TYRES

For several decades sidecarrists have been successfully fitting 15” tyres, firstly from 2CVs and then from Smarts, to sidecar outfits. There are, however a few pitfalls.


 

Nominal wheel size

15”

16”

17”

Rim height

Bead seating width


in

mm

in

mm

in

mm

mm

mm

Car

14.968

380.17

15.968

405.59

17.189

436.60

17

19.8

Motorcycle

15.080

383.03

15.978

405.84

17.080

433.83

14

16

difference


2.86


0.25


-2.77



Source: The Tire and Rim Association Inc., 2004



Firstly, as you can see from the table above, the 15” motorcycle rim is nearly 3 mm larger in diameter than the 15” car rim. (This is because car wheels and bicycle wheels have different histories.) If you fit a car tyre to a motorcycle rim you have to stretch the bead by 9 mm to get it on (2.86 x π) and you’ll have to use over 100 psi to achieve this and it’s dangerous! I think this has been fairly well recognised amongst sidecarrists in recent years, but I still come across cases of people struggling to fit car tyres to 15” bike rims.


 

 

 

 

 

 

 

 

 

 

 

As engine power and braking performance have increased tyres have got wider, and wide tyres on the front of an outfit make the steering heavier even if the trail is reduced, so there has been a movement recently toward fitting superscooter rear tyres to the front of outfits. A scooter tyre should only be fitted to a motorcycle rim. It will be slack on a car rim so it’s not just a matter of replacing the car tyre – you must replace the rim as well. There is a distinct possibility that a scooter tyre will not stay on a car rim in the event of a puncture.


With 17” rims it’s the other way round: car tyres are loose on bike rims by 2.77 mm. Everyone has been so happy to find that the 17” car tyres are easy to fit, that they have not looked any further. The tubeless tyres are sealing satisfactorily probably because the bead, being 3.8 mm wider is sitting on the safety hump, which is closer to the edge on a bike rim (see table and diagram) and the problem with that is that, once again, the rim may not retain the tyre in the event of a puncture because the bead is already on top of the hump.


The difference of 0.25 mm between the diameters of 16” wheels really isn’t significant, but even then you must bear in mind that the car bead will not sit within the safety hump on the rim of a bike wheel and once again there is a risk of the tyre coming off its seating.


The only safe option is to fit car tyres to car rims and bike tyres to bike rims.


 

 

 

 

 

 
 
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