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Discussion Starter #1
I'd like to know more about the effects and trade-offs of different tire and wheel sizes, specifically in handling characteristics and ride quality. Does anyone know of any books or internet resources that discuss this in detail?

Thanks.
 

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So get 21 inch cokers. Front and rear.
 

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A more specific question might get a more useful answer. Generally speaking I like wider tires than most people, up top a point, but I know why I like them and others might not.
 

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Are you talking differences between front and back? Or differences as a whole? Differences in width or in height?

a lot depends on the bike. For some reason twins running on dirt/flat track tend to do well with 19" front and rear, but motocross uses 21" and 16" or 18", and motards use 17" front and rear. 17" also seems to be the popular road race size but that may have to do more with access to modern rubber on older bikes.
 

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There are a million variables in this. So much so that even tires companies and the manufacturers still have a shit load to discover. A small wheel the same width may turn in faster, a taller wheel that is narrow may turn in faster. A wider tire with a certain tire pressure (I know in proddy 250 2T V 400 4T teams would run their pressure as low as 15PSI in the rear) will have a bigger contact patch. Rossi and most of the GP field run 16.5 inch tires instead of conventional 17" to get the tires wider but retain the turning speed into a corner. Stoner often ran a 16" front when he was at Ducati.

A larger rear wheel/tire will also gear the bike down. There is so much to talk about here. Whats the example you need help remedying?
 

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Even when you get your head around all this there's the possibility that putting modern sized tyres on your vintage bike is going to make it handle worse because the frame and suspension isn't up to the extra forces of stickier rubber.
 

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Usually the question is 'how big can I go' if that's what you mean, just use search
 

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Discussion Starter #9
Thanks for all the responses.

A more specific question might get a more useful answer.
You're probably right, but I was hoping to understand the basic principles and guidelines first, and then have the ability to apply them to specific situations. Having said that, I have a 2005 EVO sportster that has a 16" rear wheel and a 21" front. The 883 of the same year comes with a 19" front. I took a look at the XR1200 and it has a 17" rear and 18" front. I'd like to systematically improve the bike's handling (rear shocks, fork springs, tires and wheels).

Are you talking differences between front and back? Or differences as a whole? Differences in width or in height?
All of the above.

There is so much to talk about here. Whats the example you need help remedying?
See my response above.
 

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Then there's also the question on tire profile.

The 17" and 18" combo of the XR is a good beginning to improved handling, but tire profiles play a big part in turn-in speed. the 'round' ones are slower, the "V" shaped quicker.
 

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Dirt trackers use 19's front and rear cause that's the only size real DT tires come in, been that way for 50+ years. Most dirt bikes come / came with 21 in the front and 18 in the back so dropping in a set of 19's helped the geometry to better suit Dirt Track. by dropping the front you changed the rake and adding a bit to the rear did the same. Using same on both ends help to as one can carry less spares to the track etc.
 

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A few points to consider:

- The characteristics of the tire itself are usually more important than just the diameter.
- You have a greater selection of 17" high performance tires since this is the standard for super/sport/race bikes. Different tire profiles and aspect ratios can help you fine tune turn-in characteristics on any given rim size.
- Wider tires can impair handling and stability (even though grip is improved). A greater lean angle is needed to turn with wider wheels, which means more physical effort from the rider and less cornering clearance on any given bike.
- Run the narrowest tires you can get away with that still meet your weight and power output requirements. Anything wider adds to unsprung weight and rotational mass.
- Find a copy of Tony Foale's "Motorcycle Handling and Chassis Design." There's a whole 33 page chapter dedicated to tires.
 

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I have a 2005 EVO sportster that has a 16" rear wheel and a 21" front. The 883 of the same year comes with a 19" front. I took a look at the XR1200 and it has a 17" rear and 18" front. I'd like to systematically improve the bike's handling (rear shocks, fork springs, tires and wheels).
It's good you mentioned the bike that put you in mind for this question. I'll give you some notes on this specifically. Surprisingly, or maybe not all that surprisingly, 16" wheels are a "historical" size for HD's. Harley started using 16" wheels (front and rear) in the late 1930's I suspect because the large sidewall helped with the ride quality their larger touring bikes, the innertubes were still relatively common, and this allowed use of an older style rounded car tire in a pinch. If you look at HD's with 16" rims and tires from that era the tires are these big ballon looking bias plys - wider than anything else from that era. As the bikes got heavier tire technology really didn't keep up so to have these massive 500lb-700lb touring bikes on terrible 1930-1950s roads you needed a thick, wide, beefy tire. Even now, the bigger HD crusiers and tourers use 16" rims with large sidewall tires.

When the chopper craze happened a lot of these bikes already had 16" tires. It became fashionable and also a bit of an assist to stability to put a large 21" front rim on a long raked out chopper. Also they just looked cool - like a top fuel dragster. Also 16" rear wheels in the 1970s were a bit of a cheat to get a wider tire on the back of your bike for the dragster look because the large sidewall could reduce the pinching effect you get putting a too wide tire on a narrow rim.

your evo sporty, which I suspect is a custom or special, or anniversary model and not the standard sportster, has a 21" front and 16" rear for the harley chopper look. It has no handling benefit to you whatsoever. Standard sportster should be 19" front and either 18" or 16" rear (depending on year) and 17" rear and 18" front for the super low. The 16" front and rear of the Forty-eight sporty is I suspect just for looks as they are trying to make the bike look like an old harley (even though the sportster didn't come out till 1957 and used 18" rear with 19" front).

so to answer the question you didn't ask: the large motivating factor as to why HD runs different wheel sizes on the sportster is for looks. The XR1200 is the only performance bike in the bunch and it runs a 17" rear to get access to good modern rear rubber, and an 18" up front because it doesn't use a steering dampner and it is a cheat to make the bike more highway stable. If you look at the other performance bikes in direct competition with the XR1200 they all use 17" front and rear.

If you want your HD to handle better with tire size I would consider either 17" or 18" front and rear rims with low profile modern tires.
 

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By the time you are finished upgrading your sporty to XR1200 wheels, suspension and brakes, you could have bought an XR1200 and rebodied it. just a thought. Actually you could probably pick up a same year buell for less money than your sporty is worth right now and you get all the benefits of Erik Buell's engineering madness.
 

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Discussion Starter #15
By the time you are finished upgrading your sporty to XR1200 wheels, suspension and brakes, you could have bought an XR1200 and rebodied it. just a thought. Actually you could probably pick up a same year buell for less money than your sporty is worth right now and you get all the benefits of Erik Buell's engineering madness.
Thanks for the response. I agree with your cost benefit analysis, so I'll probably just make a few inexpensive changes, such as going to a 19" front tire, hagon rear shocks, progressive springs up front, and call it a day. I live in Dallas and the XR1200 here run around $8K. The Buell's cost even less.
 

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I've seen tube frame buells in the $3500 range. those are a hell of a bike and alot of engineering for that kind of money.

Try and see if you can find Xr1200 take off parts. Hit the Hd dealer, they sometimes backdoor customer take off shocks and such. if you can find a crashed one Having wheels laced to your hubs (if you have spokes) in the Xr sizes might not be too expensive if you use a service like Buchannans (maybe $700 all in with tires?).
 
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