Tire Question

It is to my understanding that a seca 750 can take a 130/90/18 rear tire. My question is...Does the middle number really matter that much? for instance could i run a 130/70/18?

 

This is a subject I have spent way too much time obsessing over so I have to offer up my $.02.

Middle number is ratio of sidewall length to tire width. So suppose you have a tire that's 130/90/18 these #'s indicate width(mm)/ratio/inside diameter(in.). So tire width is 130mm (5.12in.) & sidewall is (4.6in.). 130/70/18 is still 5.12in. wide but now your sidewall will be 3.58in. so your outside diameter will drop about an inch. Keep in mind that if you go measure your tire mounted in the wheel your not seeing part of your sidewall (its lipped in the wheel).

On motorcycle tires it's not as easy to picture these ratios as it would be on say a low-profile corvette tire like a 335/35/20. This tire has a lot of width and a very short sidewall hence the low ratio number. Remember that on your moto tire the width is the tread surface even though it curves downward towards your sidewall.

If you made this change your rear tire would be spinning at a higher number of revolutions per mph, but it would fit on your wheel and it would fit in-between the suspension. I never worry about making ratio changes on a car because of all the clearances and the whole 4 tires thing and in fact I usually up the width by 10 or 20mm and drop the ratio by 5 or 10% on most of my 4 legged rides. But on a bike...well I don't know it would change the dynamics some that's for sure. Lower profile does mean stiffer sidewall and you would certainly "feel" the road more, but on a possible plus side you could theoretically push harder into a turn and the tire would sit more on the tread patch at those angles. That being said I never push my XJ hard enough to need anything other than a good quality stock foot, and I do go about as low as you can go.

 

also, iv'e serached the forums, what is the absolute largest front tire width one could achieve on the seca 750?

 

Do a search for something like: motorcycle tyre rim size

Here's a page, you may find something better: http://www.dropbears.com/u/utilities/tyrerim.htm

I don't know why you would want to go larger on the front, though.

On the back, some people like lower profile tyres, but it wouldn't improve the hadling. If your legs are short, you might reach the floor more easily, or it would reduce the gearing slightly.

Or to answer the question you actually asked, YES, the middle number is important and yes it will affect the handling of the bike.
Would it be the end of the world if you had a 80% profile instead of 90% ? Probably not.

60% - you would need to check the load rating for the tyre as the weight it can carry might well be different.

It would probably cost more too, so it would be an odd thing for me to want to experiment with.