Design & Aerodynamics
The MonoCup fills the space between the forearms, reducing airflow into the chest cavity and smoothing the front-end profile of the bike. This helps improve airflow consistency through the cockpit while maintaining easy access to hydration.
Designed specifically for the Trek Speed Concept Gen 3, the system bolts directly into the existing arm cup mounting points — no bolt spacing measurements required.
Fit Type
Snug
The cups sit closer to the forearm with minimal clearance, providing a secure, locked-in feel and firm support.
Spacious
The cups allow more clearance around the forearm, reducing pressure and giving a more relaxed, forgiving feel.
Forearm Support & Comfort
The MonoCup provides continuous forearm support, helping riders stay stable and relaxed in the aero position for longer efforts.
Standard side height:
~60 mm at the rear
~40 mm at the front
The stepped profile supports the forearms while allowing natural wrist and hand positioning.
Cup length options can be selected based on rider preference and riding style, from minimal support through to full forearm coverage.
Cup Spacing Options
Standard spacing: 15 mm between forearms
This is a very narrow setup and works well for riders chasing maximum aerodynamic efficiency.
Adjustable range: 15 mm – 60 mm between forearms
Wider spacing options are available for riders prioritising comfort, breathing space, or shoulder mobility.
Spacing is selected during ordering — no post-fit modification required.
Measurements
1. Arm Cup Length
Step 1 – Identify Points A and B
Using the diagram above:
Point A is the rear reference point at the base of the arm cup mounting area.
Point B is the start of the extension upturn or bend.
Step 2 – Measure the Distance
Measure the straight-line distance from Point A to Point B along the extension.
This measurement confirms the maximum usable arm cup length before the extensions begin to turn upward.
Important Adjustment
If the measured distance from A → B is less than 250 mm:
Take your measured distance
Subtract 10 mm
This allowance ensures the MonoCup does not clash with the extension upturn and maintains proper clearance as the extensions rise.
Example:
Measured A → B distance: 180 mm
Final arm cup length to select: 170 mm
Recommended Limits
Recommended minimum arm cup length: 125 mm
Maximum arm cup length: 250 mm
2. Forearm Circumference
Use the image above as a visual guide when taking these measurements.
Step 1 – Elbow Circumference (Point A)
Measure the circumference of your forearm at Point A, which is at the elbow / thickest part of the forearm.
Wrap a soft measuring tape around your arm so it sits snugly, without compressing the skin.
Step 2 – Forward Circumference (Point B)
From Point A, move down your forearm toward the wrist to Point B.
The distance between A → B should match your chosen arm cup length (shown in the image as the approximate arm cup length).
At Point B, measure the circumference of your forearm again using the same method.
3. Arm Cup Mounting Hole Measurements
To ensure your UnderCup BTA Hydration System fits perfectly, we require the mounting hole spacing of your arm cups. This allows us to manufacture your system to match your cockpit for a precise, secure installation.
You'll need to provide two measurements.
1. Horizontal Spacing
Measure from the left edge of the left mounting hole to the left edge of the right mounting hole.
Although this is referred to as the centre-to-centre spacing, measuring from the same edge of each hole provides the exact same result and is much easier to do accurately.
2. Vertical Spacing
Measure the distance between the top edge of the upper mounting hole and the top edge of the lower mounting hole.
As with the horizontal measurement, using the same edge of each hole provides the equivalent centre-to-centre spacing.
Tips
Use digital calipers where possible for the most accurate measurement.
A steel ruler can also be used if calipers are unavailable.
Measurements should be taken with the arm cups installed or removed—whichever provides the clearest access to the mounting holes.
Double-check your measurements before submitting your order to ensure the best possible fit.
Figure 1 – Measuring Arm Cup Mounting Hole Spacing
Use the image below as a reference when taking your measurements.
1. Horizontal Spacing: Measure from the left edge of the left mounting hole to the left edge of the right mounting hole.
2. Vertical Spacing: Measure from the lower edge of the upper mounting hole to the lower edge of the lower mounting hole.