Unsprung Weight of a heavy LT tire

[Madtiger]

There was another thread on tires and there was some discussion about it. I want to create a thread on its own because i want to know more info on it. I am NOT talking about taller or wider tires. I want to focus ONLY on the consequences of a heavier tire.

My opinion (uneducated) is that a heavy LT tire will add considerable unsprung weight, which will affect handling, braking, and lead to premature wear & tear on suspension components, esp the shocks.

Opinions/facts?


EDIT: i drive like a grandma on the highway...so, handling and to a lesser extent braking are not that much of a concern for me. Wear & tear is a concern however.

 

[Madtiger]

I currently have BFG AT KO2 in stock size...love 'em! I do take my rig off-road. They are significantly heavier than stock tires. I worry somewhat because...

For example, Toyota had to re-tune their FOX suspension on the 2020 Tacoma TRD PRO because the 2020 Tacoma rims are each 4.8 lbs lighter. Imagine that...4.8 lbs difference (per rim) and Toyota had to re-turn the whole suspension.

Or in past 4runner models, there were different stock shocks depending on RIM SIZE!!

Or take the Edmunds’ comparo of Tacoma vs. Nissan vs. Ridgeline. The Tacoma’s and Nissan truck REAR shocks were leaking over washboards. The Tacoma’s were Bilstein’s mono-tubes. The front shocks on these trucks were OK i think and Ridgeline did fine. It is all about UN-sprung weight. Those rear heavy axles on these trucks put a lot of stress on any shocks, whether it be mono-tubes or twin-tubes. (Ridgeline escape unharmed because it has IFS and IRS.)

 

[Cruisin911]

From a purely engineering perspective, I would think that anything that deviates from stock would increase premature wear, as all components are designed and sized to a specification. From a realistic perspective, adding unsprung weight will likely have little wear impacts outside braking and handling. Of course, with a larger tire you have increased stopping distance, raised center mass, and greater handling impacts, to include slower acceleration (assuming no regear). As far as the shocks, I actually think with a larger sidewall, you may have less overall shock fatigue because the tire is absorbing some of those impacts, but that impact is likely negligible as when larger tires are added, that usually comes with upgraded shock/strut assemblies.

Overall, lifting and larger tires will negatively effect on road performance.

 

[Angelo1]

As a former road racing Cyclist, rotational weight is huge. My crew kept several sets of wheel/tire combos for various sections. It may seem odd to compare to vehicles but i believe it’s still valid. We would use a heavier combo for downhill and the lightest for climbing. I believe rotational weight is one of the key factors for saving energy, whether it’s your legs or fuel. However, weight is also important when the road turns to cobblestone for bikes and rough trails for vehicles. We always run a thicker, wider, heavier tire for the cobbles and road with lots of imperfections. For SuVs you have to be careful not going really light as that can cause other problems. You wouldn’t put light weight tires and uber low weight wheels if your beating up your mtn.bike as it was intended for. This is also why, proper gearing is important when running heavier wheel/tire combos.

 

[OregonLC]

Yeah, they’re heavy but that’s kind of the point. You take a tank into battle, not a bike. I haven’t weighed my 33’s on the 200, but the 295/70R18 on the 100 weighed about 95 lbs per corner. Porker on the road, brilliant on the trail.

 

[LBridges]

Not an automotive engineer. Just somethings I have reasoned out.

Unsprung weight increase is not desirable. Depending on how much is increased there can be significant impact to suspension compliance and reaction to high speed impacts (not road speed, but rate of suspension compression/rebound). The distance of added weight from the center line is what hurts gas mileage and adversely impacts braking/acceleration. Unless one wants to improve turn-in handling I fail to see the need for larger diameter wheels. I welcome corrections to my thinking here.

In the case of the 200 there are some things one can do to minimize the effect.
1. Reduce weight and move the weight closer to the center line. The metal rim of a wheel is almost always more weight than the rubber of the tire treads. So, use 17" Rock Warrior wheels. Metal is moved closer to the center line and weight is reduced.
2. Stay on passenger vice LT tires unless you need the sidewall strength. If you want a tougher sidewall, consider using something like C rated LT tires vice E - for less weight if that meets the need.

 

[Angelo1]

I average 12mpg with my 34.5x12.50/18 KO2. I would downsize to a 285/70/18 if my wheels didn’t have a 18+ offset as I was told by a knowledgeable member on here that going that size may not be great for my current setup.

 

[terdrocket]

I run LT285/75R17 Cooper AT3 XLT tires on Rock Warriors. I added the 1.25" Spidertrax spacers which gives me +18.25 overall offset.
It rides better than before the spacers.
Mild rub in reverse while turning.

 

[1world1love]

1. Reduce weight and move the weight closer to the center line. The metal rim of a wheel is almost always more weight than the rubber of the tire treads. So, use 17" Rock Warrior wheels. Metal is moved closer to the center line and weight is reduced.

Maybe I’m misunderstanding this, but I’m not sure it’s accurate. A RW weighs about 30lbs but a bfg KO2 in 275/70/17 weighs 54lbs.

Again, I could just be misunderstanding this.

 

[bloc]

Also keep in mind that a heavier LT tire in the same size as the lighter p-metric will put most of that extra mass in the very worst spot.. at the periphery. In that location it will impact any rotational changes of speed far more than if it were added at the hub.

Also whether or not the increased mass of a LT tire is brilliant on the trail depends on the trail. For low speed, yes.. mass isn’t much of a problem. But for high speed stuff like desert running unsprung weight is critical as it’s just more mass that the dampers have to control over every bump & dip.

When looking at this from the perspective of suspension wear, the closer to stock weight AND GEOMETRY you stay, the better. Things like hub spacers and different scrub radius add all kinds of different forces to the bearings, suspension links, etc.

Anything that negatively impacts mileage increases drivetrain wear. If you averaged 18mpg at one speed then switched tires and that number is now 14, you are simply requiring more horsepower to drive at a given speed. For every mile you drive. Every mile. I’d argue that people with significant mileage drops from tires (and bumper, rack, anything that increases rolling or wind resistance on the freeway) should be changing their oil more often than the minimum schedule. Or at the very least monitoring oil condition to develop a schedule with UOA. It’s not like you are hauling your max tow rating all the time, but it is added load.

Now.. how much will a stock sized LT tire wear things compared to stock P-metrics? I think that is really hard to answer. We are talking about some of the best engineered and most durable vehicles on the road. I don’t doubt that toyota would offer different valving for different rim/tire weights, because it is a very small cost for them to do that from the factory and NVH is a big deal on modern vehicles. I noticed a huge drop in ride quality from the ~75lb factory wheel/tire package to the ~90lb per corner weight of 285/65 LT-E KO2s plus stock rims. If I feel it, it’s having an impact on the suspension. Much better now with p-285/70/17 open country AT2s on rock warriors which is all within a pound or two of the stock setup, but maintains the larger diameter. At the end of the day they won’t be as durable as a LT-metric.. but it’s still a better setup for the way I travel and the trails I put this truck on.

 

[LBridges]

Maybe I’m misunderstanding this, but I’m not sure it’s accurate. A RE weighs about 30lbs but a bfg KO2 in 275/70/17 weighs 54lbs.

Again, I could just be misunderstanding this.

I was implying using the same outer diameter and brand/type tire.

Much of the weight for an AT or mud tire is in the tread. So a 33" tire for an 18" wheel will have nearly the same unsprung weight as a 33" tire for a 17" wheel. But the rim weight (not spokes) difference between a 17" forged wheel and an 18" cast wheel can be significant.

Again, I'm no engineer. This was the general philosophy (minus AT tires) that applied back when I ran an MR2 in local Autocross events. Willing to admit I've been carrying around "old wives tales" in my head.

 

[Madtiger]

Hmmmm...all of a sudden, those Toyota Heritage Ed BBS rims look good....

But damnit, i need sliders first.

 

[gaijin]

As others, I am no automotive engineer; and my logic is pretty simple:

Toyota specifically recommends LT285/70R17 BFG KO tires on 17" Rock Warrior wheels (17" TRD Wheel Data Sheet).
The engineering that went into our Land Cruisers is robust - proven daily all over the world.
With these two points in mind, I have no doubt that the additional stress of the LT285/70R17 / 17" TRD wheel package is well within the capabilities of the Land Cruiser suspension. Any additional wear/stress would most probably not be so significant as to cause dramatically premature failure - if any difference at all.

Certainly, no system is above the laws of physics. Any additional unsprung weight over stock will adversely affect braking, acceleration, mileage, and overall performance (e.g. emergency maneuvers). The extent to which these changes in performance influence one's subjective evaluation of the overall performance informs one's personal choice. For me, the benefits of increased tire durability, off-road traction/performance and overall vehicle capability outweigh the (for me minor) decreased performance.

Truer with this topic than most: YMMV

HTH

 

[1world1love]

My opinion on this is that while there can be no doubt that I sprung weight has an adverse effect in every possible way, I’m not sure if the effect is as profound as often stated. These vehicles are over engineered in so many ways and they are made to tolerate a range of uses and abuses.

Most of the factory specs are quite conservative and the factory setup is designed for maximum comfort and performance for 90% of the market.

Of course it’s not reasonable to expect to be able to use a vehicle the way many of us use them without some compromises, but I don’t think that the vehicles aren’t designed to handle extra unsprung weight.

 

[bloc]

One detail I forgot to mention: tire spring rate.

LT tires need to be run at a higher pressure for a given load capacity. This is because the extra layers and more bulky construction increase internal friction and would lead to too much heat if the tire pressure isn't higher.

That higher pressure changes how the tire/wheel responds to bumps, even if the weight were the exact same. It will transmit more of the road imperfections to the axle and suspension, which should be a surprise to no one.

Like the increased drag of LT tires, this is a 24/7 phenomenon. So not only have you increased the weight, you've forced your suspension to respond to more and more of the bumps because the tire itself is absorbing less of them.

 

[1world1love]

I was implying using the same outer diameter and brand/type tire.

Much of the weight for an AT or mud tire is in the tread. So a 33" tire for an 18" wheel will have nearly the same unsprung weight as a 33" tire for a 17" wheel. But the rim weight (not spokes) difference between a 17" forged wheel and an 18" cast wheel can be significant.

Again, I'm no engineer. This was the general philosophy (minus AT tires) that applied back when I ran an MR2 in local Autocross events. Willing to admit I've been carrying around "old wives tales" in my head.

Ok. I am operating under the same assumption then. On my LX I have 20” wheels with KO2s. The combined weight is probably 90lbs or more.

I am planning on moving to a 17” Icon that weighs ~27lbs and while a similar radius 17” KO2 weighs more than my current tire, my overall combined weight will be less.

This is your point, correct?

 

[LBridges]

Ok. I am operating under the same assumption then. On my LX I have 20” wheels with KO2s. The combined weight is probably 90lbs or more.

I am planning on moving to a 17” Icon that weighs ~27lbs and while a similar radius 17” KO2 weighs more than my current tire, my overall combined weight will be less.

This is your point, correct?

Yes

 

[OregonLC]

Not sure that the E rated mass is all at the periphery. It’s the gain in sidewall strength that I’m after for puncture resistance. True there may be an extra ply under the tread, but a lot of the weight is in the tread depth vs a highway tire. The tread weight is a constant for the same tire whether E or C.

Additionally, not many of us are racing. We want a tire that offers the best in traction and durability. An overlander’s performance metric is not getting stuck and not getting a flat. Getting to camp 10 minutes ahead of the other guy is a distant concern.

The merits of low unsprung weight are well documented in the automotive world and are without debate. The issue here is what is most important to your lifestyle. For me, job one is conquering the trail with the lowest reasonable risk of tire failure. This requires a substantial tire.

 

[TeCKis300]

As a previous road racer and autocrosser, unsprung weight was king. I remember the incredible agility and traction afforded with my 15.9lbs O.Z. Superleggera 17x8 race wheels, over my street 18" wheels. For sports cars where extreme acceleration, braking, and handling was the focus, that's where my priority was.

I still am very focused on performance with the 200-series. Balanced performance that enables more capability. I absolutely agree there are impacts, including others like gearing and braking. But the impacts are not exactly as some have made them out to be, and more rubber may offer some better trades depending on ones use.

The primary impact of unspring weight is inertia. In 2 manifestations:
1) Rotational inertia
2) Dynamic inertia

Rotational inertia primarily affects accel and deceleration. For most 200-series drivers, the focus is not on rabid acceleration and braking. We want stump pulling power or power going up a hill, but not necessarily dramatic accel/decal performance to battle a track timer. More of the accel/decal impact is more attributable to gearing or braking leverage losses as a results of larger diameter tires.

Dynamic inertia, or unsprung mass. Yes, it does effect ride, but it doesn't have the effect to ride quality that some are presuming. Nor does it impact shocks and springs to a degree that's more than the weight of the vehicle itself. I've said this elsewhere, but my heavy 71lb tires and 38lb wheels rides like a cloud down the freeway. It also take ruts better than my OEM tires ever did. Big hits may cause more secondary motions as the tire rebounds slower. Mostly, a bad ride is not directly attributable to unsprung weight. In as much as its due to poorly tuned/stiff suspension, stiff tires, tire construction, etc. MPG is also not largely due to weight as much as it is due to the rolling resistance of the specific tire design/compound and aero losses.

Less unprung mass has advantages indeed to traction and handling, as the tire can rebound quicker to follow high frequency bumps in the road, and maintain more traction. For off-road, It should equally be acknowledged that big (heavier) tires have great advantages because tires in themselves become a significant part of the suspension when aired down. The shock/spring suspension may not even have to react to corrugations or bumps as bigger tires can roll over and soak up a lot of things allowing the suspension to focus and deal with larger stuff.

Tire size is the defining factor when off-road, more-so than potentially a suspension lift. While a lifted rig riding high off-road may feel more invincible, it does nothing to lift the lowest point of the vehicle which is the rear axle/pumpkin. Tire size generally determines the class of capability, and the added weight that comes along with it may be worth the trades.

IMO, a recipe for better handling, ride, traction, and off-road capability is more tire. Less lift. 33-34" spec measurement sizes work great on this platform.

TL;DR Bigger tires are good, don't be afraid of the added weight.

 

[Angelo1]

As a previous road racer and autocrosser, unsprung weight was king. I remember the incredible agility and traction afforded with my 15.9lbs O.Z. Superleggera 17x8 race wheels, over my street 18" wheels. For sports cars where extreme acceleration, braking, and handling was the focus, that's where my priority was.

I still am very focused on performance with the 200-series. Balanced performance that enables more capability. I absolutely agree there are impacts, including others like gearing and braking. But the impacts are not exactly as some have made them out to be, and more rubber may offer some better trades depending on ones use.

The primary impact of unspring weight is inertia. In 2 manifestations:
1) Rotational inertia
2) Dynamic inertia

Rotational inertia primarily affects accel and deceleration. For most 200-series drivers, the focus is not on rabid acceleration and braking. We want stump pulling power or power going up a hill, but not necessarily dramatic accel/decal performance to battle a track timer. More of the accel/decal impact is more attributable to gearing or braking leverage losses as a results of larger diameter tires.

Dynamic inertia, or unsprung mass. Yes, it does effect ride, but it doesn't have the effect to ride quality that some are presuming. Nor does it impact shocks and springs to a degree that's more than the weight of the vehicle itself. I've said this elsewhere, but my heavy 71lb tires and 38lb wheels rides like a cloud down the freeway. It also take ruts better than my OEM tires ever did. Big hits may cause more secondary motions as the tire rebounds slower. Mostly, a bad ride is not directly attributable to unsprung weight.

Less unprung mass has advantages indeed to traction and handling, as the tire can rebound quicker to follow high frequency bumps in the road, and maintain more traction. For off-road, It should equally be acknowledged that big (heavier) tires have great advantages because tires in themselves become a significant part of the suspension when aired down. The shock/spring suspension may not even have to react to corrugations or bumps as bigger tires can roll over and soak up a lot of things allowing the suspension to focus and deal with larger stuff.

Tire size is the defining factor when off-road, more-so than potentially a suspension lift. While a lifted rig riding high off-road may feel more invincible, it does nothing to lift the lowest point of the vehicle which is the rear axle/pumpkin. Tire size generally determines the class of capability, and the added weight that comes along with it may be worth the trades.

IMO, a recipe for better handling, ride, traction, and off-road capability is more tire. Less lift. 33-34" spec measurement sizes work great on this platform.

TL;DR Bigger tires are good, don't be afraid of the added weight.

Great info., thank you. I know I’ve asked this to death but would I be fine running a 285/70/18 on my 18+ offset wheel or just stick with the 295/70 so long as it fits. I’m less concerned with MPG and just want to make sure I’m not putting the wheel bearings, etc components in any more harm and/or added wear. Appreciate the help