two. They are too closely linked to not talk about both. Most people that tow with trucks, are stuck with a fairly long overhang. The possible solution to sway is getting another truck, with the same overhang, but with a longer wheelbase (double cab, crew cab, quad cab, etc) to minimize sway. I think they would be pleasantly surprised by choosing a vehicle with a much shorter overhang and shorter wheelbase for maneuverability while towing and day to day driving, whether truck, SUV or minivan (all properly equipped with a "tow package" with payload and axle limitations met).

I believe you are going to be immensely impressed with the way your new TV and TT perform. I have high hopes for my set-up come April.

[SteveH]

I think you are asking a lot of theoretical question to which there are no absolute answers. Mostly, you have already answered most of the questions in your statements.

The facts are, with what is now days called a "bumper pull" travel trailer, you can only get the ball mounted so far forward, but your statement of the shorter the overhang, the better, is accurate. But again, without major modifications to the vehicle such as cutting away part of the rear, you can only mount the hitch head and ball so far forward, and that's it.

Wheelbase is the same thing. It is what it is with a given tow vehicle, and you can't change it. It can only be selected when you purchase your vehicle. Yes, longer wheelbase is better, and shorter overhang is better, but how are you going to change them. The only way is to get a different tow vehicle.

Some answers.....it is my opinion the ratio of WB to overhang is more important than absolute overhang. And yes, there are 20" 50 series tires/wheels available as options on some pickup models. However, I believe air pressure in the rear axle tires is just as important as aspect ratio.

[mstephens]

Originally Posted by SteveH

I think you are asking a lot of theoretical question to which there are no absolute answers. Mostly, you have already answered most of the questions in your statements.

The facts are, with what is now days called a "bumper pull" travel trailer, you can only get the ball mounted so far forward, but your statement of the shorter the overhang, the better, is accurate. But again, without major modifications to the vehicle such as cutting away part of the rear, you can only mount the hitch head and ball so far forward, and that's it.

Wheelbase is the same thing. It is what it is with a given tow vehicle, and you can't change it. It can only be selected when you purchase your vehicle. Yes, longer wheelbase is better, and shorter overhang is better, but how are you going to change them. The only way is to get a different tow vehicle.

Some answers.....it is my opinion the ratio of WB to overhang is more important than absolute overhang. And yes, there are 20" 50 series tires/wheels available as options on some pickup models. However, I believe air pressure in the rear axle tires is just as important as aspect ratio.

Right. I wasn't suggesting modifications, just looking at how different TVs compare in what are regarded as some of the more critical parameters. Clearly, not many vehicles are designed specifically around towing travel trailers.

[CapriRacer]

A couple of thoughts, then I'll talk about tires.

Usually these types of theoretical discussions are kind of moot because not only are there no adjustments for the parameters being discussed - and not only do these parameters are pretty much the same from vehicle to vehicle - but also, lots of those parameters are confounded with something else.

For example, a longer wheelbase is usually also tied to a heavier vehicle (ala, standard cab vs crew cab)

I think it is MORE important to pay attention to what the specs the vehicle manufacturer publishes. Those specs are much, much more likely to keep you out of trouble.

(An aside: I realize it is great fun to have these kinds of thoughts. Keeps the brain limber!!)

Tires? Load carrying capacity is affected by all 3 of the numbers in the tire size. Going down in profile (the second number), is usually accomplish by also going wider in width, so the net effect is no change in load carrying capacity.

And the affect stiff tires on the tow vehicle would have would be small compared to the affect the trailer has on the tow vehicle by itself.

[mstephens]

Originally Posted by CapriRacer

Tires? Load carrying capacity is affected by all 3 of the numbers in the tire size. Going down in profile (the second number), is usually accomplish by also going wider in width, so the net effect is no change in load carrying capacity.

And the affect stiff tires on the tow vehicle would have would be small compared to the affect the trailer has on the tow vehicle by itself.

The affect of stiff sidewalls is actually pretty significant with regards to sway. Load capacity is just one of the important parameters for a good performing tire on a TV. Tires of the exact same size can have very different load capacity.

My reason for the threads isn't to keep my brain limber, although that's a good effect if it happens! My reason is that the manufacturer's information about towing is limited to capacities and actually doesn't predict in the least bit how well a vehicle will be suited for towing a large travel trailer. Here's my analogy: Seeing a bunch of weightlifters perform doesn't tell you much about how well they might wrestle.

[Ron Gratz]

Originally Posted by mstephens

My C 300 has a 120" wb and 44" overhang, or a ratio of 2.72:1. My TT weight is going to be around 6000# to pick a round number. It's easy to understand that when the overhang is long, regardless of wheelbase, lateral force applied at the ball will be magnified proportional to this distance. Long is bad, short is good on an absolute basis. What's a little less intuitive is how important the wheelbase is in mitigating this issue. On one hand a long wb may reduce the effects of the lateral force, but on the other hand a long wb makes it harder to transfer weight forward with a WD hitch. And of course, we haven't even discussed the tires!

The critical "overhang" dimension is the longitudinal distance from the rear axle to the hitch ball. Depending on your chosen WDH, you might need to add 10-12" to your 44" dimension. If you would opt for a 4-bar linkage hitch, you'd have to add approximately 20-24" to your 44" dimension.

If we assume your "ball overhang" is 44+11 = 55", the overhang ratio would be 2.18. This means a lateral force of 100# applied to the ball, would require a front axle lateral force of 100/2.18 = 49#. The front axle force is directly proportional to ball overhang and inversely proportional to wheelbase. The rear axle would have to provide an opposing lateral force of 149#.

You are correct that a longer wheelbase makes it harder to transfer load to the front axle. However, a longer wheelbase also means that less load is removed from the front axle. Therefore, the amount of WD bar force required to restore front axle load loss is almost independent of TV wheelbase.

As to the weight of the TT, I have to assume the simplicity that any increase in weight increases the ease of putting lateral force on the ball. More weight, longer overhang is bad. Less weight, shorter overhang is good.

A more critical parameter is the TT's polar moment of inertia which is a function of mass times the square of the length. The less polar inerrtia -- the better. This could be a factor in comparing a longer and lighter TT with a shorter and heavier TT.

Tires. I think it is understood here that the idea is a very stiff sidewall. This is often accomplished by simply reducing the profile of the tire from something like 70 to something like 40. With stiff sidewalls and a short overhang, and a long wb, it will be hard for those lateral forces to affect the path of the car.

Here, the parameter of interest is the tire's cornering stiffness, Tire Technology, excerpt no. 2 from The Racing & High-Performance Tire. See Figure 6.4 and related discussion. Some tire manufacturers actually can tell you this number.

Questions:
Which is more crucial, the absolute value of overhang, or the ratio of overhang to WB? I would say the ratio of ball overhang to WB. I also would say that a longer WD generally means a greater TV polar moment of inertia which is an advantage.

What sort of overhang numbers are possible with the vehicles being used commonly? Yours? A very common TV -- Suburban/Yukon -- has a 130" WB and, typically, a 65" ball overhang.

Ron

[mstephens]

Nice analysis Ron. I sure am hoping not to have 11" added to my overhang. I am choosing the Eaz-Lift WD hitch, and I intend to have the ball shank made so that it is as close to the rear body bumper as possible. Maybe adding 3" or 5" at the most, not 11". Well, that's my goal. I think based on some photos I have seen that it will be possible.

[mstephens]

Originally Posted by Ron Gratz

You are correct that a longer wheelbase makes it harder to transfer load to the front axle. However, a longer wheelbase also means that less load is removed from the front axle. Therefore, the amount of WD bar force required to restore front axle load loss is almost independent of TV wheelbase.
Ron

What a cool relationship! That's a very valuable insight into the effects of wheelbase, and an interesting case of the interaction of variables in the overall solution. For me personally, it means I ought not stress too much about going from the 130" WB of my Suburban, to the 120" WB of my new Chrysler 300.

[SteveH]

Originally Posted by mstephens

Nice analysis Ron. I sure am hoping not to have 11" added to my overhang. I am choosing the Eaz-Lift WD hitch, and I intend to have the ball shank made so that it is as close to the rear body bumper as possible. Maybe adding 3" or 5" at the most, not 11". Well, that's my goal. I think based on some photos I have seen that it will be possible.

I think to keep your ball 3 to 5" from the bumper, you will have to use one of the older design weld together type hitch heads. (most all of the bolt together adjustable heads will place the ball farther back) I've noticed that most of Andy Thompson's setups use them, but I've not seen them available at any of the hitch retailers sites that I visit.

[mstephens]

Originally Posted by SteveH

I think to keep your ball 3 to 5" from the bumper, you will have to use one of the older design weld together type hitch heads. (most all of the bolt together adjustable heads will place the ball farther back) I've noticed that most of Andy Thompson's setups use them, but I've not seen them available at any of the hitch retailers sites that I visit.

Yes, exactly. I have been working with Andy to do just that. I am trying to maximize every aspect I can manage. The receiver will be a custom install with reinforcements designed for unibody (per Andy's general recommendation). The rear suspension will be enhanced, and this shank/ball piece will be customized for the setup. These small details which are within my control, are a good place to exercise leverage on getting the best results.

[Wazbro]

Originally Posted by Ron Gratz

A more critical parameter is the TT's polar moment of inertia which is a function of mass times the square of the length. The less polar inertia -- the better. This could be a factor in comparing a longer and lighter TT with a shorter and heavier TT.

Ron

2 other factors in this are center of gravity and side wind resistance.

With these in mind a 24' 4000lb 5' high popup would push a vehicle around less then a 14' 2000lb 10' high square box.

[mstephens]

Just for a point of clarity, side winds are not a necessary ingredient for sway. They may often be an instigator, but all towed vehicles are subject to sway at some speed with no wind present. You can see this in the sway models, where typically you adjust the loading of the trailer in the model, and then increase TV speed. There will be a point, as speed increases, where the rig becomes unstable.

This might be important to consider because incorrectly balanced rigs will have sway without any forces being generated by wind or pressure waves from other vehicles.

I certainly agree completely that the AS design of low CG and aerodynamic shape do mitigate the effects of side winds and pressure waves.

[SteveH]

You're right, steady side winds will not cause sway, however gusts of side winds will induce sway. Mostly winds either increase or decrease drag, and it's my opinion what I call a quartering wind, one either from 45 degrees or 315 degrees (0 degrees being head on) create much more drag than a 0 degree wind of the same velocity.

[slowmover]

One of the worst, if not the worst, situation for winds affecting the tt, is the bow wave of a big truck passing our combination vehicle on a steep downgrade. The "taut" alignment of ones combination vehicle is loose or sloppy

Pairs of big trucks while on a narrow two lane, passing in the opposite direction, is another. While the alignment of ones vehicle may be taut, the high speed winds -- which are also suddenly reversing -- are a handful. A dangerous one, at times, in both instances.

.

[mstephens]

Something to consider is how the TV on its own reacts to various gusting and other wind conditions.

[gmw photos]

All this talk about low profile tires makes me wonder if the idea of a short sidewall is such a good idea for loads and towing, why are bus and truck tires ( big trucks ) still designed and built as tall profile ?
Perhaps one of the tire engineers will comment on that ?

[mstephens]

Originally Posted by gmw photos

All this talk about low profile tires makes me wonder if the idea of a short sidewall is such a good idea for loads and towing, why are bus and truck tires ( big trucks ) still designed and built as tall profile ?
Perhaps one of the tire engineers will comment on that ?

To carry loads requires a lot of material - rubber, cords, belts, etc. And heavy loads generate heat that must be dissipated. Low profile tires trade some of that ability for more cornering (higher g's) power. Of course, every tire must carry the load that will be on it. My very low profile tires are rated for 1790 pounds each. That's 3580 pounds for the axle, or 7160 for the car. The car's curb weight is 4000 and the GCWR is 5200. The tires then are capable of the load, AND offer a very high cornering force. Good compromise.

On a large bus, you need a tire that will carry that load. Do you need high performance cornering? No. So, there is very little reason to trade off anything to get a low profile tire. Better to make a tire that carries the load, dissipates the heat well, and has a long life. e.g. High sidewall tire with lots of steel belts.

In towing, we don't race around corners, BUT to reduce sway, we need the same kind of low profile tire in order to prevent the TT from forcing the rear end side to side. We don't want tall tires with soft sidewalls. Since our load is manageable with low profile tires, we can get that advantage and still carry the load.

[CapriRacer]

Originally Posted by gmw photos

All this talk about low profile tires makes me wonder if the idea of a short sidewall is such a good idea for loads and towing, why are bus and truck tires ( big trucks ) still designed and built as tall profile ?
Perhaps one of the tire engineers will comment on that ?

I would have approached the answer to your question differently than MStephens did.

First is that industries don't make changes very rapidly - especially if it involves scrapping material. In the case of buses and big trucks, they have a lot of rims and associated equipment to fit existing tire sizes. Fleets with literally hundreds of vehicles aren't going to make changes unless there is an economic advantage to do so.

Then there is the packaging problem. Wider tires would need more space - and it's not like there isn't room as much as things would have to be moved (as in designed to be in a different location.). Again, with hundreds of vehicles, this is not something that takes place overnight.

But fleets do make changes and any a major change can be integrated by a phase-in process.

So why haven't trucks and buses gone to low profile tires? In some respects they have. In the old days, they used 100 aspect ratio tires (and for reference, passenger cars used 88 aspect ratio). Nowadays car tires are normally in the 60 series range with sporty vehicles using 45 and 40 series tires.

Today, trucks and buses dip as low as 70, but 80 is the norm. (There are replacement tires for duals with 65 aspect ratio, but that's a special case)

So what seems to be he holdup? How about need.

If there was a problem that low profile tires could fix, the industry would be on it. I suspect (and those of you who were over the road truckers should chime in at this point) that sway is just not an issue with big trucks and buses - and I suspect it has to do with tire sidewall stiffness.

First let's look at the height of a sidewall.

While people tend to think that changing to a low profile changes only one parameter, unfortunately that is not true. In order to maintain the load carrying capacity, a tire needs to go wider as it goes lower - and the net effect is only a small difference in sidewall height.

How about going to a larger rim diameter? Yup, that reduces the sidewall height, but the width increase is much greater - and this starts to get into an interesting problem.

As I change the aspect ratio - keeping the load carrying capacity the same, and therefore go wider - the tread width goes up as well - as does the belt widths. This means there is more of the most expensive part of the tire - and therefore more cost for the same load carrying capacity.

So what about sidewall stiffness? Not only does sidewall height have an effect, but so does inflation pressure. In larger trucks, the inflation pressure is pretty high, so perhaps the reason big trucks don't have sway issues is that the inflation pressure used stiffens the tire well beyond the point where it matters.

And one last point: I wonder if because the loads on big trucks are carried so high, that the limiting factor is the track width - and because of that, the sway issue is minor compared to the "tip-over" issue.

OK, one more last thought: If I wanted to reduce sway, one way would be to lower the center of gravity. In the case of big trucks, there are literally thousands of loading docks already set at a certain height and the trailers would have to be able to accommodate this. And don't forget that the common practice is to use the same tires on the trailer as on the drive unit.

So I'm thinking that what appears to be a simple change to a lower profile tire, becomes a huge change with very little benefit.

[Denis4x4]

The diameter of the tire/wheel combination is directly related to the final drive ratio of the TV. I noticed that the Grand Cherokee uses an 18" wheel on vehicles with the factory tow package vs. a 20" wheel for those without the tow package

Recently went through this exercise with a Model A and made several rear end gear changes to match the tire sizes and RPM's for max effeciency.