Stock rear tire diameter, and what's a good next oversize?

[macx]

Just toying around with tire diameters and the effect on gear ratios,
having a surprisingly hard time finding the actual diameter of the stock size rear.

And while I'm at it, what's a good oversize and how much diameter does that increase?

Oh, mostly fairly easy riding, nothing very bonzai. Fairly frequent passenger.
Minimal foul weather riding, only when I get caught in it.

Thanks!

[Airman]

Just toying around with tire diameters and the effect on gear ratios,
having a surprisingly hard time finding the actual diameter of the stock size rear.

And while I'm at it, what's a good oversize and how much diameter does that increase?

Oh, mostly fairly easy riding, nothing very bonzai. Fairly frequent passenger.
Minimal foul weather riding, only when I get caught in it.

Thanks!

I don't know what the diameter is, but your choice of sizes is limited. The Roadster has a 170/17 ZR on a 5" rim, the Rockster has a 180/17 ZR on a 5.5" rim. If you are running the stock silencer the clearance to the tire is small. I don't know what running a 180 on the narrower rim would do, but I suspect it would be fine. If someone offered me a smoking deal on a 180 I would likely buy it. I know the final drive ratio is higher on some bkes, but I've never heard the reason. Mine runs about 82 at 4000 rpm in sixth. Some RR's will be running 4500 rpm at that speed. I have the 170/17 on my bike.

[boxermania]

macx

Motorcycles, unlike cars don't have much of an option regarding oversize fitments, due to the limited wheel widths. The same tire size by a difernt manufacturer will be slightly larger or smaller on the OD and thesame happens as a direct result of inflation pressure and or load on the tire although diferences larger than .400" on the diameter are unusual.

That bing said and in answer to your question.....a 170x70 - 17 tire has a nominal diameter of

170x.7= 119 mm for wall height x 2 (for two walls) = 238mm/25.4mm = 9.37" + 17" for the rim dia = 26.37"

likewise a 180 section tire (the one fitted to the Rockster)

180x.7= 126 mm for wall height x 2 (for two walls) = 252mm/25.4mm = 9.92" + 17" for the rim dia = 26.92"

So in closing, the 180 section tire will reduce the engine rpm by ~ 2% (26.92" OD/26.37" OD) over the 170 section tire. At a given rpm, say 4000 rpm with the 170 tire the engine will run at 3920 (4000x.98) with the 180 section tire.

Hope this helps, haven't done the calculation in years so please, someone check me out......

[macx]

That sounds about right - I do it the quick n dirty way - I take the current dia and the current i.e. mph at a given rpm then do a simple ratio calcuation - "close enuf" unless I feel the need to split the proverbial hairs for some reason.

I finally did find one site that would list dia's for different size tires and diff brands - you guys are right, there aren't many options for size.

I suppose a Rockster rim wouldn't fit if the exhaust situation were "improved"?

I guess, realistically, that the perceived improvement would hardly be "cost effective" however :-)

I would like to gear my r down a little in first and OD. I often carry a passenger and am currently working out of a town with lots of hilly streets - a lower first gear like with an RTP trans would make esp uphill starts easier and be easier on the clutch. And nearly all my driving is on 60mph roads, and in the twisties and hills, I'm always "hunting" back and forth between 5th and 6th, esp again with a passenger. So the RTP trans there, too, would fit the bill. And boost my 60mph 6th gear rpm from a slightly low 2800, even with just me on it and on the flat, to a better 3200 or so, allowing me to go down to 55 before I go under 2800 or to pull hills without all the constant shifting.

A cheaper option would be a 34:11 final drive, and of course easier to swap. It would be pretty close to the stock 31:11 and an RTP / Adv trans, although would still have the wider 5 to 6 spread.

I'm always fiddling, researching and calculating. Drives my wife nuts. But I enjoy it, so

But 34:11 ABS drives that fit an 04 seem to be even scarcer than the Adv / RTP trans.

[boxermania]

macx

A 180 section Rockster tire will fit the R1150R, some owners have mounted such on the 5" wide R1150R rim, personally I will not advise the practice as mortorcycle tire design and contact angles are very specific to wheel width, a 180 tire should go with the 5.5" wheel.

In your case, the larger section (180) tire will not help you in the least and anything to improve the pulling, i.e. torque multiplication, that you want will cost you $$$$$. Regarding the 6th to 5th gear shifs on hills, in my experience with a rider any type of sustained elevation will require 5th gear and it's OK to run the engine there as well as 6th is really an OD.

Enjoy your tinkering and best of luck......

[Airman]

macx

In your case, the larger section (180) tire will not help you in the least and anything to improve the pulling, i.e. torque multiplication, that you want will cost you $$$$$. Regarding the 6th to 5th gear shifs on hills, in my experience with a rider any type of sustained elevation will require 5th gear and it's OK to run the engine there as well as 6th is really an OD.

Enjoy your tinkering and best of luck......

True. The 6th gear is high enough on my bike to make the engine unhappy under about 65 mph, or normal freeway traffic. But if you bang down to 5th you're running what, 4000 RPM ? Just fine actually.

[R4R&R]

My previous bike (Suzuki RF900) had a 5.5" rear wheel and originally came with a 170 rear tire. Everytime I ordered a replacement, it seemed that the 180 was a couple of dollars less so that's what I always bought. I didn't see (or notice) any speed/torque or mileage difference. I have to admit, the handling of that bike never made me feel too comfy with riding on the edge and I'm not sure if it was from the 180 tire. I doubt it was - I think the para-lever really let me ride faster/more comfortable. Mounting a 180 tire on the 5" wheel can be done, but the tire profile may be changed in a non-positive way. I guess if you don't wear your tires to the edges, then it probably won't really make any difference.

[CycleRob]

"Mounting a 180 tire on the 5" wheel can be done, but the tire profile may be changed in a non-positive way. I guess if you don't wear your tires to the edges, then it probably won't really make any difference."

Well said. The 180 width tire on a 5" rim will not have the ideal/optimal handling profile because the sidewalls are tucked in by the narrower rim. Your best bet at lowering the gear ratio would be with the replacement 11/34 tooth FD, but at a very high price, even for used. The tire chart that listed the circumference of each brand/size tire is the cheaper way to go, but with less actual "gear" ratio change . . . . at your next tire set change.

Me, I'd rather go for even taller gearing, like 3,300 @ 70MPH (instead of the current 3,500), for peak MPG's. For uphills 2-up, there's always 5th or lower gears, depending on the road speed.

.

[marine dad]

well, i have a question going in the opposite direction. i want to mount 120/70/17 front and a 160/60 17 rear distanzia's. what effect, if any, will the 160 rear tire have on anything, and will it be safe to run on the 5 inch rim?
thanks
mike