Head to Head Review: 2023 RockShox Zeb vs Fox 38

If you buy a phone, it's going to be Apple or Android. If you board a plane, it's probably built by Airbus or Boeing. If you're after suspension for your mountain bike, it'll most likely be from Fox or RockShox. The duopoly has been battling it out for MTB suspension supremacy for years, and while there are plenty of other brands out there, they corner most of the market because they make consistently good products.

For that reason, the Fox 38 and RockShox Zeb are natural rivals. Both have 38 mm upper tubes and are designed for enduro, ebike and (dare I say it) freeride use. Soon after they launched, Dan Roberts compared the two here on Pinkbike and concluded that performance-wise, there was no clear winner, but added that "bringing in the factors of weight, price and setup alongside performance makes the Zeb stick its nose ahead of the 38". On the other hand, I did my own testing at my previous employer and favoured the Fox 38.

But for the 2023 model year, RockShox thoroughly overhauled the Zeb with a new damper, air spring and vibration-reducing elastomers between the fork lowers and internals, called Buttercups. They also added bleed valves to match those found on the Fox 38, alongside subtler tweaks. So how do the two stack up now? I've been re-testing to find out. But first, here's how they compare on paper.

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Weight, Cost and Options

Let's start with the easy stuff. The weight of the two forks is essentially identical. At 170 mm travel with uncut steerers, my sample 2023 Zeb weighs 2,337 g, while the 38 comes in at 2,363g. That's a small enough difference that it could potentially go either way depending on manufacturing variation, and it certainly shouldn't be a deciding factor.

The Fox 38 Factory is the more expensive fork in every currency. In the USA, the difference isn't huge, but in other regions, it's much larger. It's worth remembering that Fox also offers the Performance Elite 38, which has all the same features minus the Kashima stanchion coating, for a chunk less cash. It's hard to say for sure, but my feeling is that any differences are imperceptible on the trail. There's also the Performance model, which does without the bleed buttons and has a simpler GRIP damper, which I think is a superb choice, especially for lighter riders due to its lighter damping range.

Similarly, RockShox offers the entry-level Zeb Select, but that lacks the Charger 3 damper, pressure relief valves and extra bushing overlap of the Zeb Ultimate. There is the Zeb Select+, which is identical to the Zeb Ultimate except that it lacks the damper-side Buttercup and the extra Bushing overlap. Unfortunately, that's not for sale aftermarket.

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Technology and Features

These forks could trade blows all day long when it comes to feature count. And while the Fox 38 hasn't been significantly updated since it launched in 2020, the Zeb has seen a major overhaul to almost everything but the chassis.


RockShox Zeb features and updates

I wrote a whole article going into detail about the new technologies RockShox have brought out for their 2023 forks, which you can read here. These include Buttercups (rubber pucks at the bottom of the spring and damper designed to reduce high-frequency vibration); the Debonair+ air spring with more negative volume and a higher transfer port location designed to soften the touchdown feel, and the Charger 3 damper with more low-speed compression to compliment the softer initial travel of the Debonair+ spring, and less effect of the high-speed compression adjuster on low-speed damping behaviour.

For the Ultimate-level Zeb, RockShox has also increased the bushing overlap to reduce bushing friction when the fork is loaded up, and there are now bleed buttons to match Fox's, which allow excess pressure in the lowers to escape, thereby restoring off-the-top sensitivity after big changes of altitude or temperature.

RockShox retains their Torque Cap hub interface, which is claimed to provide a stiffer overall fork-and-wheel system when used with a Torque cap hup. It's interesting that these hubs haven't become popular aftermarket or among OEMs, as it seems to me to be a relatively lightweight and inexpensive way of adding stiffness. The main complaint with the fork interface is that installing a regular hub is more fiddly, but RockShox have addressed this with bolt-on inserts which mimic a regular 15 mm fork dropout.


Fox 38 Features and Updates

The 38 may not have been revised recently but it's still big on tech.

One of the biggest tricks hidden up its sleeve is the air spring, which includes a smaller diameter tube inside the fork stanchion. This arguably reduces friction by reducing seal contact area; it also allows Fox to vary the inner diameter of the stanchion wall for structural optimisation. But the biggest advantage is probably that it allows them to have a smaller diameter tube where the piston slides, and a larger diameter above where the piston sits at bottom-out. This increases the volume of the air chamber relative to the diameter of the piston, reducing the compression ratio.

This in turn allows Fox to use more of the available volume for the negative chamber - thereby improving sensitivity at the start of the stroke and adding support in the middle of the travel - without making the fork too progressive. In contrast, when RoxckShox increased the negative volume for 2023, they reduced the positive volume, which makes the fork very progressive in long-travel versions.

Fox's VVC valving is also interesting. Rather than changing the high-speed damping by changing the preload on a spring-loaded high-speed valve, Fox's VVC valves change the stiffness of a valve spring. The idea is to avoid the on/off behaviour that preloaded valves can have and make the damping response more proportionate at the extreme ends of the adjustment range.

Fox also boasts a floating axle design, which allows the fork legs to perfectly self-align independently of the hub width tolerances. Fox claims this can cut friction significantly compared to a traditional axle which squeezes both fork legs together against the hub. The quick-release axle is particularly neat as you only have to use the pinch bolt once per wheelset, to set the fork to the exact width of your hub. After that, you just use it like a regular QR axle until you change the wheels.

Finally, there's the Kashima stanchion coating found on the Factory model. When I asked them about how much effect this really has, Fox claimed that it "cuts friction by 10-12% compared to black anodized coating … and it increases hardness, which in turn increases scratch resistance and overall stanchion durability." Take that with a pinch of salt, but it's interesting to hear Fox put a rough number on the friction reduction.

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Spring Dyno Testing

Thanks to Mojo Rising in Monmouth for letting me use their spring dyno (spring force tester) once again. This is essentially a bottle jack with a force plate, which can be rigged to compress a fork a few millimeters at a time and measure the force required to reach each increment of travel. It's intended for testing shock springs, so unfortunately, it can't be used for the entire travel range of the fork - only the first 120 mm or so.

The above graph shows how the force curve of the Fox 38 compares to the 2023 Zeb. Both were set to 170 mm travel, with the settings I've been running on the trail - 68 psi and no spacers in the Zeb; 100 psi with one spacer in the Fox 38. The Zeb was tested on the dyno after it had a lower leg service.

You can see that they're both producing the same force at about 25 mm travel, which translates to similar amounts of sag. But you can also see that the Zeb requires much more force to compress into the very first part of the travel, requiring more than 100 N of force (about 10 kg) to compress by 5 mm. But after the sag point, the Zeb is providing less resistance through the mid-stroke. It's a shame I couldn't measure all the way to bottom-out, but I suspect the lines would converge, because of how the Zeb's curve starts to ramp up after 100 mm, and because I've been using similar amounts of travel on the biggest compressions.

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On the Trail

My first impressions of the 2023 Zeb were not good. While previous RockShox forks have always felt springy and supple, the 2023 Zeb was slow on rebound even when set to fully open. The small-bump sensitivity was also poor, which resulted in me dropping the pressure below the recommended 68 psi all the way to 62 psi to try and improve traction and comfort. This resulted in the fork using too much travel on bigger impacts, lacking support and predictability.

After a handful of rides, I decided to take the fork apart and have a look inside. I didn't measure how much oil was inside because it went straight into the oil tray, but it didn't look like the specified 20 ml per leg. I cleaned the seals, re-lubed the foam rings and injected 20 ml of oil into each leg. The suppleness was dramatically improved. This allowed me to increase the pressure to 68 psi for more support, while still maintaining good sensitivity.

The Zeb is decidedly progressive. Even at 170 mm travel I was struggling to get anywhere close to full travel with the single volume spacer the fork ships with. Even with it removed, I'm rarely seeing more than 160 mm of travel. When riding the Zeb at 190 mm on a Pole Voima, I fitted a TruTune insert (which does the opposite of a volume spacer) to make it more linear and allow access to more of the travel.

Even after changing the oil, the rebound isn't that fast when fully open. With the previous Zeb, I was running about 10-12 (of 18 ) clicks from closed and you can definitely set it too fast, but every 2023 Zeb I've ridden I've ended up setting it fully open, and that is something I've heard echoed from some other testers.

With the Zeb dialled in, I jumped back on the Fox 38 for some back-to-back testing. The best way to summarise the difference in a word is that the Fox feels more "springy". There's noticeably more spring force holding you up in the middle third of the travel and the rebound (although not quite fully open) was faster. Combined, this made the 38 feel more eager and responsive, like a boxer who gets straight back in the fight even after taking a heavy hit. The Zeb feels lazier by comparison, taking slightly longer to recover.


Also, when riding without braking over rough and fast sections (like the one shown above), the fork is regularly cycling in and out of the first part of the travel. Sometimes the wheel loses contact with the trail momentarily, before reconnecting again. This can happen surprisingly often on fast, rough tracks. In these situations, the 38 feels a little smoother when it reconnects with the ground. Ironically, RockShox came up with a brilliant name for this attribute - "touchdown feel" - but on the trail, it's the Fox that scores highest in this metric. This is backed up by the force curves shown above, which I produced after doing the bulk of on-trail testing.

But sensitivity is nothing without support. If set fully open, the Zeb sometimes lacks support and uses more of its travel than expected. Interestingly I found this to be more problematic on the Hope HB916 than the Canyon Strive, presumably because of the rearward axle path putting more weight on the front in big compressions. Fortunately, adding compression (especially if you set both LSC and HSC to halfway) increases support noticeably without making the fork too harsh. The compression adjusters have a more dramatic effect on the Zeb than the Fox, and I used them more often to add support for steeper tracks, then back them off when sensitivity was a priority. But with the Fox, the more linear spring curve allows it to balance sensitivity and support well for a broad range of terrain without having to touch the compression adjusters. So although the Fox has more dials, it's more of a set-and-forget option.

One criticism that's often levelled at the Fox 38 is that it sits too low in its travel. You can see from the above force curves that it gives away the first 10 mm of travel without much of a fight, and it sags readily into the travel. In my view, though, this is a good thing as it improves the off-the-top sensitivity, giving a "stuck to the ground" sensation while providing plenty of support deeper in the middle third of the travel. To those who find the 38 (or any fork) sits too low in the travel, I'd recommend trying a higher bar height, removing a volume spacer and increasing spring pressure relative to the setup chart, or even trying a 10 mm longer air shaft so you have the desired ride height at sag without having to compromise the spring curve at the start of the travel. Speeding up the rebound from the recommended setting will also help the fork ride higher in its travel on the trail (this is true of the Zeb and Fox 38 ).

In terms of long-run comfort, both are excellent and I can't pick a winner. A lot of reviewers have suggested that stiffer forks lead to more harshness, but I really don't think there's any comfort or traction compromise when compared to a smaller stanchion alternative. Both forks offer impressive sensitivity and comfort, and in my view, the burlier forks transmit less harshness and remain more composed in the chunkiest terrain.


Durability

Aside from the issue I discussed with the initial stickiness that was fixed with a lower leg service, I also had the compression adjusters come loose and stop working on the Zeb. This was even simpler to fix, only requiring the dials to be reset and the securing bolt torqued up. Aside from that, I've put in a lot of time on this fork and a few other examples with no major issues, as you would expect.

With the Fox 38, I have even more experience, having ridden many examples, including one fork I've tested extensively since early 2020. No issues to report here, and in fact, I've been impressed with how slowly the performance degrades between services. You still notice when the oil and seals are refreshed after a few months of regular riding, but it's not the night and day difference you get with a fork that's desperate for a refresh.

Officially, RockShox recommends a lowers service every 50 hours and a full service every 200 hours of riding. Fox says 125 hours or yearly for a full service, or more regularly for extreme use or maximum performance. Honestly, though, these guidelines aren't really comparable. Both forks use at least 20 ml of oil in each leg (40 ml in the 38's damper side) which presumably extends the performance window compared to forks using lower oil volumes.

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Verdict

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Posted In:
Reviews and Tech Reviews Suspension Fork Fox Factory RockShox Fox 38 Rockshox Zeb