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Discussion Starter · #1 ·
I have been thinking of going from the stock BFG 32’1” tires to 35” and in my reading on the forums I keep running into often repeated information that 1lb of unsprung weight is equal to 10lbs of sprung weight. Using that notion, if my new wheels and tires add 10lbs of weight at each corner, that would be a total of 40lbs of added unsprung weight which would equate to the equivalent of carrying 2 substantial people around all the time weighing in at 400lbs. This formula crops up all the time, is repeated as gospel as it gets into more and more threads and seems to me to be simplistic and completely bogus.

My notes from reading about this stuff are below (disclaimer - I am not an engineer – I would be interested to hear from anyone who is!). They are a bit long winded – but I do think the 1lb : 10lb rule needs some serious debunking.

As a rule, lighter wheels will allow the suspension to respond more quickly to the terrain. In street driving they will hop less, allow the suspension to react more quickly and maintain contact with the road more effectively. Heavier wheels will bounce more and for longer over bumps, potentially impacting ride quality.

Increased rotational mass of larger wheels and tires will slow acceleration and affect mpg. (Mpg will be more impacted in lots of start / stop driving than on the highway – so how badly mpg is impacted will depend on the amount of highway vs city driving). Weight of the tires is more important than weight of the wheels when thinking about rotational mass – the weight at the edge of the circle is more difficult to turn (has more leverage) than weight nearer the hub. So any increase in tire weight is more significant pound for pound than any increase in wheel weight. Rotational mass also has a bearing on braking – stopping that increased rotational mass is harder.

That said – the Jeep still weighs 4-5,000lbs so while a lighter tire / wheel combination would accelerate noticeably faster if jacked up in the air, it all changes when you put the Jeep on the ground and factor in the huge mass it has to start moving – the ratio of unsprung to sprung weight has not changed that much even with the larger wheels and tires. Ten pounds of additional weight at each corner of a lightweight Lotus Elise (1,950lbs curb weight) would be more obviously detrimental to acceleration than on a four door JK loaded with steel bumpers, winch and recovery gear etc. Right there you can tell there is no one size fits all rule of thumb that will work when considering unsprung weight.

A bigger impact in terms of acceleration is not the wheel weight, but the increased circumference of the tire which will impact the gearing – a bigger wheel will make for a taller gear ratio. There is no getting around this without re-gearing the final drive. So going to 35” tires from stock will impact acceleration from a stoplight and hill climbing on high elevation mountain passes. (It will also increase speed / change engine braking in 4wd low rock crawling).

So - just as I am thinking well, the suspension does not have to react that quickly in a slow moving rock crawling situation and I can go and buy those Hutchinson beadlocks at 48lbs a corner and add some BFG KM2s at 68lbs, there is also the wear and tear aspect (and increased potential for bending something) of having all that additional mass bouncing up and down at the end of the axles. The stock wheel/tire on a Rubicon is a 26lb Moab wheel plus BFG KO 46.4lbs = 72 lbs. That Hutchinson combo would be 116lbs – an increase of 44lbs a corner or 176lbs total unsprung weight. While I absolutely do not believe the formula of 10:1 cited above, (that would mean the equivalent added weight would be 1,760lbs!) there is no getting around adding 176lbs of unsprung weight has to impact wear and tear and affect street handling.

My conclusion? The 1lb : 10lbs formula is bogus and not a reason to get hung up on lighter wheels and tires. It may be nearer 1lb : 2lbs but whatever that ratio is (and despite the fact the large sprung mass of the JK will diminish the effects on acceleration between different wheel / tire combos) it is still smart to go with lighter components for overall wear and tear, braking, and grip in on road situations. If our Jeep was going to be driven mostly off road, then I might approach things differently, but I’ll be looking at lighter alloy wheels (probably AEV Pintlers at around 26lbs) and good all round tires that have some siping for icy winter roads and less weight – Duratracs at 60lbs are looking promising. Those Hutchinson beadlocks are some of the best looking wheels out there but…

I have driven Lotus cars and the founder, Colin Chapman, was famous for saying “just add lightness” to increase performance rather than adding bigger engines to get more hp with the knock on effect of bigger, heavier components in the drive train, brakes, tires etc. I think the physics behind this outlook are just as applicable in the off road world. I believe I may have got it wrong with some of my Jeeping mods – my steel bumpers and heavy recovery gear would be less necessary if I just “added lightness.” I perhaps should have looked longer at the aluminum offerings. Look at the Jeep Stitch as an extreme example – it performs spectacularly. Paying more attention to weight rather than beefing everything up might help ride height, improve suspension performance, acceleration and braking and get us stuck less often!

Some guys at the BMW forum got into debating the physics/math on unsprung weight here:
Reducing Rotating Mass: Ditching Dead Weight

OK – enough of my amateur ruminations – I am keen to hear where I may have got it all wrong… ☺
 

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Discussion Starter · #3 ·
Yeah - I am a bit long winded sometimes - but there is a lot to think about if you are not someone who just bolts things on and rolls with it.
 

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Did you buy 35's yet since you just justified it to yourself (And everyone else?) lol

My biggest issue with tire/wheel weight is the added stress on axles and tailgate.
 

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Before upgrading the wheels and tires, what is the need, or is it a want? I won't be upgrading anything, unless its needed, about the only thing I see missing on the Rubicon is adequate skid plates.
 

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Discussion Starter · #7 ·
Before upgrading the wheels and tires, what is the need, or is it a want? I won't be upgrading anything, unless its needed, about the only thing I see missing on the Rubicon is adequate skid plates.
You are probably right - our Rubicon was very capable out of the box but I added a modest 2.5 AEV lift with geometry correction brackets to maintain handling. Our tires at two years old are now at the point of needing replacement. Also we just came back from Moab Easter Jeep Safari - and that always makes you want to tinker :)

No I did not buy the wheels and tires yet. We have the Teraflex tailgate hinge which looked to me to be the most elegant solution to the spare wheel thing if you ever go to bigger wheels.

The truck just dropped off my rear bumper - Easter Safari purchase - so I am going to go attach that (more weight!) before I go buying wheels and tires :)
 

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Discussion Starter · #8 ·
Was there a question in all that madness?
Yes - is that madness sound reasoning or is it flawed and there is a solid counter argument? My basic premise was that the rule of thumb about unsprung vs sprung weight is a load of hogwash.
 

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It's been a while since I did any dynamics work, but here's what I came up with:
Adding 1 lb of rotational mass requires the same energy to accelerate to a given speed as adding 1.013 lb of non-rotational mass*

*Assuming the mass of the wheel is evenly distributed

It turns out the diameter of the wheel is not actually a factor, as the terms cancel in the equations.
 

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It's been a while since I did any dynamics work, but here's what I came up with:
Adding 1 lb of rotational mass requires the same energy to accelerate to a given speed as adding 1.013 lb of non-rotational mass*

*Assuming the mass of the wheel is evenly distributed

It turns out the diameter of the wheel is not actually a factor, as the terms cancel in the equations.
Very interesting! I have argued many times that bigger tires are NOT the massive inertial issue that so many claim; and it appears you just proved it.
 

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I went through this whole thing recently too. I raced motorcycles and it's hard to get out of the frame of mind that you need to find the lightest possible combination. I've had fast cars as well and getting rid of 10lbs a corner does amazing things for your acceleration and braking...so initially i was set out to go lighter than stock, but after more reading realized for my Jeep it was a silly proposition and needless.

With that said I came to the conclusion that for me staying as close to the stock wheel/tire weight was desirable for obvious reasons, wear and tear, MPG, acceleration etc...

I didn't understand why a lot of jeepers were running around with E rated tires either...adds so much unnecessary weight to a vehicle that could never possible come close to the load rating. Yes I know, several people will say the are tougher, etc...and maybe in some special applications you could make an argument for it, but in the real world I just don't see it.

I researched for a solid month before I purchased my set up. At first the 15" route seemed a no brainer because the wheel and tire combo's are so light, however for me the backspacing required to clear stock components just sets the tire too far out from the Jeep causing even more wear on suspension. Tires for 16" wheels are almost all D and E rated so I settled on 17".

I ended up with procomp 7069's 17X9 and 285/70/17 Toyo AT2 in a passenger tire vs light truck. Even at the passenger tire rating the AT2's load rating are 2800lbs a tire...still overkill for the jeep, but in a 285/70/17 they weighed 48lbs...

The same tire is available in an E rated tire...that weighs 56lbs. Why add 8lbs of unnecessary weight for a rougher ride and more wear and tear on components.

In the end my stock setup on my scale at home was 68.8lbs. My new setup with larger sidewall and width was 69.8lbs. Very happy, took a lot of research, but for me it was well worth it.





 

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Before I got back into Jeeps, I owned a '04 F150 FX4. For it I had a set of winter wheels and tires (stock wheels, stock tire size of 275/75/18) and a set of 20x10 Ford Racing wheels shod with 305/50/20 all season performance tires for summer use. Overall height on both sets were ~32".

The summer set with the larger wheels were a tad heavier and acceleration, braking, and fuel economy were noticeably worse than with the stockers. They actually seemed to act like flywheels on the road. However, since the summer tires were low-profile, the handling improved. I wouldn't say I would have autocrossed it or anything, but you could tear up an on-ramp. And they sure looked cool.

But for me, an off-road vehicle benefits from a tire with a taller sidewall. Less chance of destroying a wheel out in the bush.
 

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Discussion Starter · #13 ·
Thanks Skippy77 - I'll take a look at that setup. I would think E rated tires might flex less around rocks with their stiffer sidewall when aired down off road (although I am still relatively new to this and may be wrong) so I was rejecting those for that reason as well.
 

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Here's my oppinion... none of us are racing F1 and the percentages we are talking about are silly. When changing tire/wheels, you are changing final drive ratio and total weight. Period. Where the weight is, is of miniscule importance.

In extreme cases maybe even changing aerodynamics.
 

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Thanks Skippy77 - I'll take a look at that setup. I would think E rated tires might flex less around rocks with their stiffer sidewall when aired down off road (although I am still relatively new to this and may be wrong) so I was rejecting those for that reason as well.
Like I said I think E rated may have applications for jeeping, but for the majority of us it's extremely unnecessary. Good luck with your search. I found 3 or 4 good A/T tires in passenger rated vs LT. Check out treaddepot.com I ordered from their and they have weights listed for everything.
 

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Here's my oppinion... none of us are racing F1 and the percentages we are talking about are silly. When changing tire/wheels, you are changing final drive ratio and total weight. Period. Where the weight is, is of miniscule importance.

In extreme cases maybe even changing aerodynamics.
I agree, the biggest factors will be aero, rolling resistance, and a different final dirve ratio. Taller tires also means the brake rotors need to generate more torque for the same stopping distance even if the tires/wheels weigh the same.
 

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Discussion Starter · #17 ·
Here's my oppinion... none of us are racing F1 and the percentages we are talking about are silly. When changing tire/wheels, you are changing final drive ratio and total weight. Period. Where the weight is, is of miniscule importance.

In extreme cases maybe even changing aerodynamics.
You may be right - but I tend to think that Hutchinson set up I wrote in my too long first post would affect street manners, mpg and maintenance more than Pintlers and the lighter Duratracs with a similar final drive ratio.

But in any case - it seems everyone so far is agreed you don't follow that 10 / 1 rule of thumb about adding unsprung weight because then you really would be talking about a huge difference between those setups.
 

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I will say this about stiff sidewalls. I have military tires designed for HMV's and there is no such thing as a sweet spot for inflation due to the fact that my Jeep weighs half the HMV. At 22 psi with my setup, I get perfect flat contact but corning on pavement is squirrely. At 35 psi they handle great but am going to wear out the center of the tread prematurely.
 

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I'm just sitting here laughing...... my 37x13.5x17 are 93 lbs plus 30 lbs for the wheel :)
I don't know for sure, but I'm closer to 150 with my double bead locks and 37's!! And I notice the final drive ratio of course, but once it's regeared I don't expect my 600 lbs of rolling rubber will make one damn bit of difference compared to someone with 200 lb shoes and a fat passenger
 
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