Flat Vs Conical Thru Axle Spacer Bike Trailer Uk

Match the spacer or axle head seat to the dropout interface: a flat seat for flat faces, and a conical (tapered) seat when the frame is recessed for a cone. The wrong stack is a frequent reason a UK trailer hitch rocks, sits crooked, or bottoms out on the threads before the wheel is centred.

Why the hitch rocks or will not clamp flush

Thru axles are made with either a flat or conical (tapered) head geometry, and that geometry must be identified before you commit to a trailer axle — the Thule UK fit guide on head shape states the requirement clearly. UK resellers likewise treat straight/flat versus coned head shape as a required measurement and note that spacers are used to take up shorter shaft lengths (Roof Box Company spacer and head notes).

Some Burley-channel 12x1.0 trailer axles are sold with both conical and flat spacers for common hub spacing (Fawkes Cycles spacer inclusions), which underlines that the seat style is part of fitment, not a cosmetic washer.

Solution sequence

1. Photograph the stock axle head and the non-drive dropout face before you strip the bike.
2. Trial-fit the replacement axle and hitch plate without the trailer arm attached so you can see gaps clearly.
3. Add only the spacer style the frame requires — flat on flat, conical into a recessed cone — rather than stacking “extra” washers to invent length.
4. Re-check that the wheel is centred and the hitch plate sits square to the dropout.
5. Confirm threads engage smoothly with no forced cross-threading.

Fit checklist before the first ride

• No visible gap at the non-drive seat once the correct spacer is in place.
• Hitch plate sits square, not tipped toward the disc rotor or derailleur.
• Axle length still suits the trailer adapter window you measured earlier (measurement checklist).
• Pitch already matched — spacers cannot fix a wrong thread (pitch comparison guide).

Who should stop and get help

• Damaged or egg-shaped dropouts.
• Frames that require an OEM conical interface that is missing from your kit.
• Axles outside the supported length band you measured for everyday 12 mm trailer adapters.

Optional next step

Once seat type and pitch are identified, compare matching 12 mm options on the ThruSports thru-axle adapter detail page. Then walk the UK hitch safety checklist before school-run or park miles.