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… ☺