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The answer for a bike with no straight top tube

Allen Adapter bar for frames without a top tube

A frame adapter bar creating an artificial top tube on step-through, ladies' and BMX frames, for bikes with 46 to 68 cm (18.1 to 26.8 in) between bar and saddle.

Allen Sports 900B bike frame adapter bar fitted between handlebar and saddle

Allen Adapter bar for frames without a top tube

A frame adapter bar creating an artificial top tube on step-through, ladies' and BMX frames, for bikes with 46 to 68 cm (18.1 to 26.8 in) between bar and saddle.

Purpose
Replaces the top tube on frames without a suitable one
Fits
Bikes with 46–68 cm (18.1 to 26.8 in) between handlebar and saddle
Tension
Internal spring maintains tension in transit
Hooks
Plastic coated, protecting the finish
Amazon reference
B000ELSSZE (identical on 6 markets)
EAN
0765271900007
  • Solves a problem nothing else here addresses — a rack arm needs a straight top tube
  • A published fitting range, 46 to 68 cm (18.1 to 26.8 in) between bar and saddle
  • Rank 23,508, so little sold — it is a niche fix

Best for: Carrying a step-through, ladies' or full-suspension bike on a rack designed for a diamond frame.

≈ $25
Check price on Amazon

Specifications

Purpose
Replaces the top tube on frames without a suitable one
Fits
Bikes with 46–68 cm (18.1 to 26.8 in) between handlebar and saddle
Tension
Internal spring maintains tension in transit
Hooks
Plastic coated, protecting the finish
Amazon reference
B000ELSSZE (identical on 6 markets)
EAN
0765271900007

A rack arm clamps a top tube. A great many bikes do not have one, and this is what fixes it.

What the manufacturer publishes

A crossbar or top tube replacement for bikes without a suitable frame, for example ladies’ or BMX bikes. An internal spring keeping tension stable during transport. Plastic-coated hooks protecting the bike’s finish. Suitability for bike frames with 46 to 68 cm (18.1 to 26.8 in) between handlebar and saddle. Dimensions of 53.34 × 15.24 × 3.556 cm (21 × 6 × 1.4 in).

The problem, stated plainly

Almost every towbar and boot rack holds a bike by clamping an arm to the top tube. A step-through frame has a sloping tube where the rack arm cannot reach, or none at all. Full-suspension frames often have a top tube interrupted by a shock.

Without an adapter, the bike simply cannot go on the rack.

How it works

The bar hooks between the handlebar stem and the seatpost, creating a horizontal tube at the right height for the rack arm to clamp. The internal spring keeps it under tension so it cannot fall out on the move.

The measurement to take

Handlebar to saddle, with the saddle at the height you actually ride. That single figure has to fall inside 46 to 68 cm (18.1 to 26.8 in).

The questions riders ask

Do I need one?

Only if your bike has no straight top tube for the rack arm to clamp. Step-through frames, many ladies' bikes, BMX bikes and some full-suspension designs all fall into that category.

Will it fit my bike?

Allen states 46 to 68 cm (18.1 to 26.8 in) between handlebar and saddle. Measure that distance on your own bike with the saddle at riding height.

More rack straps and frame adapters

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Elsewhere in the spares drawer

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    The entry road tire here: a folding 700x25C with a 180 TPI casing and a silica-based tread compound, sold singly.

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    Compound
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    Amazon reference
    DE B082VNGXDZ · US B082VNGXDZ · GB B08PP7KKXL · FR B08PP7KKXL · IT B08PP7KKXL · ES B08PP7KKXL
    EAN
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    • A published TPI figure, which is the honest measure of casing quality
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All rack straps and frame adapters