I've had a conversation with my tyre supplier, Steve Lomas of "5-1", he is the ex speedway and now sandracer I have already mentioned.
He confirms that the 20" has not been available for some years now, what he has done several times, with complete success, is to fit a 2.50/2.75 x 21" tube. When very slightly inflated the diameter is the same as the inside diameter of the 20" tyre. The extra length can be accomodated I guess.
He advised that the fitting of moto cross tubes is not advised because of the thicker wall allows the building up of heat. I guess this would apply if you travelled a long way, at high speed, in the summer. Would not apply perhaps if you just pottered about in usual British weather! For moto crossing, perhaps the heat build up is accepted.
The 2.50/2.75 is I suppose the "small" 21" tube that has been mentioned.
He confirms that the 20" has not been available for some years now, what he has done several times, with complete success, is to fit a 2.50/2.75 x 21" tube. When very slightly inflated the diameter is the same as the inside diameter of the 20" tyre. The extra length can be accomodated I guess.
He advised that the fitting of moto cross tubes is not advised because of the thicker wall allows the building up of heat. I guess this would apply if you travelled a long way, at high speed, in the summer. Would not apply perhaps if you just pottered about in usual British weather! For moto crossing, perhaps the heat build up is accepted.
The 2.50/2.75 is I suppose the "small" 21" tube that has been mentioned.