DieselD

Yea and then those 35 x 10.50 pizza cutters are gonna be sinking and slipping... 35x12.50 would be a happier medium...

Also... I'm not always concerned with HP, more so torque. Specifically low end torque. Now I'm not saying there wasn't a performance loss in my 91 with 31x10.50s but there was more loss (butt dyno) at the high end, as it still would jump off the line, so to speak. It had 3.73s. My dads has 3.55s and 33x12.50s again loss is more apparent at higher rpm, a lil laggy but still has jump from a stand still.

holycaveman

According to what I have read torque is noticed most with going with a heavier/larger tires. However gearing can compensate some for "launch"

I am a big fan of pizza cutters. For what I do they work much better than wide tires. But each of us have different terrain we drive on. My current tires are only 7.5in tread. They work awesome.

DieselD

Yea I'm on stockers on the DD maybe 7" wide lol. And yeah different terrain for sure... I'm in Indiana.... mostly mud... and any rocks are usually wet or mud covered... need that wide tire!

I think were kind of agreeing to disagree!?!?!?

I'd like to see formulas though would be interesting to know power loss in 33x12.50 vs 35x12.50 that's the tires sizes I'm I'm between for the offroader rig... numbers would be cool to know I'm just going on personal experience and butt dyno...

holycaveman

Yea we have our preferences. LOL. I would like to get over to badlands or haspin acres one if these days.

I will try and get the math equations tonight.

holycaveman

I found one formula for this. But it is so involved I am not even going to post it.

But I have found a few things that will work instead. The general common knowledge is that 1 lb savings of rotational mass unsprung is equal to 4lbs of sprung mass. Now this is important. This does not even take into consideration the distance the weight is from the wheel. Which would have to be figured on a percentage/distance basis.

So use this quote with the 4-1 just to get a basic idea.
So lets try it?

A cherokee roughly weighs 3500lbs and makes 230hp(just for example)

So we have a power to weight of. 15.21lbs per horse power.

So if we save say 100lbs of wheel/tire weight. x by 4 = 400lbs.
the new power to weight is 13.47lbs/hp.

divide the current weight by the new power to weight and you get 259.83hp a difference of 30hp. But if we had a way to figure tire height/weight it would be even more.

So in my first example of stock tires compared to 40's. You are looking at a difference of 300lbs of rotation mass. Or 1200 lbs of weight!

A 10-1 power to weight vs the standard15.21. so going to 40's would be a minimal loss of 120hp. So my hillbilly guess at first was pretty darn close! LOL

holycaveman

Good question. Lets figure it out!

Stock tire weight? Roughly 29lbs

My 32" 10ply tires. 46lbs.

difference of 17lbs per tires = 68lbs. 68 x 4 = 272lbs weight

Stock power to weight is 15.21lbs/hp stock weight 3500 - 272 = 3228lbs

3228 / 230hp stock = 14.03lbs/hp 3500lbs / 14.03= 249.5 hp - 230hp = 19.5 hp loss

Sounds about right. And well worth the trade!!

holycaveman

So taking an average of the two extremes. If you don't want to do the math.

Just pretty much figure for every literal 3lbs of rotational weight you add, you lose one hp.

holycaveman

And just for kicks I want to do my buddies yota with 37in rockers. We wheeled the other day and this one high wall he had no chance of making because of no power. Now he is locked and pretty decked out actually except for the 22r that puts out 105hp.

Weight is about the same as a cherokee. well around 3100 lbs.

Stock yota has a power to weight at 29.52lbs/hp

Additional tire weight, Stock = 29lbs, 37" rockers 88lbs. additional weight 59lbs per tire 236lbs rotational. If we simply divide by 3 = 78hp loss! LOL

Well no wonder he could not make the hill! He also doesn't pull the speed limit that well either.

Now this doesn't mean he only has 27hp. He still actually has 105hp.
But the power to weight is drastically changed.

So now instead of the 29.52lbs/hp stock. he has 38.51lbs/hp. Gearing can still get him going fine. Just this truck can ONLY crawl. Its weak as crap.

But I told him that! 105hp is not up to turning 37" pitt bull rockers. I say that and there are places he goes that gives me fits

mr white

That calculation also does not factor in friction of the larger contact patch.
My wheel/tire combos are 100lbs each. Did I lose power? Yeah. Noticably with stock gears? Yeah. Lighter tires were faster. But these are SIGNIFICANTLY better offroad. 4.88s spin them just fine. Your buddy could have made it up the same hill crawling if his tires were worth a ****. Lockers? Line? There are many other factors. What you constantly seem to do is ignore multiple factors in favor of the one that justifies your rig, you did it with the locker thread, and now this one. Just saying...
You also have to factor in other things like ground clearance. More tire = more clearance under the diffs. The ability to spin your tires doesn't get you anywhere if you are hung up on your diffs.

91 limited owner

Oh! I believe it was a worth while trade. And I believe re-gearing is vital with the larger tires. So with that hp loss, you might as well include the fact of the added overall gross vehicle weight. like the the after market bumpers, rock sliders, suspension parts, Wheels & rims, heavier spare tire. And what ever else your carry in the back (needed, for a wheeling adventure)
And the end results are: A greatly increased gross vehicle weight, with an apparent loss of horse power hitting the ground, with extremely worse fuel efficiency. But dam it, they sure look cool. This is the reason I do most of my trail riding on my dirt bikes!

holycaveman

Yea, thats what I came from. Have raced for years. Everything gets old after a while. With the jeep its new found excitement for me.

holycaveman

LOL No he could not have. I don't think your grasping what I am talking about as far as hills go. This is not some rutted steep hill. In all honestly you probably have never attempted a hill like this. You don't "climb" it. Its impossible! You don't build up speed and motor up it, " its impossible" you have to shoot it. You have to build up a lot of momentum in a short distance. Enough to defy gravity until you can let off and drop over the top. if you keep on the power you will roll your truck, its just too steep to power up. Used to climb these high walls on dirt bikes all the time. same thing with my xj except you have know when you can or can't do a hill. That or find out the hard way.

As far as my buddies truck. Its sits very low on 37" pit bulls. Locked front and rear. Articulates very well. Twin stick transfer, custom front cab and rear bed for approach angles. He has a sign on the back that says "don't follow this truck, you won't make it"

If you are running a stock xj motor with huge tires I doubt you are going to make it. I don't make the rules, your motor does. If there was a longer run at it then maybe but you would really be risking it.

Anyway all this doesn't mean a thing. You have to be happy and build your rig for you and no one else. So If I wheel with you one day mr white, don't be offended If i have a sign in the back that says "don't follow this truck, you won't make it"

holycaveman

Also mr white I am not preaching against large tires. I am just saying to look at everything and how you will be wheeling before you go build your rig. There is a lot to consider. Or if you want to change a few things, consider everything.