The tires would make a huge effect on towing just from the loss of effective gear and now you are wasting power and torque to turn those big heavy wheels. Not to mention that the computer is not calibrated for that size (unless he has a tuner) so he may not be getting the most out of his truck especially when you hook onto a load.

 

Don't you live in VA or something? The elevation in the valley here is about 4500ft. I probably topped out at 7500ft on my tow. And I gained this elevation very quickly. Altitude makes a huge difference. Less oxygen equals less fuel and less power.

As for the tires, my stockers were 32.1" and my new ones 35", so I am effectively at 3.42. 4.06 would put me back to stock. Not a huge difference when you consider I have a six speed. However rolling resistance is still unaccounted for. The main reason I bring up the gearing again is that many f150s are sold with 3.55s. Some even 3.31 and 3.15.

I think my main reason for my lack of power is...well lack of power. My effective gearing is reduced but not immensely. I think running stock tires would have yielded similar results. I ran 35" Toyos on my diesel and it pulled more weight waaaay faster. Hardly ever downshifted. Now I don't expect diesel like performance out of my lil gasser. Everyday someone asks if the F150 would make a suitable tow vehicle for their 8k# camper. On most roads it would, but if you live in the west you probably camp in the mountains. If I towed more often I would be extremely dissastisified. Not because the 5.4 is bad but there is just so much more available...diesels, V10(for now), 400hp half-tons...

 

If I towed regularly in the Rockies I also would have an F250 with a V-10 or diesel. That's just a little too much for the 5.4L to do regularly. As to the lack of power, I would definately have to agree with most that the rolling resistance and change in diameter are factors. But even with stock tires it would be working hard on the longer steeper grades I've seen in the west. 4.10's would be a better coice for gears out west for a stock F150.

 

35's just arent that big of a tire though. ive got several friends that tow with 40's on theirs and dont have the problems he is talking about. his footprint also comes from the tire width, not the height. if his 35's are roughly the same width as his stock tires then his footprint/rolling resistance is not going to change much, if any.

 

Do your friends have 40's on an 09 F150? If they are on a diesel of course they won't have as much of a problem...there is waaaay more torque available. 35's will have an effect on a 5.4 or any other smaller V-8...that is why most people re-gear...trying to get the "pep" back. I would bet they are 3 maybe more inches wider than stock.

 

a couple of them have diesels, but several others dont. one of them is running an obs f150 with 12" lift and 40's with a 5.0. he's also got a bronco with 12" lift and 40's with a 5.8. he had to regear because he went with such big tires that it cut his ratios down to the low 2's. with his regear he's back up to about a 3.55. and thats my point. broncobran's ratio is still at about a 3.40. he hasnt lost enough of his gearing that he should require a regear. all the other non diesel ones are on 70's model f150s with 302/351's and a few 97'ish 5.4's.

 

Understatement of the year I think. 3500 rpm's is nothing for a 5.4 liter.

Rob

 

I would like to throw my few sense out there...

tires do make a big difference in towing, but it is not from rolling resistance AT ALL... its all from the wieght, diameter, and the amount of AIR that has to be pushed because of width.

Its simple physics (College Student at Colorado School of Mines). The friction coefficient is not enough to even notice when you guys are talking about rolling resistance. The main deal is the weight... I even in high School designed an experiment, because I got in an argument with my teacher about a wider slick would slow me down... Turns out we did a few tests and he taught me that the contact area does not matter, but rather the weight... Take for instance... a rubber insole for a shoe. Take one, pull it across a surface. Then take it and cut it in half and see what force it takes this time... They are the same... Do the same with some weight and you will see the difference.

And the 5.4 3400 is nothing for it during a long period of time... they are high rev motors, not quite as high as a 4.6. They do not start making peak torque until around 3000 IIRC, and peak horse in the truck is around 4600 RPM... They are made to rev.

 

Wow, that's an astounding amount of physics fail.

Moving air is nothing. Rolling resistance is indeed a serious factor. Tires exhibit hysteresis - repeated cycles of deformation and recovery as the tire rotates. Tires, as with most deformable materials, dissipate considerably more energy during deformation than they produce during recovery (second law of thermodynamics). This is rolling resistance, which is dissipated as heat, and is thus energy lost.

So how do we deal with this? We can decrease the unit cost of each deformation. We can increase tire pressure and/or stiffen the sidewalls - but this causes harsh ride, and overinflation can result in an increase in friction which actually works against us. We can change dimensions - wider tires distribute the load further and thus reduce sidewall flex, but this can cause increased air resistance. We can use better materials that have a smaller discrepancy between energy consumed and energy returned. And other tricks, of course.

Ever wonder why trains use steel wheels rolling along steel rail? The coefficient of friction for a steel wheel rolling along steel CWR is in the vicinity of 0.001, where car tires on asphalt are in the vicinity of 0.3. When you consider that the force of rolling resistance Frr is given as Frr=CrrNf where Crr is the constant given above and Nf is the normal force, it's apparent that the force required to overcome rolling resistance increases linearly as the coefficient. Thus, a train requires 0.33% the force to overcome rolling resistance as a car per wheel.

As for your assertion of weight - the normal force takes this into account.

 

Big tires murder towing. tvsjr explained it well. Those big tires just suck up power to keep them rolling. I noticed a difference going from a P265/70R17 Baja AT POS tire that maybe measured 30 inches and was very light to my LT265/70-17 BFG ATs that are around 32.5 and pretty heavy.

The engine has to work harder, it's slower, and the fuel mileage is worse.

Mike

Mike

 

I like your explaination much better than even my phyisics teachers last year... The problem with that class, was that it was so simple we did not take into account of the energy lost and all the other variables. I can understand where you guys get the rolling resistance now. I appricate the clarifcation...

It sure is going to be nice taking an Advanced physics class next year... This year its all general ed classes at CSM as I am a freshman... But once I can get into my major it will be much more fun!

Thanks

 

So tvsjr, would you say that at a given speed it takes a fixed amount of horsepower to turn your wheels? And that any increase in weight would linearly increase hp loss from rolling resistance, while power loss from the wheels' inertia would remain constant regardless of vehicle weight at constant speed? And
that hp loss would increase exponentially with velocity, similar to air resistance?

So the question is which has the larger effect on power loss. A heavy wheel and tire combo with low rolling resistance? Or a light wheel and tire combo with high rolling resistance. Say 35x12.50r24 vs. 35x12.50r15.

 

Just a hint - be careful about pontificating and showing off "qualifications" (like being a college student), especially in worldwide forums. It's great you're working on your degree - but you have just enough knowledge to be dangerous at this point! :lol:

I don't claim to be a physicist myself, but I have a fair background.

Well, I think you're oversimplifying a bit. Yes, assuming everything is held constant, a constant amount of power should be required to maintain steady-state cruise. However, we're talking about wheels and tires here... your question could involve changes in air resistance, elevation, driveline losses, momentary losses of traction. There are hundreds of variables that play a part in this system.

Considering the tire system and rolling resistance, yes, an increase in weight would cause the normal force and thus the force consumed by rolling resistance to increase linearly. Unless I'm missing something, I would think that a heavy wheel/tire combination would result in a fixed parasitic loss - I don't feel this loss would increase with extra weight.

I don't think the horsepower consumed due to rolling resistance will increase exponentially, however, since it has different properties from aerodynamic drag. Horsepower is a measure of work done over time. The work to overcome rolling resistance is constant, thus the HP loss is constant. There is some component involving speed, however... as speeds increase, the tire heats up, which causes additional sidewall flex.

I believe you'd be better off with a heavier wheel/tire combo with a lower rolling resistance. But, as I said above, I'm not a physicist, and we're getting on the ragged edge of my knowledge here.

All this being said, rolling resistance typically accounts for 1-2% of total loss... if you really want to improve towing performance, minimize aerodynamic drag, which increases linearly with the exposed surface area and exponentially with vehicle speed!

 

You bring up some good points other than just the tires. Everything that goes with big tires takes away from performance. Effectively lowering your gear ratio, more rolling resistance, much more drag from the tires and wind resistance.

Now I'm not a physics expert, but the closer you keep the weight to the shaft, the easier it will be to turn. So a heavy wheel and light tire should be easier to turn than a light wheel and heavy tire. I think it has to do with the moment of inertia or something like that, but I didn't pay much attention in high school physics. (Although I really, really wish I had looking back.)

Mike

 

Here's a question if big tires kill towing because of gearing and weight issues how would getting the smallest wheel and low profile tire do to towing? Assuming that you could find a low profile tire that is still rated high enough to safely haul 8000 lbs of course.