Two Month Update and AI Bump Stop Suggestions: 39x13.5xr17 D Sidewall BFG KO3, Hyperco 2/2, Method Bead Grip 0 Offset, FMI Top Hats

[Dgchurch]

After running this set up for two months; in the Mojave, dune running, woops, etc... on the Rubicon, on the freeway and all things in between the set up has been fantastic.

--Adjusting ride height with Forscan is a must.
--A good shop needs to both dynamic and static balance the tires.
--It will handle moderate jumps and full rock crawl articulation.
--Remove the fog light section of your stock bumper and no front clearance issues.
--I'm running an ADD Phantom rear bumper and have plenty of rear clearance. I did rub once on a full Rubicon day but @JohnGalt and I were not able to repeat the issue.
--Stock steering rack, tie rods, UCA, LCA, etc... have handled everything like a champ.
--Longer term plan is to put some 5:13 or 5:38 gears in to regain some low end rock crawl and torque but the stock 4:7 gears still light 'em up and crawl well.
--What it needs is a bit taller bump stop to allow full on jumping, woops, etc...

I entered my set up into deep thinking Chat (extended pro) and the output is pretty useful.

I am going to extend my bumps by 1.25" and send it... fingers crossed. I think the FMI set up is probably the best but it is pricey.

Chat Output:

Practical answer​

For your setup, I would start with about 1.25" taller rear bump stops than stock, and I would consider 1.5" the safer starting point if your goal is to protect the rear flares during harder, near-full-compression jumps.

I would not jump straight to 2" taller rear bump stops unless cycling the suspension proves you need it, because that gives away a lot of the Bronco Raptor’s rear uptravel. Ford lists the Bronco Raptor at 14.0" rear suspension travel, so taking 2" out of the rear is meaningful. (Ford From the Road)

Why the number is around 1.25–1.5"​

The important part is not the nominal tire size. It is the actual radius increase.

The stock Bronco Raptor tire is listed by Ford as 37x12.50R17 BFGoodrich KO2. (Ford From the Road) Tire Rack lists the 37x12.50R17 KO2 at 36.5" overall diameter. Your 39x13.50R17 KO3 is listed at 38.5" overall diameter. That is a 2.0" diameter increase, which means the tire reaches about 1.0" higher into the fender at full bump. (Tire Rack)

So the baseline bump-stop correction is:

1.0" extra tire radius + 0.25–0.5" safety margin = 1.25–1.5" rear bump-stop extension

The extra safety margin matters because you are jumping the vehicle, the tire carcass deflects, the flare is plastic, the axle does not always land perfectly square, and your wheel/tire package is also farther outboard than stock.

The 0-offset wheels are the flare-risk multiplier​

Assuming you came from the common OEM Bronco Raptor wheel spec of 17x8.5 +34 mm, going to a 0 mm offset wheel moves the tire centerline about 34 mm, or 1.34", farther outward per side. The 39x13.50 tire is also about 1" wider than the stock 37x12.50, adding another roughly 0.5" outward sidewall/shoulder envelope. That means the outside tire shoulder is roughly 1.8" farther outboard than stock, before tire deflection. (Bronco Raptor Forum)

That outward movement does not automatically mean you need 1.8" more bump stop, because bump stop height controls vertical travel, not lateral tire position. But it does mean the tire is now much more likely to catch the outer flare lip when the rear compresses hard or when the axle shifts laterally through the track-bar arc.

The 2" rear lift does not protect you at full bump​

The Hyperco springs increase ride height, but they do not necessarily change the body-to-axle relationship at full compression. In other words, the truck may sit 2" taller in the driveway, but during a hard landing the axle can still travel to the bump-stop limit unless something else stops it first. Hyperco/Foutz list the 2"/2" Bronco Raptor spring kit as a ride-height lift using factory shocks, not as a bump-travel limiter. (Foutz Motorsports LLC)

So for flare protection, set bump stops based on full bump clearance, not parked ride height.

How I would set it up​

Start with 1.25" rear bump-stop spacers/extensions or an adjustable bump pad set to reduce rear uptravel by about that amount. Then cycle the rear suspension with the springs removed or unloaded, both tires installed, and the axle raised to full bump. Check the tire-to-flare clearance on both sides.

For jumping, I would want at least 3/4" of real clearance between the tire shoulder and the plastic flare at full bump. 1" is better if you are landing hard, aired down, or landing slightly crossed up.

Use this rule:

Required extra bump-stop height = target clearance − actual clearance at full bump

So, for example:

• If you cycle it and the tire has 0.25" clearance, add about 0.5–0.75" more.
• If the tire just barely touches, add about 0.75–1.0".
• If it overlaps the flare by 0.5", add about 1.25–1.5".
• If it needs more than 1.5–1.75", I would stop trying to solve it with bump stop alone and look at trimming/flare changes or less outboard wheel fitment.

My recommendation for your exact case​

Given 39x13.50 KO3s, 0-offset Method Bead Grip wheels, and rear lift springs, my best starting setup would be:

Rear bump stops: +1.25" over stock for testing
Rear bump stops: +1.5" over stock for harder jumping / flare protection bias

I would avoid going taller than 1.5" until you physically cycle the suspension, because too much bump stop will make the rear hit the stops sooner, ride harsher, and potentially kick/buck more on landings.

Two checks I would not skip​

First, make sure the rear axle is centered at ride height. A lifted Bronco rear axle can sit slightly off-center because the track bar locates the axle laterally. A track bar’s job is to keep the axle centered side-to-side, and adjustable bars are commonly used to re-center lifted vehicles. (Rough Country)

Second, verify your exact wheel width. The 39x13.50R17 KO3 spec is listed with a 9–11.5" rim-width range and an 11" measuring rim. If your Method wheels are 17x8.5, that is narrower than the published range for that KO3 size, which can change sidewall shape and deflection under hard landings. (Tire Rack)

If you are really going to use full capability​

For repeated jumps, I would rather see you use a proper rear hydraulic bump-stop setup than just a taller hard stop. Foutz’s Bronco Raptor rear kit, for example, uses full-size 2.5" diameter, 2.5" travel hydraulic bump stops and mounts over the stock rear bump-stop area. That type of setup lets you engage the bump earlier while absorbing the last part of travel instead of just slamming into a taller block. (Foutz Motorsports LLC)

My bottom line: set the rear up 1.25" taller first; move to 1.5" if you want a safer jump setting; anything beyond that should be based on full-bump cycling, not guesswork.