JasonC SBB

http://truckyeah.jalopnik.com/videos-show-the-jeep-renegade-has-a-scary-problem-under-1787687990



So can someone make a free body diagram explaining why when the rear end comes off the ground, it doesn't keep going and go end over end?

B6Tfastiva

iTrader: (1)

Of course I see this when I'm driving a renegade rental. Kinda wondering if I should try this lol.

Braineack

iTrader: (62)

at least it doesn't roll over like the Cherokee...

fooger03

iTrader: (2)

I'm not a betting man*, but I'm pretty sure the front tires are supposed to break traction first under heavy braking...

*Actually, I am a betting man.

shuiend

iTrader: (24)

I don't understand why Jeep makes anything other then the Wrangler. Or why anyone would buy anything other then a Wrangler.

acedeuce802

iTrader: (1)

Going end over end would require a steady state acceleration that would cause the rear tires to lift. If you take the wheelbase, and divide it by the CG height, then multiply this by the rear axle weight, this is the acceleration level in G that the rear wheel will have zero load. Using values from the internet, this would require 1.53 G to remove all load from the rear, and if the front tires could hold it, it could go end over end. Obviously the Renegade tires can't handle 1.5 G, especially on just the front tires, so there's something else going on.

This could be a pitch center that's low and forward, which would cause a relatively high pitch moment (due to a low pitch center), and the rotation point being towards the front would cause more linear displacement of the rear suspension. Having a forward pitch moment would also mean there is more braking inertia since it's further away from the CG. Combine this would relatively soft front springs and dampers, and a rear damper with high rebound force. Now when the vehicle reaches a high acceleration, the body pitches forward and keeps pitching forwards because of the high inertia, and the rear wheels either run out of travel, or have a high enough damping force to keep the springs from forcing them to stay on the ground.

This is just a speculation on how this may be possible. It definitely looks like it's just hopping off the ground really quick at the end of it's pitching, and not anywhere near going end over end. There are several other things to consider with braking dynamics which can all play a factor, such as anti-dive, spring/damper characteristics, tire characteristics etc. I guess these all just add up to a load case that FCA didn't simulate or test, but it's also possible that the car in the video didn't have OEM tires or dampers.

fooger03

iTrader: (2)

The initial "jump" up into the air is a function of both inertia trying to pull the car over the front wheels and the springs constantly trying to push the rear of the vehicle away from the ground. As the rear of the vehicle accelerates away from the ground, the upward pressure exerted by the rear springs decreases until eventually upward spring pressure is equalized by gravity (maintaining upward speed but no acceleration) and then overcome by gravity (still traveling upward, but now accelerating downward). The rear axle is also being pushed upward, but with far less room for upward acceleration provided by the pneumatic springs we call tires. At some point, there is a net zero force applied by the rear axle just before it leaves the ground, when the upward spring force applied between the tires and the road equal exactly the gravitational force pulling the rear axle and associated fixed components down. At this point in time, acceleration of the rear axle is zero while speed is upward - if there is enough upward speed in the rear axle, the rear axle will continue to move upward (while accelerating downward) all the way through lift. With regard to the whole vehicle, the rear axle and associated components now begins to provide a substantial downward force on the rear of the vehicle as gravity pulls them down. Given that this is a 4wd vehicle, we suspect that there is substantial downward force on the rear of the vehicle that was not there when the rear axle was originally pushing upward on the vehicle at steady state.

This should explain why it could be very easy to lift the rear end off the ground compared to the relative force it would require to continue to rotate the entire vehicle completely about the axis provided by the front tire.

The primary safety issue though is "what happens when the vehicle conducts emergency braking while not traveling in a straight line?"

aidandj

iTrader: (5)

Please do it! And video.

Now I want one. That is awesome.

JasonC SBB

^ OK it makes sense that it's forward rotational momentum that momentarily causes the rear lift, and not just CG vs. decel.

This quote made no sense to me:
Unless Jeep knew about the problem and the workaround was to reduce the braking G's. Which would be lame.

Fooger wrote
Slalom test here. I wonder if he'd hit the brakes while it was unsettled, if it would have rolled:

aidandj

iTrader: (5)

You mean like this?


I'm with lars. Buy a wrangler or don't buy a jeep. They do the wrangler better than anyone else. Everything else sucks.

Erat

iTrader: (5)

y8s

iTrader: (8)

Jeat?

Fiep?

Reneg00X?

Five hunegade?

how do you combine these names, I'm uncertain.

renegade ~= fiat 500x which means it's practically a miata.

TurboTim

iTrader: (9)

are these things full time AWD with a locking center diff? why don't the rear wheels stop immediately once airborne? The inertia of the wheel/tire is relatively tiny.

B6Tfastiva

iTrader: (1)

The one I'm driving is but I believe the video of the red one is a fwd version. Tried it when I got close to home and off the highway. Abs just kicked in and I didn't notice anything weird happening.