flatbase wheel Flatbase wheel:
A wheel with centre section and rims in the same diameter is named a flatbase wheel.
BBS flatbase wheels are available in 13˝ (16 bolts) and in 15˝ (20 bolts).
Because of the equal diameter of the flatbase wheels shape, it is not possible to mount or remount a tyre on a flatbase wheel as usual! The wheel has to be disassembled for tyre changing and then reassembled on the tyre! Otherwise damage is caused!

First the bots should be tightened lightly and the wheel centres should be centred exactly (there may be 0.7 to 0.8 mm / 0,028 - 0,031˝ space between center and rim). Then the bolts/nuts have to be thightened crosswise up to 10 Nm (7,4 lbf ft / 88,5 lbf in) and finally to 15 - 16 Nm (11 - 12 lbf ft / 133 - 142 lbf in).
To change tyres later, first the tyres flanks should be pressed over the rims hump (by machine), than the wheels can be disassembled to take the tyre off.
Note: Use new sealing rings and new rim bolt sets everytime the wheels are reassembled.

Flatbase wheels always were delivered in pieces – for self assembling.
drop-centre-wheel Drop-centre wheel:
A wheel with rims in a bigger diameter than the centre section makes a drop-centre wheel.
BBS drop-centre wheels are available as 13˝ (12 bolts), as 14˝ and 15˝ (16 bolts), as 16˝ and 17˝ (20 bolts; centre section E87) and as 18˝ and 19˝ (24 bolts; centre section E88).
Because of the smaller diameter in the middle of the drop-centre wheels shape a tyre can be mounted in the same way as on a road wheel.
Drop-centre wheels will always be delivered completely assembled at no extra charge.
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