When moving from a 21" wheel to a 19" or smaller, does it change the geometry of the bike at all? I imagine since the bike was initially set up for a 21", it has to affect something in the stance, handling, etc.

I've got a '13 Blackline, and I really want some bigger rubber up front. Ideally, I'd like to stick with a 21", but I haven't seen many in that size with larger widths. Not looking for Fat Bob size tires, but something in between would be nice.

 

i'd assume it effects the stance..you're taking 2" of diameter away from the front wheel, which is going to drop the nose down. Not sure if you can compensate for that with thicker walled tires, short of putting Fat Bob tires on it

 

the only way to know is to calculate using rim diameter and aspect ratio. I am not sure about motorcycle tires but for car tires the aspect ratio is a percentage of the width. so 100-90 would suggest 90% of the 100 mm wide or 90mm of sidewall times 2 plus the rim diameter.

 

it all comes down to "rolling diameter"


http://www.csgnetwork.com/tiresizescalc.html

 

21x3.0 or 21x3.5 isnt fat enough? Their both 120/70/21

 

I'm confused. A 21 diameter can be a narrow width or a large width. Wheel size really has nothing to do with it in regards to rubber on the road. But I get there aren't many width choices in a 21 diameter wheel. To your question Yes a smaller diameter wheel will obviously lower the front end of the bike.

 

Lowering 1" in front will make the bike want to turn quicker, less stable feeling. And during heavy breaking the rear tire unloads faster, less rear breaking. To compensate you could lower the back 1". But loosing ground clearance becomes another issue.

I would not consider a 120 width narrow. They just look narrow on a 21" rim.

 

Front tire diameter has a large effect on your trail number. Trail is figured by the right triangle that is formed by taking the center line of the neck bearings and the center line of the front axle at a right angle and extending them to the ground. Because the center line of the neck bearings is so long a few degrees of change can result in a change of a couple inches in trail. Take the numbers of the tire sizes and plug them into this calculator

https://www.rbracing-rsr.com/rakeandtrail.html

Here is one for tire size

https://www.rbracing-rsr.com/tirediamcalc.html

 

Get a 21x3.25 or 21x3.5 with a 120 tire. That way your overall diameter will extremely close to what it is now. Not sure if the front fender will fit with the wider tire and wheel. the 19 will sit slightly lower, probably about 1/2" to 3/4".