Tread and tire width?

What is the ideal tread width to rim width ratio? for road tires.
If a 5" rim can seat tires with tread widths from say 150mm to 190mm, what are the pros and cons of narrower vs wider, given a constant sidewall aspect ratio? and excluding clearance issues.
Also, what part does sidewall aspect ratio play in handling? I know a shorter sidewall is stiffer, and that would seem to give quicker steering, but front bike tires never seem to go below 60 or 70. Would 50s just be too harsh?

How does the weight and Hp of the bike figure into all of this? obviously a Superbike is going to need a lot of tire to transmit the Hp to the road, but they don't use the same tires and wheels on 250 GP machines.

and how bout diameters? Why do sportbikes all have 17" rims and not 18"? or 16"?

I know this is a lot of ques. and I certainly don't expect all of them to be answered , but I've seen a couple of other tire threads and never seen these questions discussed.
Steve

Er, yeah. What he asked...

Hopefully some of the smarter folks will chime in. I've *REALLY* wondered about all the different wheel diameters myself.

Ok, what I really want to know is what to put on my rear wheel. I switched from an 18" rear to a 17" and now have to put on a tire. A 160/60 will fit but I also may be able to squeeze on a 170/60. Any advice or recomendations would be appreciated.

A 170 will be fine on a 5" wide wheel. Be sure that the bigger tire will clear all the hard parts around it throughout the full stroke of the suspension travel.

I'm no expert on these matters, but here's what I understand about the other questions you've asked:

Rim width - there's an ideal rim width for every tire that a manufacturer makes. They will post that in their sales literature and on their websites. That width is what they have determined gives the best fit and profile for a particular tire. A rim that is substantially wider or narrower will either pinch the sidewalls in or flatten the profile, both of which can cause a reduction in performance.

Aspect ratio - taller sidewalls are more compliant but they can also squirm a bit when you push them hard, making handling feel somewhat less secure. Short sidewalls, in addition to being stiffer will also give a smaller overall diameter (all other things being equal) and provide lighter, quicker steering. On a 17" front wheel you won't find anything smaller than a 60 profile because there's too little sidewall remaining. In fact, some riders have switched to a 65 or 70 profile because a 60 can be more prone to rim damage from hitting pot holes or other sharp edges in the pavement.

16" vs. 17" vs. 18" wheels on sportbikes - it's the current fashion and/or paradigm that follows racing trends. Both 16" and 18" wheels were standard sizes in previous years. As chassis and tire technology have improved, the 17" wheel has been found to strike a good balance between stability and manueverability. A lot of racing bikes are using 16.5" wheels these days but there doesn't seem to be any movement towards fitting those on streetbikes as standard equipment.

Maybe someone who knows more can add their thoughts to this.

Wow, where to start? Rim width, tire height / rim diameter affect a lot of things: ride height, rake, trail, unsprung weight, turn in etc.
If you have a specific bike or fitment I could better give you some guidelines.


Patrick

Rim width, tire height / rim diameter affect a lot of things: ride height, rake, trail, unsprung weight, turn in etc.

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I think this is exactly what he was getting at. Not for a specific bike but on general principle.

I was looking more for generalities, rather than a specific app.

The specific which got me thinking about this is: I switched the rear wheel on my beemer from an 18" to a 17". The rim width is the same at 5". I now have a large range of 17" rubber to pick from, including a 160/60 and a 170/60. The 170 may have a clearance issue, so is there any real reason to go wider (excluding looks....everyone who watches Amer.Chopper know fatter is better). Then since the total wheel height is now lower, would this then increase the rake? thus slowing turn in (minimally)? Seems to me that trail would only minimally affected.

This got me thinking about the front rubber. Seems front tires rarely go below 70's. Wouldn't 50's give less sidewall flex, therefore more front end "feel" and quicker turn in? Yet, racers don't use them, so there must be disadvantages ie harshness? The frnt on my bike is 17" so I'll stick to the stock spec rubber.

Then I wondered about wheel D. in general. Old board tracker racers ran 21" on frnt and back. Why are cutting edge race bikes now running 16.5"? Larger D = less rolling resistance, but more difficult to turn (gyroscopic)? Why not 14" wheels. Dirt bikes run big F and small R, why not big on both ends?

Kinda thinking out loud, hoping to learn something from the smarter folks on the list, without going into a long seminar on tire and wheel tech.

Steve

For a given diameter, generally a fatter tire will turn in slower than a thinner tire. So for a racer, fatter is not better. But for a cruiser, fatter is not as big a deal and looks cool

If both front and rear are changed, I don't think the rake angle changes, but the trail would get smaller. I know big rake makes for slower steering and makes it harder to hold a bike into a turn. However, I am not positive how the trail affects the steering. I think it increasing trail has the same affect as increasing rake, it makes the bike more straight line stable. Less rake and trail makes the front end more flickable and potentially twitchy, which is why steering dampers are useful on sportbikes.

Don't forget that wheel diameter also affects gearing ratios and engine rpm. This may be part of the reason for not wanting to go to small.

For dirt bikes, the big front goes over obstacles better: ruts, rocks, logs, etc,... This makes it easier to control the steering when things are trying to push the front tire all over the place (that and the wide bars). With the back end, this is not as big a concern. However, having room within the suspension travel range for the tire is a big concern. If you go with a bigger back tire and want the same travel in the suspension, you have to raise the height of the bike. Most dirt bikes are pretty tall already so that really isn't an option. You can get away with the big front end though because there is more room for the suspension to travel without having to raise the bike.

As with most things, compromise is the name of the game. Change one thing and it affects other things. Gotta decide which behaviors are more important and how much negative change is tolerable in other places. It is unlikely we will ever get each and every component/aspect of a bike to its' full potential.

Cadaver said:

The specific which got me thinking about this is: I switched the rear wheel on my beemer from an 18" to a 17". The rim width is the same at 5". I now have a large range of 17" rubber to pick from, including a 160/60 and a 170/60. The 170 may have a clearance issue, so is there any real reason to go wider (excluding looks....everyone who watches Amer.Chopper know fatter is better). Then since the total wheel height is now lower, would this then increase the rake? thus slowing turn in (minimally)? Seems to me that trail would only minimally affected.

Click to expand...

There's no practical benefit to going wider. Assuming that the stock tire size for your BMW is 160/60-18, the back end of your bike will drop @ 1/2" to 3/8" by swapping to the 17". Raising the rear ride height (if possible) or pulling the forks up in the front might be needed to get the steering geometry and weight distribution right. But if you pull the forks up, you will lose some ground clearance and may touch down earlier when leaned over. Your overall gearing will also change some with the smaller diameter combination, making the motor spin faster.

Just my humble opinion, but I'd keep the 18" wheel. There are fewer tire choices, but at least you wouldn't have to worry about all these other variables.

I also changed the torque arm to a GS arm which is 2 cm shorter than the stock arm, pushing the back end back up. Over all the ride ht has been raised approx 1/2-3/4". I'm at the limit of the side stand and both wheels touch when on the center stand. The "flickability" is noticeably increased, without giving up much straight line stability and gaining ground clearance.

I switched to the 17" to get more tire selection (plus I like the 5 spoke wheels better than the 3 spoke). BMW only put the 18" R on 2-3 years of the R11xxR bikes and then switched to 17"s, so I feel like I'm fixing their initial mistake.

Unfortunately for me there is little I can do about the front end, the telelever precludes ANY adjustment. Next comes shock$$$$, Ohlins, Wilbers or BiTubo? They don't seem to show up on Ebay often.

thanks for the input

Steve

Cadaver said:

I also changed the torque arm to a GS arm which is 2 cm shorter than the stock arm, pushing the back end back up. Over all the ride ht has been raised approx 1/2-3/4"...

Click to expand...

Then it sounds like mounting the 17" rear combo should get you close to stock ride height again. Post back and let us know how it works out.

I would pay a visit to or have a conversation with David at http://www.bibmen.com/.

David is a Michelin motorcycle tire dealer who attends races with his trailer outfit and installs and balances tires for competing motorcyclists. He also has racing experience himself.

In the past, I have taken my BMW wheels to him for tire fitment. The last visit, he was examining the new tire that he was about to install on my rear wheel. He had just received it from Michelin. He explained to me that I would have to return another time. The tire was of the wrong construction for my application. He indicated that the tire would not provide maximum performance and tire wear would be excessive.

I trust Island Racing’s experience and savvy.

Thanks for the info, I'll keep the bibmen.com in mind when I finally buy a tire. The 17" wheel (from a K1200) came with a Battlax 160/70 Bias tire. The !8" wheel had a Pilot Road on it (160/60). I'm trying to set up my bike up in more of a sport direction, so the touring tire won't do. I am wondering what tire to buy...which is why I started this thread.

I've already done the wheel and torque arm switch and am happy with the results. The ride ht. has been bumped up between a 1/2 to 3/4 of an inch. I don't like the extra stand over ht, but the bike does feel more "flickable" and it is still very stable on the straights. I want to switch the tire to a radial as quickly as I can. The Battlax is fine for my commute, but I don't want to test the edges. Also, I'm going to try to get at least a couple of track days this year, so the K bike's touring tire is probably not going to cut it.

I'm probably going to stick to a 160 w and drop to a 60 sidewall.

Next I'm swaping out the handlebars.

Raising the rear is preferable to lowering the front so you have gone the right way even if unknowingly. The downsides as you've found are a higher seat height, greater side stand angle, and less effective center stand.

I don't know about the bmw setup but on "regular" rear suspensions, one could try a slighty shorter shock and use shims/spacers to adjust ride height.

Or you could try different rear tire sidewall heights. This could be potluck as one companys 60's might me more like anothers 70 since that is a ratio not necessarily a measurement thus subject to rounding off. Measuring tires on the tire rack is also potluck as they will change profile once mounted.

My recommendation would be to go to a 160/60-17 in the rear. If it's for sport / track use, any of the current crop of sport tires are good: Bridgestone BT014 (what I'm currently running myself), Dunlop Qualifer or even the 208, Metzler M1, or Michelin Pilot Power.
I've gotten some good mileage out of the BT014, the grip is good, even in the rain, and it is a lighter tire than the rest which gives quicker side to side transition.
A 70 series front tire will give you a more linear turn in and will be less likely to push.

Patrick

Lighter sounds like an excellent way to go, since I'm not in danger of overwhelming the tire with my Hp.

Any body want to buy a slightly used Battlax BT020 160/70 Bias, only about 300 mi. on it. Or even less likely a, Pilot Road 160/10 18?

I heard of the GS torque arm switch on a different site, IBMWR maybe, so I can't claim any credit for that trick. I'm not the tallest (5'9"), but I don't really mind the extra ht. I am going to have to do something about the side and center stands. It looks pretty precarious on the sidestand. I should be able to find a GS stand that will fix this. The last track day I did, the center stand was the first to hit. I think the extra ht will get me a little more lean, but I'll see if/when I get to the track. Otherwise I'll pull it. sure is nice having the centerstand for working on it, though.

Next comes the handlebars. The stock bars feel like ape-hangers at the track, but they're perfect for commuting and long rides. The converti-bars look like the perfect solution, but just switching back and forth with clip-ons (provided there are no cable issues) for the track would be cheaper. But thats for a different thread.

These charts might help, it states rim width and tire diameter/tire width matches. See link.


http://www.steelthundercc.com/tiresizes.html