Tire Size Calculations? changing both front and rear

[BrianShaw]

I am debating between tire sizes for my 98 Kawasaki ZX6R.
I am not even sure I am calculating this correctly.
The stock size should be 120/60-17 (23.07") and 170/60-17 (25.59").
The previous owner installed a 180/55-17 (25.35") on the rear.
I am considering going to a 120/70-17 (24.09") front and a 190/55-17 (25.83") rear.

The only downside I see is a slight loss in acceleration that I could correct, or even do much better than stock with a gearng change. And about a +1/4" rise in ride height from stock when corrected, and about a +1/2 of where it is now if it had been corrected.

The tire I am going to go with is the Michelin Pilot Road 3. Optimal is a 6" rim for the rear, but compatibal with a 5.5" rim as well.

So here is the decision I need to make, and you can stop reading after this if you not interested on how I got to this point:

Changing to the 120/70 front from the 120/60 front.
And on the rear:
180/55 with a 9.5mm drop in the trees to keep geometry stock.
-OR-
190/55 with a 4.5mm drop in the trees to keep the geometry stock.








I was looking at the calculations in this manner.
-For the Rear, 1/2 the diameter of the installed tire diameter difference, between the 170/60 stock size and 190/55 size on the rear.

-For the front, calculating if I needed to lower the front. 1/2 the diameter of the front tire difference. Then taking the difference of the front and rear tires than dividing that by 2. I have read somewhere that 1mm drop at the tripple tree is a 2mm drop (or close to) for geometry calculations.

Here are my calculations: (all measurements taken from Michelin fitment chart)

Rear: 25.83"-25.50"=.33", .33/2=.155" rise in the rear of the bike.
Front: 24.09"-23.07"=1.02", 1.02/2=.51" rise in the front of the bike.
so the differnce between the the front and the rear tire axle rise is .355" or 9.017mm in the front. So for a compensation I would need to drop the forks in the tripple tree half that amount. 9.017/2=4.5mm to remain at the stock geometry.

Currently for the front to have stock geometry with just the 185/55(r) I should drop the front 3mm in the trippletrees and it currently is not.

I was going to drop the front 6mm in the tripple trees (1.5mm from stock) to get a slight increase in turn in. The Profile of the front and rear tire combination should help even further than that with turn in. I would loose a little trackion in the rear compared to the 180/55 because of the contact patch (maybe) but be able to lean the bike further over, +.25” increase in ride height, and wider tread patterns on both the front and rear tires.

This all started because I will be running the 120/70 front tire and not raising the rear of the bike with aftermarket adjustable dogbones, Or have to excessively drop the front of the bike in the tripple trees like I would have to with the 120/70, 180/55 combination. (to remain stock that would be a 9.525mm drop in the tripple tree)

So here is where we are at with my 2 options: (I will be putting the 120/70 on the front)
180/55 with a 9.5mm drop in the trees to keep geometry stock.
-OR-
190/55 with a 4.5mm drop in the trees to keep the geometry stock.

Am I completely off base here, or is there not enough difference to even care?

[Wrider]

Yes you can do it, but your turn in is really what is going to be affected. It's also going to handle weird by going 20 mm wider in the front as well as a taller front.

If you're dead set on going to the taller front tire, at least stay with the 180 or go back to the 170.

[BrianShaw]

I am not sure going from a 120/60-17 to a 120/70-17 in the front is going to make it handle weird. I would expect faster turn in.

The 180/55 is a shoter and wider tire than the 170/60. And the 190/55 is taller and much wider than the 170/60.

[Wrider]

No I'm saying the combination of a taller front and a wider rear is going to make it handle weird. Even if the rear is also taller, it won't turn in as easily as it does now.

[kevplus2]

problem with going that wide on such a narrow rim is that the width combined with the shorter sidewall will pull the sides in and down causing the tire to be really tall in the middle with short, steep sides.

remember that all the manufacturer's measurements assume a specific rim size.

[TFOGGuys]

I see a possible issue with a 190/55 on the rear(aside from reduced contact patch due to the undersized rim): Swingarm clearance at the front of the tire. We've run into this on a number of bikes, including RC51s and GSXR1000s.

[BrianShaw]

Yes, the swingarm clearance at the front could be an issue. I will have to check that out. Looks like I have enough room between the chain an the tire.

I do not tink the tire size for the PR3 on the 5.5" rim will be a huge issue. Preferred 6", Compatible (5.5").