Having to choose nonstandard tire sizes for the wee Ninja; help me decide?

Trying to figure out what to do in lieu of correct-size tires for the little Kawasaki Ninja 250R track day/kart track bike.

Wheels are 17" diameter , 2.75" wide front, 3.5" wide rear.

Stock tires are 100/70 front, 130/70 rear. Nobody makes a 130-width track day tire, nor in fact a track day rear tire narrower than 140.

There are two camps of local riders on these:

1. Folks on the full size track in the Ninja 250 Cup, running Bridgestone slicks in 120/600-17 at both ends (Bridgestone does F and R construction in that size)
2. Folks at the kart track running 120/70-17 front, 140/70-17 rear

I don't want to deal with tire warmers and so the Real Slicks aren't for me. Trying to run 120 at both ends in a DOT Track Day tire (e.g. Pirelli Diablo Rosso Corsa or Bridgestone Battlax RS10) means running a front-construction tire on the rear.

The most trusted person I could ask locally didn't want to make a specific recommendation, but felt that 140 was mostly too wide for a 3.5" wheel. Note that Pirelli won't recommend anything to fit the Kawasaki, and Bridgestone says the ideal fit for the RS10 is 3" front, 4" rear, but the recommended range includes 2.75" front and 3.5" rear, though that's not true of the tire one notch less aggressive: their Hypersport S2 wants a minimum 3" front, 4" rear.

I'm a little confused that going up 20% in front tire width appears to be dandy, while going up 8% rear is too much tire, though I don't have enough command of normal relative wheel and tire widths to know whether 2.75" is wider relative to the 100 front than the 3.5" is relative to the 130 rear.

My tentative plan is to go RS10s 110/70 front 140/70 rear, on the grounds that that's all within Bridgestone's recommendations for rim widths, and is "one step" up both front and rear. I'd almost be tempted to stick to a 100 front, but given that both the wide-rear and skinny-rear groups have settled on a 120 front, going up one step feels safer. The case could be made for running a 120 front, too... But then the RS10 isn't available in 100, and the 110 front and 140 rear are both from their H-series and the 120 front is from the W series as is the rest of the larger rears. Probably a carcass construction mismatch?

Actually, that's a great spot to stop babbling about what I've gleaned and guessed and ask whether anybody has any opinions or related facts.

I know very little, but agree with your +1 assessment. Should be fine.

I don't ride trackdays (at least not yet) so I may not know what im talking about, but I do ride little bikes and I like grippy tires, so I feel your pain. Would stepping down to the grippiest high-performance street tires available in the correct size be completely unsuitable at a trackday? I'd think it might allow you to get out on track and enjoy, perhaps at a few less degrees of lean angle, or a couple mph less at the apex, but that would be my approach.

My Suzuki Goose 350 calls for 110/70-17 front and 140/70-17 rear and I'm using Bridgestone's Hypersport tire in that size and am happy. (S22? The little sizes are only H speed rated, but that works for my 350 single.) The previous owner did trackdays on it using bias-ply Pirelli Sport Demons. ?!?

My 80's JDM GSXR250 is more of a challenge with 17" wheels, 2.5" front and 3" rear, and calls for 100/80-17 front and 130/70-17 rear. It isn't running now, but I found some bias-ply Michelin Road Classic in the correct size. No idea how they will ride yet...time will tell.

I can't add anything past upping sizes in my street bikes has never been a problem, but I am not the pointy end of that spear. In regards to bias vs radial, I recall a time when my stepdad was trying to figure out a handling problem on an early Concours. Getting rid of the quality-not-an-issue radials and getting it back on bias ply tires it was spec'ed for brought it back to good handling. You can make anything with sticky tires corner well, but the optimal steering geometry is not always the same for both tire types due to the differences in carcass flex.

In reply to ae86andkp61 (Forum Supporter) :

The most important thing from your post first: You've got a GSXR250!? Rad!

I think I have a possibly-overdeveloped and underinformed desire to run "conventional" and contemporary performance stuff because a big part of what I want to be doing right now is making up for lost time on my riding skills with a particular eye to a better understanding of motorcycle chassis. To that end, I *think* I'd like to stick to a radial, and if I did decide to run some Ninja Cup races, I'd also like to have been riding something not wildly off in terms of performance. I'm not trying to "win the track day," but  before too long I want to start looking at times and possibly data to get a little scientific about making headway.

My only track day experience was underutilizing some Pirelli Diablo Rosso Corsas on an Aprilia. I mean, I got out to the edges of them, but I was never moving at a pace that caused the bike to move around. OTOH, I'm hoping to knock the rust off and move forward from where I left off. I had a big jump forward on my last track day, a bunch of stuff clicked, I was suddenly a lot faster than the rest of the slow group and got signed off to move up, and then I got a job that didn't give me weekdays readily and I haven't ridden since.

If I could get the original sizes in something properly performance-oriented but more streety (like those S22s or the non-Corsa Diablo Rossos) I might give that a shot, but I don't think I want to get any further away from a current performance tire than that.

Anyhow, between the feedback here and knowing from elsewhere that these bikes don't seem to fuss over needing radial or bias ply specifically, I think I'm going to give the 110/140 RS11s a shot. Thanks, all!

Canoe brought this up, but a silly update:

Nothing is available. Hopefully the RS10s will be available 7/15. Well, the rear is available, but the front isn't. And that, the opposite, or neither being available is also true for four types of Pirelli, Dunlop Q3+, and, uh, whatever else I could think of.

I can get Bridgestone HyperSport S22s, and if the RS10s don't surface I'll do exactly that, but I'm so busy with sudden progress on the garage (hooray!) that I don't the wait is actually keeping me from the track.

Looks like another canoe got torpedoed, but just noticed this had been bumped recently. The good news is RS10s finally came in and I got a 110/140 set.

The bad news is that the other good news is that I've been in headless-chicken mode getting the new shop ready for the next inspection and haven't had a chance to even put the wheels back on, let alone go helmet shopping and get registered for a track day...