I have a 1986 K100RT 2V with the stock wheels ( not 3 spoke ). In the owners manual if you look under tires for the rear tire it shows the 140/80x17 for radial tires and for bias tires 130/90x17. I'm aware the actual diameter for each is very similar with something like 5-10 MM difference. The front tire either bias or radial keep to the same size 100/90x18.

I have a couple of questions.
1. Can you run a 130/90x17 radial tire on the stock rear rim of that year bike?
2. Should you run a 140/80x17 if you are going to run radial tires on that year of bike? Are there any clearance issues with the center stand?
3. Any reason why they show different tire sizes for the rear? Was if for the later K100's 4V?

I have a 130/90x17 radial that I will at least this tire change, run unless I hear some good reasons why I shouldn't. I will be replacing the front tire with a radial at the same time. 
Thanks for your input.

I have run 130 and 140 on the RS and RT, standard stock Y spoke wheels.

I mostly used 140/80 and about 2 years back decided to use 140/70. No clearance issues at all. I have been using Bridgestone Battlax BT45/46.

The plus-one sizes for the Y spoke wheels are 110/80-18 and 140/80-17. These have been used successfully for decades. It is difficult to find a 100/90-18 radial so, if it suits you, use the plus-one size as they're heaps easier to find.

Thanks for the comments. I've used a 110/90 x18 tire on the front before and it affected the steering, making it heavier and slower to turn in so I switched back to the 100 size. Yes limited numbers of 100/90x18 in radial.

cycleman wrote:Thanks for the comments. I've used a 110/90 x18 tire on the front before and it affected the steering, making it heavier and slower to turn in so I switched back to the 100 size. Yes limited numbers of 100/90x18 in radial.

Yeah, agreed, a 90 series that's 110 could feel a bit heavier and slower, that's why it's recommended (even by BMW) to use an 80 series if you use a 110. That's your plus-one.

110/80 is a good front size. I like them.

Ever wonder why you're told to use caution after fitting new tyres/tires? Here's one motorbike shop's answer.

                                                                     CAUTION:
New tires may not offer the traction to which you are accustomed for the first 100 miles or so. Additionally, between the months of May thru September, baby unicorns are drawn to the scent of new rubber and are more likely to be near the road side due to said smell. For the sake of the baby unicorns, please exercise caution and allow for extra stopping distance and less radical leaning. Also, always keep a supply of candy corn available as baby unicorns hate artificial sweeteners.
                                                                     Thank you,
                                                                    Management

Two Wheels Better wrote:

cycleman wrote:Thanks for the comments. I've used a 110/90 x18 tire on the front before and it affected the steering, making it heavier and slower to turn in so I switched back to the 100 size. Yes limited numbers of 100/90x18 in radial.

Yeah, agreed, a 90 series that's 110 could feel a bit heavier and slower, that's why it's recommended (even by BMW) to use an 80 series if you use a 110. That's your plus-one.

So will that fit the fender on a 86 and older bike. I know on the later model K's they went to a two piece fender so don't know if the different fender would have any affect on tire size. When I compare the two tire sizes there is about .4" wider on the 80 series, but the diameter is .2" less so maybe that makes it fit.

cycleman wrote:

Two Wheels Better wrote:

cycleman wrote:Thanks for the comments. I've used a 110/90 x18 tire on the front before and it affected the steering, making it heavier and slower to turn in so I switched back to the 100 size. Yes limited numbers of 100/90x18 in radial.

Yeah, agreed, a 90 series that's 110 could feel a bit heavier and slower, that's why it's recommended (even by BMW) to use an 80 series if you use a 110. That's your plus-one.

So will that fit the fender on a 86 and older bike. I know on the later model K's they went to a two piece fender so don't know if the different fender would have any affect on tire size. When I compare the two tire sizes there is about .4" wider on the 80 series, but the diameter is .2" less so maybe that makes it fit.

Early Ks have a one piece fender and no fork bridge. The later 2v ones do have a fork bridge and a 2 piece fender.

I would like to put radial tires on my 86 K100, and the choices are limited. One that does show tire sizes that will fit the old K bike are the A41's. I realize they are a 90/10 tire but for my purposes would work fine. I was wondering if there is any experience out there about these tires or any comments as to noise, tread wear etc.

cycleman wrote:I would like to put radial tires on my 86 K100, and the choices are limited. One that does show tire sizes that will fit the old K bike are the A41's. I realize they are a 90/10 tire but for my purposes would work fine. I was wondering if there is any experience out there about these tires or any comments as to noise, tread wear etc.

These are a quality, light dual-sport tyre which should deliver expected traction (not really an offroad tyre) on dry and wet paved surfaces with above average mileage since the tread grooves are deeper - also good for water displacement. I see you can get a 110/80R18 and 140/80R17 so ideal as a plus-one size for BMW Y spoke wheels on early Ks. They're one of the OEM tyres on BMW F750GS.

[url=https://www.bridgestone.com/products/motorcycle_tires/products/detail/pr142/#:~:text=Rear BATTLAX ADVENTURE A41 Tire, 7 more rows]A41[/url]

I am unable to make the link a clean one-touch here this morning but this is the website for any of you who might be interested.

I might give 'em a whirl myself.

Digging deeper to see whether the US distributor(s) have these rubbers in stock anywhere and note that the plus-one sizes of 110/80-18 & 140/80-17 are available in bias as well as radial. Radial part numbers, front 303970 & rear 303977 if getting them through the largest B'stone distributor/supplier in US, Tucker Powersports - not a retailer of goods but a supplier.

Out of total co-inki-dink, we just whacked a set, this morning, onto a 2014 R1200GS wasserboxer for a customer who 'never' goes off-road and they look the goods. The rear tread grooves are quite pronounced and the front less so but quite a street-able tyre, IMHO.

Thanks for that I'll likely order a set after Christmas.

I checked on the Bridgestone tire site and the A41 140/80x17 requires a 3.50-3.75 rear rim so it won't fit on the stock 2.75 rear rim of the K100. So these radial tires can't be used on the bike in question. Thought I'd post this for info purposes.

Back to the drawing board, looks like I'll have to stick with a bias tires as I can even find a current 130/90x17 radial tire.

I'm using a 140/80-17 on my R100 Mystic with a cross spoke 2.5 x 17 rear wheel, as well, a 110/80-18 on the front 2.5 x 18 cross spoke. Back Down Under I have used multiples of plus-one 140/80-17 & 110/80-18 on Y spoke wheels on my K100RS with zero issues of clearance or handling. You can do it, like many others before you.

As TWB has advised on the plus 1 sizes, I am one of many who have run the plus1 sizes of 110/80 x 18 front (bias) and 140/80 x 17 (radial) rear, in my case on the standard Y spoke wheels of my K75 - now have been using those sizes for around 60,000kms on that bike.

A front and rear radial combination that you may not have thought of is a Pirelli Angel GT on the front in 110/80 x 18 and a Pirelli Scorpion Trail II in 140/80 x 17 on the rear. The "adventure" Scorpion is a very good road tyre (rated 90% road, 10% off road). Pirelli use the same carcass for the Angel GT and the Scorpion. I ran that combination on my K1100LT and was very happy with them, albeit that the rear wheel is 3.00inch compared to 2.75 on the K100 but would have no hesitation using them on a K100 as per my experience on the K75.

Cheers

From what I can see there is no problem running a bias 140/80x17 on the rear because they fit a 2.75 rim.  It is when you go to the radial 140/80x17 that it calls for a wider rim, at least with a Bridgestone. The 3.00 rim will work on a radial 140 but I wouldn't think a 140 radial on a 2.75 rim would be a good idea with the softer sidewall of a radial. Also I believe the early RS did go to a wider rim on the rear later on in the run of K100RS, so on those you could run a wider radial rear tire. Maybe I'm all wet on this one.

Have either of you had experience with running the110/80x18 & 140/80x17 in bias on the K100RT. Does it make the ride harsher.

cycleman wrote:I believe the early RS did go to a wider rim on the rear later on in the run of K100RS, so on those you could run a wider radial rear tire. Maybe I'm all wet on this one.

Have either of you had experience with running the 110/80x18 & 140/80x17 in bias on the K100RT? Does it make the ride harsher?

'Later' K100RS-16V used a 4.5 x 18 three spoke rear wheel, same as K1100RS, both of which used a 160/60ZR18 tyre, so different beast entirely. The three spoke wheel used at the back of later K1100LT (dealer installed option on other K75/100 models) and later K75 was a 3.0 x 17 and, yes, it was spec'd to use a 140/80-17 tyre. 

Y spoke rear wheels on early K were always 2.75 x 17 except for the drum brake rear K75 which used a narrower wheel and 120/90-18 tyre.

I have used bias 140/80 series tyres out the back of my K100RS when I ran the original Y spoke wheels and they fit and function fine - I really liked the Pirelli Sport Demon (bias). There is no harshness in them. These were on an RS, not an RT, but, IMO, there would be zip difference in feel, except, perhaps, for the great handling.

You are being cautious in your reasoning and there is nothing wrong with that, but I reckon you'll find a radial in 140/80-17 will fit and function on a Y spoke 2.75 rear wheel without changing the profile much - or at least not enough for you to actually notice. My K1200RS is spec'd to use a 170/60ZR17 tyre but a 180/55ZR17 spoons on like it was made for it and this is a superior road-holder - as well as vastly faster - as compared to a skinny tyred older K.

Thanks for the info, helps me make my decision.