Why 84.39m for the straight on the track and field ?

Just out of curiosity, why 84.39m why not 80, 85 or 90m instead ?

http://track.isport.com/track-guides/track-field-dimensions

I thought it might be an English system measurement, but it's not quite.

Do you mean "Imperial system measurement".

LOL! Whatever the bogus system of weights and measures the US is stuck with.

Yeah you are right DrSpudnik, it got noting to do with the Imperial system. I understand it wouldn't make sense (or look right) to have a 100m straight, the curves (or bends) will be narrower. 

You really want to consider how many kilometres a litre you're getting, rather than miles per gallon?

Your head would explode! (at a velocity measured in metres per second)

anyway I could be wrong about this.....

The 400 metres, or 400 metre dash, is a common sprinting event in track and field competitions. It has been featured in theathletics programme at the Summer Olympics since 1896 (1964 for women). On a standard outdoor running track, it is exactly one lap around the track. Runners start in staggered positions and race in separate lanes for the entire course. In many countries, athletes previously competed in the 440 yard dash (402.336 m)—which is the length of a quarter of a mile and was referred to as the 'quarter-mile'—instead of the 400 m (437.445 yards), though this distance is now obsolete. An athlete who competes in the 400 m may still be referred to as 'quarter-miler'; the distance or event might still be called the "quarter."

https://en.wikipedia.org/wiki/400_metres

I'm confined to quarters!

You maybe right there about the imperial thing. In America in those days may have been using the 440 yrds track and are still being used in some high schools. See dia. (note: is from the army)

Note the radius here is 40 yrds = 36.570m which is the same for the 400m track but the straight has to be shorter for the 400m. 440 yrds =  402.336m.