The back end: subframe, Tossa R line, solo or pillion

Where it stands

I’m leaning to Tossa R’s line over Lakic’s. Tossa R (a workshop in Sofia, Bulgaria) built a 2021 CX500 on a CX500E Eurosport, the same Pro-Link frame as mine, and all their CX650s are CX650E frames from the same family. So their rear end copies straight across. Nothing was grafted on.

What they published:

• Cut off the rear subframe and the section above the centre posts.
• Built a seat support from steel plates, two inner and two outer, MIG-welded and bolted along the backbone. The seat pan’s front bolts double as the tank’s rear mounts.
• Kept the factory Pro-Link, with a CBR1000RR-pattern Öhlins. On the CX500 they moved the top mount so the shock shows from the side.
• Stock rear wheel painted black under an aluminium moon disc.
• Their own Eurosport tank, cut at the rear and tilted until the top is level. From the photos that’s about 3–6° nose-down, not Lakic’s 10°.
• Short solo tail ending ahead of the rear axle.

They haven’t published rake, trail, offset, shock length, plate thickness or seat height. The “6 mm laser-cut plates” figure in my original build manual has no source.

Is the stock rear frame removable?

No. Honda lists the whole frame as one part, 50100-MC5-610ZA. The seat, grab rail, battery band, rear guard and taillight all bolt to a welded rear loop. It has to be cut.

The concept

• Cut the two seat rails only, about 60 mm ahead of the rear axle line. Keep the rear strut, the shock top mount and its cross tube. That leaves Honda’s rail, strut and pivot-plate triangle to carry the load.
• Weld one interface plate to each side, once.
• Solo and pillion subframes bolt to the same four M10 bolts.
• The pillion subframe needs a brace. The concept maths gives it a safety factor of about 2.6 with the brace and 1.3 without.
• Pillion pegs go on bolt-on hangers on the kept rear strut, blanked off in solo trim.

Design loads: me at 100 kg (checked at 130), a 100 kg pillion and 10 kg of luggage, a 3 g bump, and a pillion standing on one peg.

Three risks to settle first

1. I haven’t found an LVVTA document that allows two seat configurations on one certificate. Ask the certifier first.
2. Two-up at those loads is about 30 kg over Honda’s maximum total weight for the bike.
3. The pillion tail runs about 170 mm past the axle, so it’s longer than the Tossa look. Solo keeps the look.

Rider fit (I’m 183 cm)

• My knee clears the heads. The risk is my shin brushing the back-top corner of each valve cover.
• Sitting about 50 mm further back than the Tossa seat line, with the pegs about 40 mm higher than stock, gives about 60 mm of clearance.
• Full rearsets make it worse. Flush clip-ons work. A clubman bar is the one to avoid with the stock tank.

What would change it

Every number above is a concept, built on frame positions estimated to ±20–30 mm from Honda’s drawings and photos. Before anything is cut:

• Measure the bike: swingarm pivot, shock top mount, seat-rail heights, rear strut, tank mounts.
• Tape a valve cover and sit on the stock bike to calibrate the fit.
• Mock up a plank seat and an adjustable peg plate.
• Talk to the LVV certifier, and show them bare welds before powder coat.

The shock choice is on rear shock.