FMX street wheels swap with 21" off road wheels

[duonas]

Hi there!

I currently have Honda FMX650, which is kind of supermoto or a street adapted XR650L.
Engine is same as XR650L, NX650 Dominator, Vigor, etc.
Frame and suspension is quite high, so maybe it is not hardcore offroad bike, but you can take it off the tarmac provided you have appropriate tires.

However, wheels are 17" size, with 150 rear and 120 front tyre widths - for city/tarmac riding.
I fitted TKC80 so I can go offroad more, and I keep hearing how people praise 21" rims over 19" or smaller ones.

I could buy a set of NX650 Dominator wheels, which are 17" rear, 21" front, and probably they will fit. But when I do that, front fork angle with ground will be changed. Currently if you put two pictures of FMX650 and NX650 together, the fork angle is exactly same. But if I fit 21" wheel in front, angle will be affected, with front being slightly raised.
Is this going to affect bike handling, drivability somehow? One person that I know, is a biker, and he said that it will change angle and this is "bad", but didn't say what exactly is bad with it. So I'm asking you, forum people, is this going to really be somehow bad and how? Or can I fit this wheel with no problems?

P.S. I haven't heard about any problems when you fit 17" supermoto wheel into XR650R for instance.

Sincerely,
Duonas

[Grant Johnson]

Measure the actual diameter of the wheels - 21" tires are typically quite small, and 17's may well be so much fatter that the height difference is surprisingly small.

Basically you want to maintain roughly the same "attitude" of the bike - front end height against read end height, which DOES affect the angle of the forks as you mention. A steeper fork angle - rear end high, front low - quickens the steering - perhaps TOO MUCH, and the bike becomes unstable. Raise the front, lower the rear, and you get a cruiser style - slow steering, very stable. It's, as always with bikes, a trade-off. MotoGP riders tweak by 1 mm increments!

You'll want to research actual wheel heights for each, current fork angle, XR650 etc fork angle, and aim for whatever they're running. You can make a lot of difference by sliding the fork tubes up and down in the triple trees, and tweaking shock spring height and sag. Research "shock sag" and learn how to set it up to start. You will then want to set up fork sag too.

It takes some work, but you can have a sweet ride in the end if you do it right. And along the way, you'll learn a LOT about suspension and handling.


Hope that helps!

[duonas]

Thanks for the info, that explains something to me.
I'm not sure if I'll dive very deep with this, but one thing that is clear now, that I can use 21" rim and it won't make any disaster.
I looked up shock sag, but my shock or forks aren't adjustable, except rear shock spring tension can be adjusted.

Thanks!

[Dick_FMX650]

Hi, I am curious to know if you ended fitting your 21" wheels and what you experience is so far?

I'm thinking of doing same on my fmx. Any help /aadvice would be great.

Thank you