The options for radial 140/80/17 rear tires are vanishing.  That narrow size is not popular, so manufacturers seem to be dropping it.  Is there a good reason why I cannot fit a front tire to the rear wheel?  In the past, the tread designs differed between front and rear tires. Today, not so much.  I understand that the forces of acceleration and braking are different, but would that mean a front to rear swap is dangerous?  Or, am I missing some other important factor? 

After reading here about fitting radial tires on my bike I tried them and was very pleased with the result.  I'm thus reluctant to go back to bias ply tires. I'm been riding on Bridgestone T30 tires, but I cannot find a replacement for them now. 

You can get 160/60/18 if you use the K1100RS rear alloy.

brickrider2 wrote:  I understand that the forces of acceleration and braking are different, but would that mean a front to rear swap is dangerous?  

If you understand that, why would you contradict that? It would probably mean that a cobbled-up tire combination will perform less than optimally when such performance is critical.

If you go with an aspect ratio of 70 for front and rear, you'll have more choices; however, it's late in the season for shopping for favorites.

Well, bias ply tires do not perform optimally when compared to radials.  That is why.

brickrider2 wrote:Well, bias ply tires do not perform optimally when compared to radials.  That is why.

From your sentence composition I misunderstood you to be referring to the juxtaposition of front tires to the rear. Optimally refers to the tires performing as a matched pair rather than a mixed pair or in comparison with tires of a different construction.

Clearly, you're at the top of your game and I need sleep.

If the 'front' tyre has two arrows on it saying 'front fitment' and 'rear fitment', well, no problem. Otherwise it can be done, but it's not advisable and is known to the State of California to cause cancer.

https://www.avontyres.com/en-gb/tyre-care/motorcycle-tyres/

Years ago, upon sending my original R75/7 back to OZ from the US, I was trying to be preemptive/smart/clever/economical, and mounted a heavy duty, front-intended Metzeler ME880 on the back. My reasoning was simple: Metzeler didn't make a 120/90-18 rear, but they did make a front version. I planned to run across the undulating but not particular curvy Nullie, in the heat, the price was right, and the Metzeler rep gave his more than tacit approval by stating, "what could it hurt, just flip it and run it against the directional arrow." I'm up for an adventure. I matched it to the correctly facing ME880 110/80-19 tyre up front.

Within three month's time I'd slid out and down in roundabouts, one dry, one wet, after years of never having a prang. 

I have not experimented in such a manner since.

On my formerly narrow wheeled & tyred K100RS in OZ I have an 18" three spoke K75S front wheel with a 110/80ZR18 Michelin Pilot Road III fitted. Out back I have an 18" three spoke K1100RS rear wheel with a 160/60ZR18 Michelin Pilot Road II fitted. The bike is nimble, albeit you can feel the fatter rubber back there, the steering is accurate, direction changes are spot on, though I've only very slightly changed the rake/trail/wheelbase/ride height numbers the bike was intended for. Until that slightly oddball 160/60-18 size goes away, I'm in fresh radial rubber for a while yet.

The link to the Avon site seems to open the door to the possibility of using a front tire on the rear of the bike, provided it's mounted to turn in "reverse." I think it may be impossible to make a general statement applicable to all tires on such tire swapping being safe or dangerous.  
I see that Avon makes a tire in 140/80/17 size.  However, I can't determine if it is a radial or bias ply construction.  Further investigation is needed.  
If a wheel from an RS is fitted to a K1100LT, will the larger rear wheel allow the tire to rise off the ground when the main stand is deployed?

A 160/60ZR18's radius is not far off a 140/80-17's. I use a K11 main stand, too.

In Avon's alpha-numeric description of the tyre, do they suggest 'B' or 'R' anywhere? Provide a link. I'll give it a peruse.

brickrider2 wrote:I think it may be impossible to make a general statement applicable to all tires on such tire swapping being safe or dangerous.   

True but it's possible to state generally the resulting performance might not be optimal. I know because I've already stated that

If the tire size description doesn't have R preceding the diameter numeral—like so, it isn't a radial

Back when I still owned my K1100LT I ran a set of Pirelli radials in the standard sizes.

They were a Pirelli Angel GT radial 110/80 x 18 on the front and a Pirelli Scorpion Trail II radial in 140/80 x 17 on the rear. Somewhere on the Pirelli website I noted that those two tyres were built on the same carcass and the Scorpion Trail II is rated by them as 95% road/5% off road. That's near enough to a road tyre for me.

I am no road racer but do my share of touring and I found that combination worked well.

Cheers

P.S. I have taken Avon at their word and run two sets covering 40,000km of a rear bias ply 110/80x18 tyre on the front of my K75 mounted opposite to the direction arrow and a matching radial 140/80x17 on the rear of my K75 with no issues - just my experience.

I have K75s three spoke rims on my k100rs and have run radials for a lot of years, front  110/80zr18 rear 150/70zr17.
At the moment it has Avon sprint st tyres on it, though I have used many other brands.
Yes the 150 tyre isn't recommended for the three inch rim, all I'm giving is my experience.
I ride under all conditions through the ranges and the bike performs great, turns in and changes direction easy.

Avon's website gives very little info about the details of a given tire.

https://www.avontyres.com/en-gb/tyres/roadrider-mkii?searchtype=tire&cartype=motorcycle

I was looking for an indication of bias ply or radial in the description(s).  What Avon reveals is: Timeless appearance; New tire construction; and, Enhanced sidewall design.  One gets the feeling that this description is the work of the marketing gurus, without any help from engineers. 

brickrider2 wrote:Avon's website gives very little info about the details of a given tire.

https://www.avontyres.com/en-gb/tyres/roadrider-mkii?searchtype=tire&cartype=motorcycle

I was looking for an indication of bias ply or radial in the description(s).  What Avon reveals is: Timeless appearance; New tire construction; and, Enhanced sidewall design.  One gets the feeling that this description is the work of the marketing gurus, without any help from engineers. 

The Avon RoadRider series is a bias ply tyre, for sure.

Don't worry for Tyre extinction as these dimensions are very popular on trail bikes. I've sometimes ran a F650 GS Bridgestone Trail Wing (radial) on my K11LT and was very satisfied of it. It appeared to be the most long lasting Tyre I ever had on this bike:more than 7000 km, twice the average.

jbt wrote:Don't worry for Tyre extinction as these dimensions are very popular on trail bikes. I've sometimes ran a F650 GS Bridgestone Trail Wing (radial) on my K11LT and was very satisfied of it. It appeared to be the most long lasting Tyre I ever had on this bike:more than 7000 km, twice the average.

Did you mean 17,000 km? 7,000 sounds rather pathetic. I can get over 12,000 km on cheap as chips Shinko bias ply tires.

My comment above about info regarding front tyres on rear or vice versa on the Avon site is in the Frequently Asked Questions. Here is a link to the page. (you'll have to scroll down a little)

and here is a screenshot of the text on that page. Naturally it has the "we would not normally recommend" but it does say why the rotational direction should be reversed if you choose to do it.

So, if all that the arrow indicates is the orientation of the scarf joint in the rubber of the tread, what is the big deal about reversing it when the tire is used on the other end of the bike? The only issue that might arise is that the rain grooves will be running in the "wrong" direction. The risk there might be a slightly greater propensity to hydroplane.

But,but...Omigod, the arrow is backwards!!!

Point-Seven-five wrote:But,but...Omigod, the arrow is backwards!!!

Bravado is the last bastion of the elderly biker.

Gets you an automatic annual vehicle test failure in the UK, whatever the manufacturer says.

Dai wrote:Gets you an automatic annual vehicle test failure in the UK, whatever the manufacturer says.

That eliminates worry about the outcome and saves a trip. 

Point-Seven-five wrote:

Did you mean 17,000 km? 7,000 sounds rather pathetic. I can get over 12,000 km on cheap as chips Shinko bias ply tires.

No, 7000km with a tyre is my personal record. Usually, they're worn at 3500km, whatever the bike or the use. At the opposite, I had a brand new METZELER tyre on my K1100LT, a sport touring tyre mounted to go on a week end with my girlfriend. 300 km later, it was worn out. I had to change it to come back home...
I'm heavy and ride fast on twisty mountain roads. Tyres would probably last longer on flat straight roads, but here in the Alps, this is called an airport and I'm not allowed to ride on.

jbt wrote:At the opposite, I had a brand new METZELER tyre on my K1100LT, a sport touring tyre mounted to go on a week end with my girlfriend. 300 km later, it was worn out.

To paraphrase Alphonse De Lamartine:
Let us enjoy the transient delight
That fills our fairest day
And fill it with smoking rubber.

A. De Lamartine in "The lack (of gum thickness)"