Crazy rear end

[Gr8ful]

Thanks for all the imput. I run 36psi in the tires. I will drop them down to 34 psi cold. I have the Goodyear Eagle F1 tires that come with the 20 inch tires.
Again thanks for the help.

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[Skye]

A general rule of thumb when changing tires is to stay within 3% of the original's circumference.

FWIW, when I changed mine, I was +/- 3% in volume from factory. My thinking is I'd be +/- 3% from the door placard, putting me at 32-34 psi / 2.2 - 2.34 bar. Monitoring the tread pattern over time, I've found my sweet spot to be between 32 and 33 psi, cold.

YMMV.

 

[Q6543]

One of the best and 1st mods I did was steeda cradle lockout bushings and locating dowels
Instantly made the rear end “tied in” tight to the chassis. So worth for a couple hundred bucks and an hour of your time.

 

[DCS]

I just turned 1,000 miles a few days ago. I haven't run the engine above 4,000 rpm, but I have accelerated a few times.
I have noticed the rear end feels like it's about to break loose when I accelerate from a dead stop while entering a roadway from a shopping center. Therefore I am turning and not going straight.
Could this have something to do with the Torsen rear end?

 

[Mr Hyde]

It's a kit bmr and steeds make to tighten the bushings.

What is a subframe lockout?

https://www.bmrsuspension.com/?page=products&productid=2850

or the level 2 kit https://www.bmrsuspension.com/?page=products&productid=2847

Both add stiffness and stops the cradle bushings from deflecting. Steeds also makes a kit. The s550/650 both have the same issue being unibodies that don't have their front and rear cradles locked together by an x brace under the car or at least tied more stiffly to the unibody allowing the car to move and flex in a uniform manner front and rear. You may also be feeling wheel hop.

 

[GhastlyTT]

I just turned 1,000 miles a few days ago. I haven't run the engine above 4,000 rpm, but I have accelerated a few times.
I have noticed the rear end feels like it's about to break loose when I accelerate from a dead stop while entering a roadway from a shopping center. Therefore I am turning and not going straight.
Could this have something to do with the Torsen rear end?

Yessir, that's a characteristic of the Torsen.

 

[RLE55]

From Maximun Motorsports:

High bias Torsen T-2R torque-biasing differential is a full-time torque-biasing system, not just a limited slip differential. The Torsen differential has a unique helical-gear design with no clutch plates or preload springs. It provides continuous, uninterrupted torque output, torque biasing, and differentiation in one integral unit. The Torsen greatly improves traction because it is continuously and instantly responding to torque feedback from the tires, sending more torque to the tire with the most traction.

The 4:1 TBR of the Torsen T-2R means that this differential is capable of delivering up to four times as much torque to the tire with better traction than is delivered to the tire with the least available traction. Excellent for the tight turns typical of autocrossing.

By comparison, the standard Torsen T-2 can deliver about 2.5 times as much torque to both rear tires, collectively, as an open differential can, given the same conditions.

Torsen differentials have NO clutches or preload springs.

Ford equipped several Mustangs with a Torsen differential as original equipment:

• FR500S, Boss 302R, Boss 302S
• Boss 302 (optional)
• Boss 302 Laguna Seca
• 2013-14 Mustang with the OEM Track Pack option

 

[Wiley Marmot]

Thanks for all the imput. I run 36psi in the tires. I will drop them down to 34 psi cold. I have the Goodyear Eagle F1 tires that come with the 20 inch tires.
Again thanks for the help.

Keep in mind your cold tire pressure will fluctuate with ambient temperature as well. I lose/gain about 1 PSI with every 10 degrees (F) of ambient temperature change.

Like Skye; I've considered the OEM pressure recommendation to be just a reference point for more than 30 years. I've also got the 265/35R/20 Goodyear all seasons; based on my current driving style, I've got 37 PSI in the fronts and 35 PSI in the rears, and have a nice flat wear pattern across the entire width of my tire tread on all four corners.

Also, carry an accurate air gauge in your Mustang and use it all the time. Very few air gauges read the same. It's really common to see 5 PSI or more difference between 2 gauges.

 

[Col F]

Tire pressure should generally conform to the pressure specified on the door pillar placard. A deviation of +/- one, two or even three pounds is generally no big deal. Of course, tire pressure must only be adjusted on cold tires (neither driven nor sitting in direct, hot sun or baking hot pavement/surface). Typically, adjust the pressure on the undriven car in the morning if parked outdoors, or any time of day if garaged. Use a good quality digital gauge.

A few psi of deviation from spec (which is very common depending on changes in ambient temperature as well as over time from the last pressure adjustment) should not make any difference in drivability or handling for street/road operation; especially at "normal" (aka "legal") speeds. (For an example, a 36/37 psi cold tire will be around 40/41 psi on the highway, and will feel and handle exactly the same. Typically, only severely overinflated / underinflated tires will perceptibly affect handling.

I suspect that RLE55's post regarding the Torsen rear-end possibly affecting driving characteristics may be valid - particularly if the rear axle tires are not withing 1-2 psi of each other or there is another cause of reduced traction or coefficient of friction on the road surface.

 

[Gr8ful]

I do monitor my tire pressure regularly and I agree that this should not be a factor in the loose/twitching rear of the car. I am sure something is amiss, but what. I'm not ready for the dealer tech to be hot rodding my new car to duplicate the issue.

 

[cosmo73]

I agree wholeheartedly with the recommendation to install the Steeda “Stop the Hop” kit. It makes a very noticeable improvement to the rigidity of the rear subframe and improves handling/composure under power.

https://www.steeda.com/Steeda-S550-Mustang-Stop-The-Hop-Starter-Kit-555-4455

 

[REV745DH24]

I agree wholeheartedly with the recommendation to install the Steeda “Stop the Hop” kit. It makes a very noticeable improvement to the rigidity of the rear subframe and improves handling/composure under power.

https://www.steeda.com/Steeda-S550-Mustang-Stop-The-Hop-Starter-Kit-555-4455

But in the instructions says:
Note: Does not fit 2024+ Dark Horse !!!!

 

[Randi]

When you have a moment, check the placard in the door jam; this will list the PSI/BAR the tires should be set to. 41 psi / 2.81 bar was their shipping pressures when the car left the factory; yes, these were way to high. When cold in the AM, 35 psi / 2.4 bar still seems a bit much.

If you're using the Tire Pressure Management System screen to confirm pressures, it rounds the actual number, but should be within 1 psi of actual. If using a physical pen-style or some other gauge, confirm its accuracy by checking against another.

Thank you! I checked the door this morning and it says they should be 32 PSI. After lowering them to 35 from 41 the day before yesterday, they were still showing 36 and 37 PSI on the TPMS screen. It’s hot outside but it was under the carport out of the sun. I’m going to get my boyfriend to lower it again to 32 tonight after it cools off. I ordered a physical gauge for myself off of Amazon last night just to have so I can check them myself and reduce if needed. Thank you so much!

 

[Randi]

Along with confirming the correct, cold pressures, check the wear of the tires themselves. It reads that you might have been operating at high pressures for some time. Do a visual review of the tread pattern, so see what they look like.

Being over-pressurized previously, you might see the center of the tire wearing more than the rest. Getting down to the specified pressures will help relieve wear on the center and generally flatten the pattern over time. As that happens, traction and drivability will continue to improove.

When I get off of work, I’ll check the tread. I glanced this morning and they looked fine but that’s not saying much lol.

 

[Randi]

If your tires are at 40+psi cold, they will be 44 or more psi hot. That is above max pressure for the tire. Keep this in mind.

Yeah, they have been running between 42-43 on the interstate/highway. I’m going to lower the PSI some more tonight. Thank you!

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