Wide rear tire r90 /6

[8ball]

Hi, I'd like to put the widest rear and front tires possible on my 76 r90 /6. What is the widest I can go and what modifications would it take? I currently have the snowflake wheels with stock type Metzler tires. I've heard I could get a wired wheel made that would fit a 140 wide tire. Anyone have experience doing this?

I bought a solo police seat, went to install and instead ended up tearing apart a perfectly good r90 6. The wheels are off and the front end is completely dismantled and I see no end in sight.

Bryan

[Roy Gavin]

How wide are you rims? That should determine the max size of tire.The Hagon wheel building site has a good chart of rim/tire sizes.

I tried a 130 on my R100 GS , but didnt repeat it, as I prefer the feel of the 120.

The heavier 140 will also conflict with your spring and damper rates.

[Rob Frankham]

Hi, I'd like to put the widest rear and front tires possible on my 76 r90 /6. What is the widest I can go and what modifications would it take? I currently have the snowflake wheels with stock type Metzler tires. I've heard I could get a wired wheel made that would fit a 140 wide tire. Anyone have experience doing this?

I bought a solo police seat, went to install and instead ended up tearing apart a perfectly good r90 6. The wheels are off and the front end is completely dismantled and I see no end in sight.

Bryan

If you're running a stock swing arm, you'll find it virtually impossible to go any larger than stock on a /6 unless you offset the wheel. Later swing arms have a bit more clearance but not much.

I don't know what you're trying to achieve but there are several excellent reasons not to fit wider tyres on a boxer:-

1) You'll upset the neutral handling of the bike.

2) It will be necessary to totally deflate the tyre in order to get it between the brake shoes and the other side of the swing arm when removing the wheel

3) In order to fit a wider rear, you're faced with with major surgery on the swing arm or an offset rear wheel. Some have done that but I can't say I like the idea very much.

4) If you fit a much wider front tyre you're likely to have clearance issues between the caliper and the fork leg and betwen the fender mounts.

In short, If you're doing it for performance or handling reasons, you're flogging a dead horse... if it's purely for aesthetics, then it depends how much you're willing to spend. Anything can be made to fit if you throw enough money at it.

Rob

[Duane Ausherman]

I just have no words for this idea.

[vanzen]

The old flakes and frame will accommodate a 100 front and 120 rear. I say this knowing full well that aspect ratio (height of tire) plays an important part in the equation, and that actual measured width will vary between make and model of tire. In other words, these given sizes (100 & 120) may either approach, reach, or surpass a practical limit. This situation will vary depending upon tire choice, as well year and model of the bike in question. Any "wider than stock spec." tire choice should be installed on the rim, inflated, and checked for clearance.
Just because it says "100" or "120" on the sidewall does not guarantee a fit.

Rim width always determines maximum possible tire width. Disregarding the proper rim to tire width ratios
changes the tire's engineered and designed profile, perhaps significantly, and may result in a revised shape that is ill-mannered in the best case scenario, or dangerous in the worst.

My experience dictates that "neutral handling" (better known as "zero steering effect" and only too often confused with "light steering effort") will be a function of many chassis design factors – and not simply or directly related to tire width. Later BMW models, other makes, and even old airheads have achieved this goal without resorting to 1970 tire specifications. On the old airheads, I have found that the ratio of front tire width to rear tire width will be more important to preserving "neutral handling" than the actual measured widths of the tires.
The primary, critical, and determining factor of "nuetral handling", however, will always be combined CofG.

Airheads can cope with fairly significant wheel offset. This fact does not pretend to imply that increasing wheel offset will not create it's own handling consequences. To fit a 120 rear, it is most likely necessary to offset the wheel. The factory makes a wheel spacer that is used for this task on many models, and it is available at your favorite parts source.
I have yet to see an original, unmolested, and stock late 70s BMW with co-linear wheel alignment.

Increased horse-power output and improved braking technology will be the practical REASONS that tire width has increased throughout the years. The kids on the track needed that extra rubber down to brake late and accelerate out of the corners more quickly without wasting valuable traction – in order to reduce lap times. Production models supported the trend with more powerful engines, more efficient brakes, and tires that were commensurate to the task. One key element to justify the need to increase tire width becomes readily apparent: Acceleration and braking that would compromise the traction capability of a given tire. The power must first be available, ... and then it must be put to use.
The rest of it will be nothing more than fads & fashions "American Chopper" style.

These facts do not dismiss the reality that increasing front tire width will typically also require increased rider input to affect a curve, or the reality that a wider rear tire will proportionately increase the lean angle necessary to affect a given curve. Increased tire width also results in a heavier tire, which increases unsprung mass to the detriment of handling.
Motorcycle chassis dynamics is a complex jumble of intimately related and interacting physics –
There will be NO free lunch.

The use of "wide rubber" should be a choice that will be the considered estimation and balance
of all pertinent factors.

[DanielMc]

A characteristically lucid, informed and insightful post there Vanzen.

I did have reason however to recently speak to Bridgestone's UK technical department: my most recent bike has a post '81 swingarm fitted, and I wanted to know whether this would affect tyre choice. I was advised that whilst recommended fitment for a '74 R90S was 3.25x19 front and 4.00x18 rear (Bridgestone being one of the few manufacturers still making those sizes) the "fatter" swingarm resulted in slightly reduced clearance, and I should therefore fit a 120/90x18 to the rear. Bridgestone's own figures state their 4.00 is 119mm wide and their 120/90 is 118mm wide.

[vanzen]

... Bridgestone's own figures state their 4.00 is 119mm wide and their 120/90 is 118mm wide.

Precisely.
A quick trip to your favorite metric conversion source will reveal that 4" = 101.6 mm.
Obviously, 119 mm ≠ 101.6 mm, and 120 mm ≠ 118 mm.

I have in front of me a 120 Dunlop that measures 124.89 mm (4.917")
and a 130 Avon that measures 120.35 mm (4.738")

Valuable lessons regarding listed tire size vs actual measurement,
and the need to either source accurate information or MEASURE for yourself.

Also consider that the measured width of any given tire will vary (somewhat)
depending upon the specific width of the rim to which it is fitted.

[8ball]

Thanks everyone for your insightful comments, much appreciated. I measured and photograpged the wheels; they appear to be the stock 3.25 front and 4.00 in back. I also uploaded all the r90 pictures thus far here http://www.flickr.com/photos/digitalb/s ... 950295162/ The pristine bike shows the condition it was in when I purchased.

I'm not trying to gain performance here, I would of kept my GSX-R 750 if I wanted to go 130 mph. I'm trying to create that look. This bike will just be used to drive back and forth to work and occasional rides around town. I really like the old BMW bikes that have been redone bobber style. I'm going for a simple, loud, matte black style bike.

Again, thanks for all your helpful comments. This is my first foray into such a project.
Bryan

[vanzen]

"Style" is what appeals to the eye,
"function" is what rolls down the road ...
Do keep your eye on the road, eh ?

And good luck with it !

[Major Softie]

Yeah, you are VERY limited in how much width you can put on the back, but offsetting will gain you some. On the front, if you really want to go bobber look with a fat front tire, a wider triple clamp is what it's going to take to really accomplish your goal, and there isn't anything out there you can pick up off the shelf. You would have to decide for yourself if a custom project such as you are contemplating would be better off with a custom part machined to your specs, or with simply replacing the entire front end with a different assembly off another make or model to give you that width. The latter would make it easy to very cheaply get your look AND greatly improve your brakes at the same time, but may not fit your "vision."