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Center of Gravity & Weight Distribution

Reprinted with Permission:

Weight Distribution the Hidden Advantage 

  Have you ever been on a trail ride that included some sort of up-hill obstacle(s) that really seemed to give your vehicle fits - with front tires in the air, rears struggling for enough traction, and maybe some white-knuckle-inducing pitching and leaning?  And worse yet others just walked right up?  These differing performances on the same obstacle can seem even more puzzling when the vehicles are nearly identical in all seemingly relevant specs from tire size to suspension lift, etc.


  If the difference is dramatic - such as one rig requiring a winch while the other doesn't even slip a tire, a lot of 'trailside engineering' might take place:  someone will blame the tires; the guy that walked right up will blame his friend's choice of lift kit, someone will joke about his funny paint job, and the driver will often want to blame the spotter or himself for missing the line.  In addition there could be talk about tire pressures, gear ratios, spring rates, locker type or usage, the list of possibilities is so long and yet one of the most likely causes is rarely mentioned:  weight distribution (I'll call it WD for short).  I'm amazed how many supposedly seasoned off-roaders still struggle with the consequences even after decades of driving and multiple vehicles, yet they still get themselves frustrated and/or embarrassed when they can't conquer an obstacle because they still have the same problem!


  It's also important to understand that WD affects much more than just climbing a nasty obstacle, bad WD can affect your on-road handling making your rig unsafe in emergency maneuvers - and it affects other off-road aspects such as traction in loose sand (if one end is much heavier it is more likely to get stuck, miring the whole vehicle), and even while descending tricky terrain (causing 'endo's or dangerous sideways slips, etc.).


  The problem with weight distribution is that it's not very tangible nor easy to separate from other factors and most people don't think enough about it because it's not tied to a conscious choice to be made like what tire to buy, it just 'happens to you' (your rig, actually, that beer gut is a whole different weight distribution problem!).  In addition to usually not making specific choices about weight and were it should be, it's also hard to 'see' unless you have a very good grasp of what each item weighs, which most of us don't.  So while you can do a quick visual assessment of your vehicle and guess if it's 'heavy in the rear', etc. the only way you're really going to know is to actually go to a scale and weigh your rig.  When you do this, be sure to get a weight for the front and for the rear separately so you can tell 'where the weight is' - but before you rush off, read on.


  You've probably heard of the term 'center of gravity' (Cg), but do you really know what it means?  The textbook definition is a little hard to digest, so I'll put it this way:  Cg is the single imaginary point within your vehicle where any push or pull will only move it, without causing it to spin or rotate, but since almost all forces on your rig except gravity occur somewhere 'around the edges' (usually at the tire-to-ground contact patches) it's the 'spin or rotate' part that causes the problems.  If your rig was a uniform hunk of material - like a brick for instance - the Cg would be right in the middle, but that's not the case for your rig, and it gets shifted according to the relative weight and position of every item you add to your rig.  It's probably still somewhere near the middle, but a fairly subtle shift can have major effects, and that's the tricky part!


  You may have also heard that the higher your Cg is, the less stable your vehicle is.  That's true, and longer wheelbase and wider track width can restore some stability for a given Cg height, but they still don't mean you can put weight anywhere you want.  All forms of racing including competitive rock crawling have learned and proved that a low, centrally located Cg is desirable, but what's not readily apparent is exactly where that Cg should be to get the best results possible, that is what we'll talk about here.


  If your rig winds up with tires in the air at times when it's not expected - or when others don't over the same spot - it may be suspension-related (bad geometry or wrong spring rates), but it may just be your suspension showing you that it can't cope with where you've put the weight you've added your rig.  Most rigs started out in stock form with a pretty decent WD (50/50 on Wranglers and similar vehicles, and slightly front-heavy on most other vehicles) - so take a look at the things you've added to your rig either permanently (like bumpers, tire carriers, etc.) or semi-permanently (like tools and spare parts).


  Because most vehicles have most of their empty room for cargo at the back - roughly over the rear axle - this is where you've probably added the most weight, which shifts the Cg rearward.  Also, the most popular place for large items like spare tires on most SUVs is external - hanging off the rear bumper via some beefy tire carrier (which itself tends to be heavy to handle the bulk of a matching oversize tire) these items shift the weight even more dramatically and actually take weight off of the front tires.  Adding weight up high - such as on a rooftop carrier - is even worse yet, because it reduces your on-road stability by moving your Cg up (and usually rearward) and the higher your Cg is, the more weight you loose off the front tires when pointed up a steep obstacle.


  To test for WD problems there are several things you can do:  The most direct is on the trail: If you have plenty of time at a troublesome obstacle, try repeating the obstacle (same line, etc.) with as much weight removed from the vehicle as possible (especially things that are high or behind the rear tires - just stack things off to the side for the test).  If you now make it with ease, you're on to something and if your friends are really patient, you can try adding things back (maybe even in different places if that's easy to do) to get a better picture of what matters and what doesn't.  Conversely, you can also try removing/adding weight to the front (this may not be practical to try on the trail) - such as removing your winch, etc.  Weight at the front usually is good weight - as it helps maintain traction so the front tires can contribute to pulling you up and over a steep obstacle.


  You can also do some 'exercises' at home to educate yourself on WD and study the actual on-trail problem: Weight Transfer (WT) - which is the dynamic effect of WD.   You will need to take some careful measurements of your rig's ride height - but make sure they're measurements right over the axles (such as frame-to-axle tube or whatever you can measure easily).


  Start this test by leaving all of the stuff on your rig; measure the heights while parked on flat ground and then while parked on a slope.  Notice how the measurements change when parked pointed uphill on the slope - the rear gets lower and the front gets higher, but how much is 'okay'? That’s the hard part to answer because it depends on so many things.  


  To find out how much your WD is influencing things, add/remove items and see how the measurements change - you're shooting for the least amount of change on the slope vs. flat ground as you can manage (it will never be zero).  You might be stunned at how much that spare tire on the back lowers the rear, but you'll see that it also raises the front - this is not what you want for maximum trail performance! (Other items will also make dramatic changes, such as heavy things on the roof).


  To appreciate the numbers, you need to understand how much weight is represented in the height changes, this depends on your vehicle's spring rates - which you may or may not know, but you can still get an idea of your WD/WT problem even if you don't know them.  As an example, if your front springs are 150 lbs/in each, then a 1.0" rise means that 300# (remember you have two front springs) was shifted off of the front axle.  To put this in trail-performance perspective: if the front of your rig weighs 2200# (a typical trail-ready Wrangler) and raises one inch as in this example, you've moved about 13.6% of your traction-making weight off of the front tires - that's a LOT!


  It's hard to give a 'good/bad threshold' for what's acceptable or not since it depends on how steep the slope is, how long your wheelbase is, etc., but keep this in mind:  every fraction of an inch that the rear goes down or the front goes up means that weight has shifted off of the front tires and onto the rear ones, this means less traction on the front tires and (quite possibly) too much load on your rear springs - overwhelming their capacity and causing poor stability that makes uneven climbs into heart-pounder’s.  If your WT is extreme, it can actually 'top out' your front shocks (or in extreme cases even bottom-out the rears), leaving you with no available wheel travel.   That means no ability to keep the tires in contact with the (uneven) ground and if they're not touching, they're not helping!


  Excessive WT also means that your rear tires are where all of your traction needs to be - but if the ground there is loose and/or uneven, you may need more traction than what's available for each tire, making a locker, etc. necessary (or maybe even still not enough) while your buddy's 'low-WT' rig can walk right up easily.


  So what to do about your tail-heavy rig?  The answer depends on what you've already done to make it tail-heavy, and the basic answer is to 'undo' as much as you can - but since you probably don't want to leave a bunch of things at home, you can compensate/minimize the WD/WT problem by moving things around.  Mounting heavy items that were on the back or roof to the front of your rig isn't easy, but it will help a lot.  For instance, if you can mount your hi-lift jack to the front bumper, it will go a long ways toward 'canceling' that heavy spare that needs to stay on the back.  If you have no winch on the front but have a full-size spare already, then adding the winch will help as well.


  Avoid placing anything up higher than the 'belt line' (windowsill height), and never use a spare tire carrier that holds the tire away from the body to make room for jerry cans, etc. as this exaggerates the WT effect of the spare.  If you have a real WD/WT problem, you will find after some experimenting that you can make a dramatic difference in your trail performance by correcting your WD problems and your on-road handling and fun-to-drive factor will improve as well!

Jim Frens


Nth Degree Mobility 

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