This is a very interesting one.
First a bit of history.
Back in the Bias Ply era, it was common for manufacturers to dual mark tires for Front and Rear use, the respective indicator arrows being opposite.
Such tires were raced in this manner, Isle of Man included, with perfectly acceptable results.
This said in terms of also including heavy powerful race bikes with twin disc brakes, and not just low powered drum braked single cylinder 4 strokes.
If one accepts that the primary reason for doing so is to avoid stressing the tread overlap joint the wrong way, it makes sense
My recollection is that any such tire of the day also had what I will call a non directional tread pattern.
In other words, no matter which way the tire was mounted, the tread looked the same in terms of how it's footprint first contacted the pavement on the basis of the bike going forward.
A good example of this is the ancient set of Pirelli Phantoms that are on my sleeping ‘73 SOHC CB750, and the Dunlop K81s I used to run before the Pirellis, were just the same in terms of the above mentioned.
However, such tires were available in sizes "correct" for fronts or rears, meaning there was also a(n acceptable) match of tire size to rim width.
From the above, the key points are carcass design and tread pattern suitabilities re dual rotation use, and tire/rim width matching.
I’ll also add in something here about so called “V’d” front tread orientation.
I just happen to have a current and originally tread patterned dedicated front Metzler Lasertec ready for my ‘73. (they were an awesome wet tire back in the day, some say the definitive wet tire)
It’s V pattern is arranged as are modern sport tires, as in the outboard end of the tread openings lead in terms of contacting the road surface, with the root of the V trailing.
As for the use of a 160/60R18 Rear backwards instead of a 130/70R18 dedicated Front, a few observations and comments.
The ideal rim width for the 130/70R is a 3.5”, with an allowable range of 3.0 to 4.0”.
My guess is the m109r’s front rim is a 3.5”, and would be surprised if it wasn’t.
Some quick fishing about indicates that the ideal rim width for the 160/60R18 is 4.5”, and that the allowable range is 4.25 to 5.0”.
My guess is that 160/60R18s come on 4.5” wide factory wheels.
So the use of a 160/60R18 tire on a 3.5” factory m109r front rim will result in “rim pinch” of 1.0” in terms of ideal rim widths, and 0.75” in terms of the 160/60R18’s minimum allowable 4.25” rim width.
Both are huge amounts of rim pinch, and there is no way that carcass stability won’t be adversely effected.
Said rim pinch will also change the profile of the tires contact patch, by effectively narrowing it and steepening it’s sides, effectively making it more front tire like.
There will be rolling circumference difference, but likely not all that much, and my guess is the 160 will be a bit taller, including the effect of narrow rim pinch.
At least the treading, if V’d in any way, will be in the right direction, that said in terms of tread overlap and water evacuation.
Textbook it’s not, but for those that are doing it, it seems workable.
I’d run away from it personally, especially because of the extreme rim width mismatch.
My question is “Why are they even doing it?”.
I hope it’s not just for looks, because there is plenty of tire varied and good tire choice.
Quadiak, I’ll bet there’s a bunch of us that would like to hear from you about the “Why?” of your friend.