JL Rubicon 2 Door - Roof Load Capacity? Fitting out with Frontrunner Slimline II.

[OffroadWrangling]

Hi all,

Currently I am fitting out the 3.6L Wrangler Rubicon JL 2D MY22 with a Frontrunner Extreme Slimline II (please see the photos below).
Frontrunner cannot provide me with a roof load capacity as they suggest I need to go by the requirements suggested by Jeep.
However, Jeep flatly states in the user manual that there cannot be any roof loading on the hardtop whatsoever.

The Frontrunner has zero roof load on the hardtop as it is installed onto the roll cage underneath as well as support bars to the front bonnet.
Does anyone know what the potential roof load on the 2D would be with the front runner installed? The best I can find is a static capacity of 300 kg which might suggest a dynamic capacity of about 100 kg (from YT videos I've watched a rule of thumb appears of be a multiple of 3 between the two).

Interms of what I'd like to be able to load is a 270 awning (19 kg), king titan swag (16 kg, not an RTT just want to tie it down) and a rolled up inflatable queen mattress (9.8 kg). The installed weight of the frontrunner is 45.3 kg. The swag could easily be placed across the back of the boot but the mattress unfortunately won't fit very well, so I could minimise the weight by 16 kg but I'd prefer to have the swag tied on the opposite side of the awning so as to help balance the weight distribution.

Any help would be very much appreciated!

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[Reinen]

It's odd that they won't provide static & dynamic load capacities. That would be a red flag to me. I also can't find any pics of the interior brackets so I'm not exactly sure how the rear is supported. I see several manufacturers of unsupported racks give a dynamic weight limit of 75kg, but that doesn't coincide with Jeep and the Frontrunner rack does have some support.

I have the MAXIMUS-3 JL 2 DOOR ROOF RACK SYSTEM. It's similar to the Frontrunner rack (albeit shorter) and it's very strong as its supported by the roll bars with the HT sandwiched between brackets. 900lb(408kg) static / 300lb(136kg) dynamic.

But the Frontrunner rack has some key differences that will definitely give it a lower dynamic weight limit. I can tell you the weakness of the HT as far as carrying dynamic weight is resisting side-to-side rocking. The rear end of the HT has a propensity to wag off-road carrying weight. That wagging can cause the HT to crack in the upper corners of the rear window. The mounts of the rack will have to resist that movement because the HT won't.

I'm concerned that the long bars supporting the front will allow the rack to torque side to side in the rear where the HT is weak, so it all depends on how well the rear brackets resist that movement. In comparison, the front of the M-3 rack is bolted to the roll cage above the sound bar by short thick brackets. The rack will get ripped apart before the front brackets allow any side to side movement in the front or torqueing in the rear.

So the key question I have is how well do the rear brackets of the Forerunner rack resist side-to-side rocking? The answer to that will give an idea whether you're closer to the 75kg or 136kg dynamic limit of the M-3 rack.

 

[OffroadWrangling]

It's odd that they won't provide static & dynamic load capacities. That would be a red flag to me....

Hi Reinen,

Thanks so much for the detailed response.

In regards to the static load capacity, I have found out the static capacity of this particular rack is 660 lbs (or ~300 kg) from other distributors of the product and a lot of internet trawling. However, I think to alleviate any potential legal activity as a result of having the rack installed, the company will not provide dynamic capacity. Although after watching plenty of youtube reviews, reading many comments spread across various websites/blogs/forums and such, this particular frontrunner product is highly recommended so I'm not too worried about the product itself thankfully.

I also can't find any pics of the interior brackets so I'm not exactly sure how the rear is supported.

Here are some pictures to illustrate the attachment of the frontrunner backets to the tray, bonnet and roll cage under the HT:

In the first picture, it can be seen that the brackets are attached to the foot rail at the rear of the tray then drilled into the rear of the roll cage. This can be seen more closely in the last photo. Roughly in the middle of the tray appears to be a bracket which connects to the foot rail and is drilled into near the middle of the roll cage under the HT. This connection and the rear bracket connection to the roll cage can be seen in the second last photo.

Below are the connections that I can find for your M-3:

It appears that the frontrunner has the same amount of roof top connections as the M-3 with the additional support from the front pillar brackets.

I'm concerned that the long bars supporting the front will allow the rack to torque side to side in the rear where the HT is weak, so it all depends on how well the rear brackets resist that movement.

Personally I am unsure if the additional pillar support will facilitate equivalent dynamic load capacity and you certainly raise some excellent points regarding torque side to side movement. The question is also whether or not the four structural supports of the frontrunner on the roof attached to the roll cage provide similar support to the four supports of the M-3. If so, it may be the case that the front pillars supplement this support.

I've read on some older posts around here that a general rule of thumb is as follows:
dynamic loading is ~1/2 of the static loading
offroad dynamic loading is ~1/3 of the static loading

If so then with the static rating being 660 lbs (~300 kg) then:
dynamic loading = 330 lbs (~150 kg)
offroad dynamic loading = 217.8 lbs (~99 kg)

Although of course the M-3 rating does not match this rule of thumb. If I were to be far more conservative, the dynamic loading could be much closer to 1/3 of the static rating and therefore would be 220 lbs (100 kg). This actually aligns with the dynamic rating of the M-3 (900 lbs / 3 = 300 lbs or 136 kg). I must note this was purely coincidence and I didn't work backwards from the M-3 dynamic rating.

My total kit is 203.5 lbs (or 92.5 kg) which I think could be acceptable as a dynamic load, I don't have plans of ever going above this (in fact, my upper limit would be 220 lbs (100 kg)) and would probably sit more around ~132 lbs (60 kg) if I were to off-road (although in saying this, I would most likely stick to easier, flatter off-road and would have next to nothing on the roof in heavy off-road situations).

Of course, I will be sticking to the slow lanes and not hitting anywhere close to highway limits in Australia (~68 mp/h or 110 km/h).

Thoughts/queries/suggestions on this would be very much appreciated.

I think if anything taking it out on the road, taking it easy and feeling it out might be the best way to approach it but I've seen some solid RTT setups on the frontrunner for the JL that definitely are over 209-220 lbs (95-100 kg).

 

[dragoneggs]

Subscribed! As a 2dr guy that oscillates between rock crawling, overlandings, and yeah, a bit of in town 'mall crawling' as necessary... I love threads like these.

 

[Reinen]

Ah, it has 6 mounting points. I was thinking it was only the front and rear 4. That makes all the difference. The center mounts will resist a lot of side-to-side movement and the torqueing that can damage the HT. They're attached to a very sturdy mounting point and they are too short to flex.

I think you'll be fine at 100kg. The center and rear mounting points are very similar to my M-3 rack (although the pics are of the 4dr mount, the 2dr rear mounts are larger and sturdier) and it's well within the load limit of my rack. Just make sure you distribute the weight and keep heavier items near the center of the rack by the short sturdy supports. I'd keep lighter items in the front as those support bars are long enough to flex side to side a bit. That end won't be as strong as the rest of the rack in regard to dynamic weight. Medium weight items towards the rear.

Also remember it's not highway speeds that are the problem. That's smooth sailing assuming you don't crash. It will be the slower speed rutty, rocky, potholed dirt roads and trails that rock you back and forth. That's where the rack will be stressed the most. The bouncing and rocking you feel in the driver's seat will be amplified on the roof. If you ever went up the mast of a sailboat you'll know what I mean. If it's ever going to break it will very likely be at slow speed in off road conditions, not on the highway.

 

[OffroadWrangling]

Ah, it has 6 mounting points. I was thinking it was only the front and rear 4. That makes all the difference. The center mounts will resist a lot of side-to-side movement and the torqueing that can damage the HT. They're attached to a very sturdy mounting point and they are too short to flex.

I think you'll be fine at 100kg. The center and rear mounting points are very similar to my M-3 rack (although the pics are of the 4dr mount, the 2dr rear mounts are larger and sturdier) and it's well within the load limit of my rack.

Awesome, thanks for the feedback mate. Feeling far more confident in my purchase now. I called a business that specialises in fitting racks to jeep wranglers and he said 100 kg would be easy as on my rig without any issues so I think we are definitely in the clear!

This will probably be a good thread for other people in the future who are looking into purchasing frontrunner extremes for their 2D JL's because as far as I know, this thread is probably where its been most extensively discussed.

Just make sure you distribute the weight and keep heavier items near the center of the rack by the short sturdy supports. I'd keep lighter items in the front as those support bars are long enough to flex side to side a bit. That end won't be as strong as the rest of the rack in regard to dynamic weight. Medium weight items towards the rear.

Yep 100%. I am way too excited to take this badboy out once its fully fitted. I'll definitely provide an update in the future via this thread with pics of the fittings as well as the feel of the car with the load on top in case anyone is interested.

Also remember it's not highway speeds that are the problem. That's smooth sailing assuming you don't crash. It will be the slower speed rutty, rocky, potholed dirt roads and trails that rock you back and forth. That's where the rack will be stressed the most. The bouncing and rocking you feel in the driver's seat will be amplified on the roof. If you ever went up the mast of a sailboat you'll know what I mean. If it's ever going to break it will very likely be at slow speed in off road conditions, not on the highway.

For sure, that will certainly be the case. As I mentioned previously, I won't be doing any serious off-roading with a roof load anywhere near to the intended 92 kg and will for sure minimise to the more essential items if I were to do some more intensive off-road (probably just have the swag on the top, no awning or mattress).

On a side note, I love the side ladder you've got going on. I'll have to get one installed in the future, for the moment frontrunner only do a telescopic ladder w/ additional bracket supports but I'm more interested in a permanent solution that would be placed similarly to yours.

Here's to some great adventures!

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