large section tyres on narrow rims .

[canny colin]

 Is a Schwalbe marathon plus 54/599 ( 26X2.10) , Mounted on a  599x 18 rim a safe combination . Looks weird to my untrained eyes .   

[LittleWheelsandBig]

https://bikerumor.com/tech-story-match-bicycle-tire-width-rim-width-best-results/

[canny colin]

Thanks LwAB . That combination is on a S/H mountain trike ,I have acquired . I bought it for the new  Trykit conversion axel  . Apparently built up by a professional bike shop . Certainly felt weird on fast corners .   

[LittleWheelsandBig]

Wide tyres on narrow rims and high side loads need higher pressures and stiffer sidewalls to keep the tyre stable on the rim. Marathon Plus tyres have very stiff sidewalls.

Max tyre pressure is limited by tyre width (and rim strength). Check what pressure your rim manufacturer advises.

[canny colin]

Thanks LWaB . I am in the process of striping it down , the axel is going on my old Claud butler frameset re rimed with 622 . I was going to fit the rims and tyres on a friends bike , but the rims  are too narrow according to the tables   . I would feel happier fitting wider rims  or fit different tyres . He just use the bike to get about with minimum maintenance . So the tyre pressures will only be checked when I look at it once a  years .