Can anyone tell me why the kingpin is placed where it is, and what determines the distance from the front of the trailer?
Are they set all the same or differently.
There yougo good buddy. Anthing else over and above that just let me know and il try to help. Over and out .
Saves explaining it all really.
That doesn’t really answer the OP’s question. Whilst specifics between our countries may vary, the basics remain the same.
The placement of turntables and kingpins can vary to alter or optimize axle loading. When coupling care has to be taken to ensure there is adequate clearance between the back of the cab and front of the trailer and between the legs and light bar/rear of the prime mover. Don’t forget these clearances reduce when turning.
The Kingpin happens to be the Chinese Leader at this point in history - as it is the only superpower leader needing a politbureau to stay in power, rather than just jailing the leader of the opposition sooner or later… 
There are dimension laid down in Construction and use regs, I believe a standard trailer, its a maximum of 12m from the pin to the rear of the trailer and a 2m arc from pin to the front, so that gives you 1.6m at each front corner and so you get a 13.6m trailer.
These are maximum you can have short pin trailers and there are exceptions to the standard dimensions regulations.
Can you still adjust them depending on the trailer your pulling, I remember if you worked a lot of different types of trailers you could bring it back or forward in a sliding motion.
At one time king pins were fitted a lot further forward than of now on full length trailers.
Trailer lengths have increased and so have tractor cab and wheelbase lengths, sleeper cabs and multi axle tractor units.
Kings pins have to be in place where correct weight distribution is achieved, but also to allow the trailer to run as close as practicable to the cab for both length limits and aerodynamics.
Drivers of old motors will recall the days when their short day cab tractors couldn’t manage to pull a 48" pin tilt, in extreme cases couldn’t even hitch up let alone turn the thing, where they were used to having lots of space to fit suzies without any of this split coupling or Mavis rail cobblers.
Drivers with modern rear tag axle tractors will now and again come up against an old design trailer they too can’t hitch up to because the legs are too far forward.
Juddian:
At one time king pins were fitted a lot further forward than of now on full length trailers.
Trailer lengths have increased and so have tractor cab and wheelbase lengths, sleeper cabs and multi axle tractor units.Kings pins have to be in place where correct weight distribution is achieved, but also to allow the trailer to run as close as practicable to the cab for both length limits and aerodynamics.
Drivers of old motors will recall the days when their short day cab tractors couldn’t manage to pull a 48" pin tilt, in extreme cases couldn’t even hitch up let alone turn the thing, where they were used to having lots of space to fit suzies without any of this split coupling or Mavis rail cobblers.
Drivers with modern rear tag axle tractors will now and again come up against an old design trailer they too can’t hitch up to because the legs are too far forward.
And that’s why there was a sliding fifth wheel
I know it’s silly but I wouldn’t work at a firm in Which I had to regularly adjust the fifth wheel unless the pay was great.
It’s a pain in the ■■■ and I just know one day I’m gonna forget to readjust the fifth wheel and hook up to a reefer only to get my wind deflectors caved in. 
adam277:
I know it’s silly but I wouldn’t work at a firm in Which I had to regularly adjust the fifth wheel unless the pay was great.It’s a pain in the ■■■ and I just know one day I’m gonna forget to readjust the fifth wheel and hook up to a reefer only to get my wind deflectors caved in.
Takes 2 minutes to shift a slider once you get your head around the sequence, the secret is to shift it now and again even if not needed, and keep the slider mechanism greased but the sliding plates not overgreased if you follow, don’t want the slider guides too slippery but if rusted solid it might as well have been a fixed option, just use it now and again to keep it working.
As for hitting the deflectors, that’ll be the same crew who miss the pin, who reverse up and under in one movement by mirrors alone, safe to say if it was their owner driver wagon and trailer they’d be doing the job properly or going out of business.
A slider isn’t always the answer by the way, even with a slider there will be times when the tractor and trailer can’t play ball, these days its usually because the trailer is old especially if its a shorty (there won’t be room between fifth wheel and rear of chassis to clear the trailer legs), when a rear tag tractor meets and old shorty they can’t always mate.
For most of us there’s a sweet spot in the middle of the slider locks where we can operate 99% of trailers we’ll ever come up against, for myself i set mine for ideal tractor unit axle weights and judge handling accordingly, i make use of an axle weigher for this but it can be done on a normal weighbridge via moving forward one axle at a time when fully loaded and working out what each one weighs that way…rather be 6" overlength than driving a motor that doesn’t handle right or has axle weight issues.
You should be looking for that job where the pay is great anyway 
Juddian:
adam277:
I know it’s silly but I wouldn’t work at a firm in Which I had to regularly adjust the fifth wheel unless the pay was great.It’s a pain in the ■■■ and I just know one day I’m gonna forget to readjust the fifth wheel and hook up to a reefer only to get my wind deflectors caved in.
Takes 2 minutes to shift a slider once you get your head around the sequence, the secret is to shift it now and again even if not needed, and keep the slider mechanism greased but the sliding plates not overgreased if you follow, don’t want the slider guides too slippery but if rusted solid it might as well have been a fixed option, just use it now and again to keep it working.
As for hitting the deflectors, that’ll be the same crew who miss the pin, who reverse up and under in one movement by mirrors alone, safe to say if it was their owner driver wagon and trailer they’d be doing the job properly or going out of business.
A slider isn’t always the answer by the way, even with a slider there will be times when the tractor and trailer can’t play ball, these days its usually because the trailer is old especially if its a shorty (there won’t be room between fifth wheel and rear of chassis to clear the trailer legs), when a rear tag tractor meets and old shorty they can’t always mate.
For most of us there’s a sweet spot in the middle of the slider locks where we can operate 99% of trailers we’ll ever come up against, for myself i set mine for ideal tractor unit axle weights and judge handling accordingly, i make use of an axle weigher for this but it can be done on a normal weighbridge via moving forward one axle at a time when fully loaded and working out what each one weighs that way…rather be 6" overlength than driving a motor that doesn’t handle right or has axle weight issues.
You should be looking for that job where the pay is great anyway
I never drive the same cab. I’m a permanent cab hopper agency bod. So ■■■■ keeping it greased.
I should really try a tramping job / permanent job at some point.