What is the effect on performance from installing a different size rear tire?

My 650 Seca needs a new pair of shoes, and as it is with our old bikes, it's hard to find a matching set of tires for the front and rear except for a couple brands.

So I started thinking about different tire dimensions, since the 130/90 x 16 from the Maxim is a direct fit, and thought if that size would fit without rubbing the drive shaft, then what about a 130/80 x 18? After reading the forums, doing a calculation online, and some pen and paper work, I'm at a loss to explain to anyone why you wouldn't want a slightly wider tire (not overly wide) for better overall performance. That brought me here, as I'm looking for more information. Specifically, how does the slightly smaller, but wider tire affect acceleration, top speed, handling, and braking? From my comparison chart, it appears that the speedometer difference is smallish, but I'm not just worried about reported speed.

In my mind, the 130/80 x 18 should have slightly higher acceleration potential, lower top speed by 2-3mph (hey, that's still 124mph, right?) and better handling due to more surface area of the tire being on the ground, especially in turns. Braking I'm not sure about because I haven't really been able to figure out a standard weight to use in comparison, but the braking would be better for whichever tire has a lower rolling mass.

If I have anything wrong with my thoughts, please take the opportunity to educate a noob on the pros and cons of using a wider tire, but also please explain why anyone would put the 16" Maxim rear tire on the Seca... Wouldn't there be a huge loss (10+MPH) in top speed because of the much smaller tire, while getting a fair boost to acceleration? I only ask because I have a Maxim rear wheel sitting next to the Seca in my garage.

Thanks!!

 

Changing tire size can lead to handling issues , a wider rear tire you have to be cautious of clearance to shaft and swing arm . Some have successfully put the front forks and wheels from modern bikes so it can be done but the rear wheel is probably biggest thing to overcome . You stated you were having problems finding tires where are you looking ? JP cycles, cycle outfitters, just to name a couple companies I'm using Shinko front and rear on my Seca , had good luck using these on a couple other bikes with good mileage . Just a note on my Buell went thru rear Dunlap got 6k and front replaced at 8k switched to shinko sold bike to a friend at 12k tread still looked good .

 

I understand all the fitment issues I'd have, I'm looking specifically for the performance changes associated with using a smaller/taller/wider/narrower/etc rear tire, not necessarily on our exact bikes, but on a motorcycle in general. What happens when I stick a 16" rear wheel and tire on the back of a bike that is intended to have an 18" rear wheel and tire? (Assume that clearance is not an issue.)

 

The circumference of the tire is the variable in top speed, acceleration or grip. Your seca was designed to run 120-18 rear and mine runs just fine in high speed corners. Bridgestone spitfires on the xj650rjc and on my Xj750rl I have Dunlop 404’s.

 

A 10mm wider rear will have the effect of reducing the turn-in rate of the motorcycle (steering will be less 'snappy').
A 130mm rear will just barely clear the swingarm, and some 130mm rear tires will rub on the swingarm.

Putting a smaller overall diameter rear tire/wheel on (the combination of wheel circumfrence and tire sidewall height makes the overall circumfrence a bit different depending on the tire that is chosen) will change the rake of the front end slightly. Smaller diameters will increase rake, improving stability but sacrificing quick handling. Larger diameters will reduce rake, sacrificing stability while quickining handling (and possibly making it dangerous).

The small change that you suggest is common to make, and makes little difference. You'd only notice the change if you rode back-to-back with the stock sizes.

One ting that fitting a wider than stock tire does not do is increase grip. The increase in footprint is marginal (even a 50mm wider tire sees very little increase in road contact area), and squeezing the tire onto the rim changes the crown radius slightly. To get the benefits of a wider tire (increased lean angle) the motorcycle has to be designed (or modified) to use a wider tire.

I suggest doing a bit more shopping. All of the major brands (and most of the others) offer at least one model of tire set in the stock sizes (many offer two or more models).

 

Thanks! My question was more informational than practical. I wanted to verify what I was thinking, and you guys have done that for me. I asked because my friend who has been riding a lot longer than me, told me that he lowered his Ninja 250R and put smaller, wider tires on it so he could feel like he was part of the bike. He also consistently argues that any sport bike can stomp a Seca or 650 Nighthawk in the Twisties or in a straight line. I had to educate him on displacement, acceleration, and top speed, and am still trying to convince him that his wider tires made him more sticky and stable, but also slower. Besides the fact that his old 250R had a top speed of 105 with stock size tires, and ran a quarter mile almost 20mph slower than either of the 650s my son and I have.

It's basically a constant battle with him always thinking he's right about everything, and me having to either go with it and let it go, allowing his ego to get even bigger, or educate him on his errors. He still thinks his 250 would beat both 650s in the turns, but I think once either 650 got out in front, he'd only be able to catch up in the corners, but never pass.

To clarify, I do actually have a Maxim wheel sitting next to my Seca, and was debating which one to use just because they both fit, and I'm trying to keep my costs down so I want to get the best tire for the bike. After confirming my suspicions, I'll be sticking with the 18" rear wheel because of all the things you guys said. Thank you.

 

Yep, the maxim wheel size is all for aesthetics, does nothing for handling. I removed one and fitted a seca wheel.

 

I like Shinko tyres good tread pattern and long life.