From Single Crown to Double Crown: all You Need to Know Before the Swap
Thinking about upgrading your mountain bike with a #dualcrown fork like a Fox 40, RockShox Boxxer, or Rux38? You’re not alone. Riders of #enduro, #freeride, #Bikepark and #gravity bikes are increasingly making the switch from single crown forks to dual crown setups – for more stiffness, confidence, and fun.
But this upgrade isn’t just a plug-and-play. There are a few things you need to know first – especially about your bike’s head tube, fork steerer, and wheel compatibility.
1. What Kind of Head Tube Does Your Bike Have?
Before you choose your new fork, you need to understand your bike’s head tube. This is the part of the frame where the fork goes through.
There are three main types:
- Straight head tube:
Same internal diameter top and bottom (often 44 mm or 49 mm). Common on older bikes. - Tapered head tube:
Narrower at the top (usually 44 mm), wider at the bottom (usually 56 mm). This is the most common setup on modern bikes. - Integrated or semi-integrated (IS, ZS, EC):
These terms describe how the headset bearings sit inside the frame. For example:- ZS = Zero Stack (bearings sit inside the frame)
- EC = External Cup (bearings sit outside the frame)
- IS = Integrated (bearings sit directly in the frame, no cups)
Not sure what you have?
Remove your stem and top cap, and measure the inside of the head tube:
- Top should be around 44 mm
- Bottom usually 49 mm, 56 mm, or 62 mm
You can also send us a picture and we’ll help you identify it.
2. Will a Dual Crown Fork Fit My Head Tube?
Most dual crown forks (Fox 40, Rockshox Boxxer, SR Suntour Rux38) come with a straight 1 1/8” steerer tube, but most modern (Non DH) bikes have a tapered head tube.
To make it work, you’ll need:
- A reducer headset (e.g. to fit 1 1/8″ in a 56 mm lower head tube), or
- A reach-adjust headset to gain some extra cockpit length (+5mm/-5mm)
There are solutions for almost every head tube standard – even older straight 1 1/8” bikes can take dual crowns directly (if there’s clearance).
3. Is the Steerer Tube Long Enough?
Dual crown forks are designed for downhill bikes, which often have shorter head tubes. As a result, the steerer tube may not be long enough for your bike if you have a taller head tube, spacers, and a stem.
Measure it first:
Add up the head tube length + headset + spacers.
Compare that with the fork’s actual steerer tube length.
If it’s too short, the conversion might not be safe or possible without major modifications.
4. Frame Clearance for the Upper Crown
Dual crown forks have two crowns: one at the bottom and one near the handlebars.
Make sure there is enough space between your top tube and down tube for the upper crown to move freely when turning the handlebars.
If your frame is compact or has a steeply sloped top tube, test-fit the fork before committing.
5. Will My Front Wheel Fit?
Dual crown forks usually come with a 20 mm axle. Most trail/enduro wheels are 15×110 mm Boost – and won’t fit directly.
Instead of buying a new wheel, use the Freakynuts adapter to fit your 15 mm Boost front wheel safely in a 20 mm fork.
This way, you keep your current wheel and ride right away.
6. Brake Mounts and Rotor Size
Dual crown forks come with Post Mount 20 brake mounts, meant for 200 mm rotors or larger.
Check if your brake caliper fits, or use an adapter. Smaller rotors are not ideal for heavy DH use – consider upgrading to 200 or 220 mm for maximum power.
Summary: What to Check Before the Swap
| What | Details |
|---|---|
| Head tube type | Know your headset standard – tapered, straight, ZS, EC, IS |
| Steerer compatibility | Use a reducer or reach-adjust headset if needed |
| Steerer tube length | Confirm it fits your frame + spacers + stem |
| Frame clearance | Enough room between top tube and upper crown |
| Front wheel fit | Use a 20 mm wheel or a Freakynuts 15→20 mm axle adapter |
| Brake setup | PM20 mount, 200+ mm rotor recommended |
Final Thoughts: Dual Crown = Big Fun
Upgrading to a dual crown fork can completely transform your ride. Ride with more stability, More confidence, More fun – especially on steep trails, park laps, or gravity missions.
With the right Freakynuts headset and the Freakynuts axle adapter, you don’t need a DH frame or a new wheelset to make it work.
Got questions? Send us your setup – we’ll help you make it fit.
➡ Shop adapters, hardware, and tech tips:
www.freakynuts.com