Independent Suspension vs Solid Rear Axle??

[CRobin]

I am sure there's an ongoing discussion about suspension and design. However; I wanted to serve this as more of a question out of curiosity for our engineering friends and suspension friends.

I am intrigued to understand what the main difference is and why people LOVE a solid rear axle vs the independent rear coil spring set up?? I've watched a lot of Youtube videos of both and I guess I don't see why people and manufactures prefer solid rear axles for trucks and SUV's vs the other set up. IF they are going for ride and comfort wouldn't it make more sense to be independent and coil set up for a better ride and handle?? Or am I missing something???

I guess I am also just curious as to why this was the decision for these Scout vehicles vs the other options, especially coil spring suspension. My F150 has leaf springs obviously you all know that is a Ford staple. Even with brand new shocks on front and rear it bounces around A LOT on rough roads and highways. I am assuming a solid rear axle would do the same would it not???

I've driven the new Tundra with the rear coil spring suspension and to me it handled like a car. I figured Scout would do the same based off the luxurious amenities and quality of building they are doing?? IDK I am just very curious. But curiosity killed the cat so forgive me for rambling nonsene!!

 

[J Alynn]

I am sure there's an ongoing discussion about suspension and design. However; I wanted to serve this as more of a question out of curiosity for our engineering friends and suspension friends.

I am intrigued to understand what the main difference is and why people LOVE a solid rear axle vs the independent rear coil spring set up?? I've watched a lot of Youtube videos of both and I guess I don't see why people and manufactures prefer solid rear axles for trucks and SUV's vs the other set up. IF they are going for ride and comfort wouldn't it make more sense to be independent and coil set up for a better ride and handle?? Or am I missing something???

I guess I am also just curious as to why this was the decision for these Scout vehicles vs the other options, especially coil spring suspension. My F150 has leaf springs obviously you all know that is a Ford staple. Even with brand new shocks on front and rear it bounces around A LOT on rough roads and highways. I am assuming a solid rear axle would do the same would it not???

I've driven the new Tundra with the rear coil spring suspension and to me it handled like a car. I figured Scout would do the same based off the luxurious amenities and quality of building they are doing?? IDK I am just very curious. But curiosity killed the cat so forgive me for rambling nonsene!!

Great question

 

[cyure]

I am sure there's an ongoing discussion about suspension and design. However; I wanted to serve this as more of a question out of curiosity for our engineering friends and suspension friends.

I am intrigued to understand what the main difference is and why people LOVE a solid rear axle vs the independent rear coil spring set up?? I've watched a lot of Youtube videos of both and I guess I don't see why people and manufactures prefer solid rear axles for trucks and SUV's vs the other set up. IF they are going for ride and comfort wouldn't it make more sense to be independent and coil set up for a better ride and handle?? Or am I missing something???

I guess I am also just curious as to why this was the decision for these Scout vehicles vs the other options, especially coil spring suspension. My F150 has leaf springs obviously you all know that is a Ford staple. Even with brand new shocks on front and rear it bounces around A LOT on rough roads and highways. I am assuming a solid rear axle would do the same would it not???

I've driven the new Tundra with the rear coil spring suspension and to me it handled like a car. I figured Scout would do the same based off the luxurious amenities and quality of building they are doing?? IDK I am just very curious. But curiosity killed the cat so forgive me for rambling nonsene!!

I’m so used to it in my Jeep it doesn’t bother me anymore, but then my favorite ride at Disneyland is Indiana Jones and I can pretend I’m on the ride when I’m in my car because it has a similar bouncy feeling as the ride.

 

[SrfnFly227]

In the off road world, solid axle would usually equal stronger with more articulation. You can have a solid axle with leaf springs, coil springs, or air suspension. Leafs would give the harshest ride. Depending on the exact set up, coil and air are probably similar for ride quality. Air would allow for changes in height for the conditions.

My 16 Grand Cherokee has an air suspension. I can raise or lower it with a dial in the vehicle. It also lowers itself to a "aero" mode at about 60 mph.

Independent suspension would be best for straight ride comfort, but it would have drawbacks off road.

 

[Logan]

In the off road world, solid axle would usually equal stronger with more articulation. You can have a solid axle with leaf springs, coil springs, or air suspension. Leafs would give the harshest ride. Depending on the exact set up, coil and air are probably similar for ride quality. Air would allow for changes in height for the conditions.

My 16 Grand Cherokee has an air suspension. I can raise or lower it with a dial in the vehicle. It also lowers itself to a "aero" mode at about 60 mph.

Independent suspension would be best for straight ride comfort, but it would have drawbacks off road.

This.

Solid rear axles are preferred, and are used on almost all trucks, for strength. They also work better for rock crawling, usually with better articulation. They also make the vehicle easy to lift.

The solid axle can be suspended via different methods as mentioned. A leaf spring is super simple, but has lots of friction, which makes it ride rough. most pickup trucks use this in the rear axle.

You can also use coil springs to suspend a solid axle. Or airbags.

Fun fact, the Jeep Cherokee (1984-2001) used leaf springs in the back, and coils up front. While the grand cherokees of the same era (1993-1998) used coil springs all around for their solid axles. That’s one of the reasons why the Grand Cherokee rides better (less friction in the springs).

Independent suspension usually rides better, but also is harder to lift, and easier to damage. It’s also usually better at speed (ie, trophy trucks use independent front suspension, while competition rock crawlers are usually solid axles front and rear).

This is why for a long while now most trucks have used independent front suspension, with solid axles out back. The cabin and steering are primarily suspended with the comfy IFs setup. While the solid axle handles the payload out back.

Personally I’m expecting a coil sprung, multi-link solid axle setup on the traveler. Not sure about the terra though. I’d assume the same, but it could be different.

 

[6-ScoutFamily]

I’m sure SM’s choices here are designed to give the Scouts some serious off-road capability along with ride comfort and handling. It’s not a purpose built rock crawler after all.

 

[J. Sweat]

I really hope the Scout doesn’t get watered down for the masses. There are plenty of comfortable, boring SUVs to buy. Let us have a real Scout.

 

[CrazyOldMan]

Solid axle usually better articulation on trail, simpler to lift, and better towing capacity. Also more durable under hard use. Finally, and super important - solid axles allow for mechanical lockers between the driver side and passenger side wheels on each axle. Otherwise, software manages that for you either with controlled braking or power application to each side. JLR became legendary for their innovations in this space. But - it’s a computer. Something else to break, and something where you have less control. Only the Wrangler and the Grenadier have live axle for both front and rear (and triple locked - front, center and rear). GWagen, Defender, and Toyota Land Cruiser were the last holdouts, and they went to IFS for the front (also Bronco). I think the Defender and the GWagen both switched in ‘19, and the LC when Toyota relaunched last year. It allows them to market to a wider customer base and provide for more of a soft-roading scenario (which fits the needs of most consumers TBH).

The suspension is separate from the axle design - you can get adjustable King or Fox racing dampers with remote reserves on a Grenadier that soak up hard hits at high speed - kind of like a Raptor setup - or trail tuning - or add air bags (Aussies love their air bags) - it all depends on what kind of offroad death wish you have, how much towing you do, etc.

Or - if you really want to go nuts on the lift and axle option - portals. Google “Arctic Trucks” and see what you think ;o)

 

[Iguy]

For context I am a suspension design engineer, so this is right up my alley.

Pretty much everything said is correct. Solid axle is stronger and allows for more articulation so it’s better off road. Independent can be very strong. Look at Baja buggies for example. It’s got better kinematics so the control and ride quality is better. It’s also less unsprung weight so better wheel control that way. You also typically have better ground clearance because you can tuck the bowl of the axle up a little. But, solid is still better off road. One of the other main reasons is the suspension mounting points act as a fulcrum so loading one tire actually helps load the other side.

One correction I’ll make is CrazyOldMan implies you can’t have mechanical lockers with independent suspension and that’s not true. Many vehicles have mechanical lockers with independent front suspension. And some have them in the rear too, you just don’t see it often because if you are getting serious enough to need lockers you pretty much get something with solid rear axle.

 

[oldgeeksguide]

For context I am a suspension design engineer, so this is right up my alley.

Pretty much everything said is correct. Solid axle is stronger and allows for more articulation so it’s better off road. Independent can be very strong. Look at Baja buggies for example. It’s got better kinematics so the control and ride quality is better. It’s also less unsprung weight so better wheel control that way. You also typically have better ground clearance because you can tuck the bowl of the axle up a little. But, solid is still better off road. One of the other main reasons is the suspension mounting points act as a fulcrum so loading one tire actually helps load the other side.

One correction I’ll make is CrazyOldMan implies you can’t have mechanical lockers with independent suspension and that’s not true. Many vehicles have mechanical lockers with independent front suspension. And some have them in the rear too, you just don’t see it often because if you are getting serious enough to need lockers you pretty much get something with solid rear axle.

Can you explain how a solid axle leads to better articulation? That seems counterintuitive, given that it seems like there would be fewer constraints on an independent system. I understand the fulcrum bit, that makes more sense to me but I’m confused by the articulation thing.

 

[Iguy]

Tough to do via text, but I’ll try!

Essentially it boils down to space constraints. An independent suspension has a mounting point somewhere at best near the center of the vehicle (side to side). The wheel is in a fixed location on the outside edge of the vehicle. That means your arm sweep only has that much length to travel. Within that you have to consider side to side movement as it travels and a bunch of other stuff. You could make the control arm go vertical but then your tire would be in the center of the vehicle! Obviously that won’t work! This is why you have “long travel” kits which just make the control arms longer for more travel, but it makes it wider. Baja cars work on this by making the arms longer and moving the pivot point to the center. Ford got around this with the “twin torsion beam” and “twin traction beam” suspension. Very cool, but they had their own issues.

A solid axle either has leaf springs which when designed right with good shackles have better ride and travel than most realize, but are still limited. First the mounting points of the spring also control the axle position. A longer limb length on the spring allows for more travel typically, but you still have to package it on a vehicle and the longer the spring, the harder to package (front or rear overhang gets large, gets into passenger space, spring angle gets hard to manage and this dictates axle planing angle, nothing else can be mounted in this area, but they are more robust!).

So instead you go to coils or air. But these can’t control the position of an axle so you need control rods to control the axle. This is what’s typically called a 3 link or 4 link. Now the control rod lengths dictate the travel path. Additionally the control rods are roughly fore and aft on a vehicle and inboard of the wheelbase so you have lots of space to make those rods longer. The longer rods result in less for and aft movement of the axle for the same amount of travel and therefore also allow more travel. More critically here though, independent suspension have a fixed amount of travel no matter the travel type (cross articulation or “Jounce” (up) and “rebound” (down)). A solid axle can have MORE travel in cross articulation than just jounce and rebound travel because the wheel is further out from the fulcrum. This is really valuable in off road where you usually don’t have isolated jounce and rebound travel, but mostly cross articulation.

Hopefully that helps and with a bit of looking at some of the suspension types I mentioned you get a good understanding! Let me know if you have further questions!

Please forgive spelling mistakes, I did this on my phone.

 

[J Alynn]

Tough to do via text, but I’ll try!

Essentially it boils down to space constraints. An independent suspension has a mounting point somewhere at best near the center of the vehicle (side to side). The wheel is in a fixed location on the outside edge of the vehicle. That means your arm sweep only has that much length to travel. Within that you have to consider side to side movement as it travels and a bunch of other stuff. You could make the control arm go vertical but then your tire would be in the center of the vehicle! Obviously that won’t work! This is why you have “long travel” kits which just make the control arms longer for more travel, but it makes it wider. Baja cars work on this by making the arms longer and moving the pivot point to the center. Ford got around this with the “twin torsion beam” and “twin traction beam” suspension. Very cool, but they had their own issues.

A solid axle either has leaf springs which when designed right with good shackles have better ride and travel than most realize, but are still limited. First the mounting points of the spring also control the axle position. A longer limb length on the spring allows for more travel typically, but you still have to package it on a vehicle and the longer the spring, the harder to package (front or rear overhang gets large, gets into passenger space, spring angle gets hard to manage and this dictates axle planing angle, nothing else can be mounted in this area, but they are more robust!).

So instead you go to coils or air. But these can’t control the position of an axle so you need control rods to control the axle. This is what’s typically called a 3 link or 4 link. Now the control rod lengths dictate the travel path. Additionally the control rods are roughly fore and aft on a vehicle and inboard of the wheelbase so you have lots of space to make those rods longer. The longer rods result in less for and aft movement of the axle for the same amount of travel and therefore also allow more travel. More critically here though, independent suspension have a fixed amount of travel no matter the travel type (cross articulation or “Jounce” (up) and “rebound” (down)). A solid axle can have MORE travel in cross articulation than just jounce and rebound travel because the wheel is further out from the fulcrum. This is really valuable in off road where you usually don’t have isolated jounce and rebound travel, but mostly cross articulation.

Hopefully that helps and with a bit of looking at some of the suspension types I mentioned you get a good understanding! Let me know if you have further questions!

Please forgive spelling mistakes, I did this on my phone.

Thanks for sharing that info. My brain hurts absorbing it but I love members who understand things I don’t

 

[Iguy]

Happy to! I love this stuff!

 

[CrazyOldMan]

For context I am a suspension design engineer, so this is right up my alley.

Pretty much everything said is correct. Solid axle is stronger and allows for more articulation so it’s better off road. Independent can be very strong. Look at Baja buggies for example. It’s got better kinematics so the control and ride quality is better. It’s also less unsprung weight so better wheel control that way. You also typically have better ground clearance because you can tuck the bowl of the axle up a little. But, solid is still better off road. One of the other main reasons is the suspension mounting points act as a fulcrum so loading one tire actually helps load the other side.

One correction I’ll make is CrazyOldMan implies you can’t have mechanical lockers with independent suspension and that’s not true. Many vehicles have mechanical lockers with independent front suspension. And some have them in the rear too, you just don’t see it often because if you are getting serious enough to need lockers you pretty much get something with solid rear axle.

For context I am a suspension design engineer, so this is right up my alley.

Pretty much everything said is correct. Solid axle is stronger and allows for more articulation so it’s better off road. Independent can be very strong. Look at Baja buggies for example. It’s got better kinematics so the control and ride quality is better. It’s also less unsprung weight so better wheel control that way. You also typically have better ground clearance because you can tuck the bowl of the axle up a little. But, solid is still better off road. One of the other main reasons is the suspension mounting points act as a fulcrum so loading one tire actually helps load the other side.

One correction I’ll make is CrazyOldMan implies you can’t have mechanical lockers with independent suspension and that’s not true. Many vehicles have mechanical lockers with independent front suspension. And some have them in the rear too, you just don’t see it often because if you are getting serious enough to need lockers you pretty much get something with solid rear axle.

sure - Bronco has IFS and a front locker (on some trims and with the Sasquatch package). It’s important to provide the “buuut” so people know why all SUV’s with IFS in front don’t offer a front locker from factory (I’m trying to think of an exception in addition to Bronco, but am not coming up with one). Not that the Scout will need it - it will cover what 99.9% of owners want with just center & rear, and without the compromises of a solid front axle.

 

[Iguy]

Every ford raptor (f-150, ranger, and bronco along with some non raptor trim broncos), the Chevrolet ZR2’s (2500, 1500, and Colorado) all have mechanical lockers and IFS. I guess those are trucks, but I’m not sure why it would have to be an SUV specifically. Lockers inherently can put more stress into the half shafts/axle shafts, that is independent of Independent suspension or solid axles. Independent is really only weaker because of the two CV or U joints required on the half shafts. They can be made to handle plenty though and honestly it often not that that fails on the trail, it’s tie rods that I see.

Yeah, we can put an asterisk on that and let people know it’s not offered often because it’s often not needed and higher stress. It wasn’t really the question or point of discussion though which is why I didn’t get into it. I agree though, IFS is better in nearly every situation, but a good linked solid rear axle does not provide much worse ride quality and has much better off road performance which is why I think they went with it.

Scout motors will not have a “center” locker because there is no center differential or transfer case for that. They’ve also already said it will have front and rear mechanical lockers.

 

[CrazyOldMan]

Every ford raptor (f-150, ranger, and bronco along with some non raptor trim broncos), the Chevrolet ZR2’s (2500, 1500, and Colorado) all have mechanical lockers and IFS. I guess those are trucks, but I’m not sure why it would have to be an SUV specifically. Lockers inherently can put more stress into the half shafts/axle shafts, that is independent of Independent suspension or solid axles. Independent is really only weaker because of the two CV or U joints required on the half shafts. They can be made to handle plenty though and honestly it often not that that fails on the trail, it’s tie rods that I see.

Yeah, we can put an asterisk on that and let people know it’s not offered often because it’s often not needed and higher stress. It wasn’t really the question or point of discussion though which is why I didn’t get into it. I agree though, IFS is better in nearly every situation, but a good linked solid rear axle does not provide much worse ride quality and has much better off road performance which is why I think they went with it.

Scout motors will not have a “center” locker because there is no center differential or transfer case for that. They’ve also already said it will have front and rear mechanical lockers.

My guess is it’s because of the “sport” in “sport utility vehicle,” and maybe truck buyers have different priorities. The manufacturers for SUV’s want a better all-around platform and think their buyers won’t be happy with the trade-offs? I guess that just runs counter to the typical intended use for SUV’s. Given your background, you know what would have to be done to avoid the typical IFS/locker vulnerabilities, and you’ve probably seen your fair share of trail carnage from aftermarket lockers on IFS rigs. I still wake up in a cold sweat with memories of that “POP!” In fairness, automation is delivering really solid results. The new Defender and Lexus GX 550 are good examples. Not my cup of tea, but they can do a heck of a job with the right tires. OK, and maybe bumper swaps ;o)

Anyhooo - lockers and solid axles make good buddies, to the original question. You’re right (of course) - you can have a locker on IFS - many people add them in aftermarket, and many have paid a price for that. The manufacturer would just have to compensate to offer it stock - and most choose not to. Whether it practically makes any difference . . .

 

[Jamie@ScoutMotors]

The issue for manufacturers is that there is a ton of data showing that the average person in these segments (the two biggest in the US) doesn't take their vehicle off-road. Or if they do, it is fireroads and light-obstacle kinds of stuff that isn't too stressful or dangerous/damaging - especially when you need that vehicle as a daily driver. So it gets difficult to justify the crappy ride of a solid front axle in a half-ton truck for the 2% of the market that want it, versus the rest of the market that wants their truck to ride and handle well. We are building a capable, rugged truck, and it will be able to take you to 95% of the places most people would want to go. Beyond that, you might have to add a lift kit and some mods for specialized use. I've been to Moab numerous times, and any vehicle with decent approach and departure angles and locking diffs can do most things out there. Heck, we took a first-generation Touareg in and out of Micky's Hot Tub. Guys with $30k in mods on their Jeeps were just shaking their heads.

We have some deep off-road enthusiasts employed at Scout with some serious rigs. We have had major aftermarket companies out to our engineering and design office in Michigan. The vehicle itself is body-on-frame and can handle a lift kit. The bumpers are designed to be removable. Front and rear locking diffs are available, and a front sway bar disconnect. We expect some robust aftermarket support down the road. I know some of you would love a solid front axle, but the tradeoff in ride and handling was too much.

 

[Iguy]

Strong (possibly overkill) tie rods and some heavy duty half shafts with good shaft/joint angles and you can prevent most issues. I agree though, IFS is inherently weaker and a locker highlights that more. The “POP” is indeed common but when well designed from the factory, it’s less and less of an issues these days. Lifts change your half shaft angles and put more stress on those joints, bigger tires and lower gearing puts more stress on everything. That’s the common cause of the pops.

The minor tradeoffs of IFS are worth it for most vehicles these days.

 

[Jamie@ScoutMotors]

You also get into discussions about which part should be the weak link. Do you want to break expensive and difficult-to-repair stuff? Or cheaper/easier to repair items?

 

[CrazyOldMan]

This is the right choice and exactly what I expected. It’s got to meet many user needs - and be a great daily. Have you guys released any info about your approach to off-road assist?

Sometimes the guys with the biggest mods just get themselves in more trouble. It’s all an integrated system. Would have LOVED to see the Touareg video. Awesome.