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Discussion Starter #1
I'll start with this.

I'm about to dive into a street build and was wondering how much larger I can go on the front end of a 2000 Honda cr125r. I'm trying to keep cost down so I'm not trying to drop the money on a set of SM wheels.
 

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There aren't many 21" fronts wider than 90mm, and even if you find one I doubt it'll be a decent profile on your wheel.

Out back, I'm not sure what options you have. 19" rear has very few tire options available for the street. As I said elsewhere, a flat track tire might be your best option there.
 

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Discussion Starter #3
Yeah. Just trying to get things more active from what I've been reading. Thought I'd cross post to see what happens.


You're right though, making it a tracker instead might just be the ticket.
 

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I've seen people go really fast on 19s on kart tracks. But the reason for suggesting a flat track rear was more that there aren't any other options that do decently on pavement that I know of for 19" wheels. Generally, on a 19 rear your options are pretty much full knobbies or flat track.
 

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There is also the sportsman class/setup keeping the stock size rims. Re-lacing the front rim to a 19 is definitely the cheapest option. Keep in mind too the smaller that front rim gets, the less effective that stock brake rotor size will be.

Have you sorted out a charging system on that thing?
 

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There is also the sportsman class/setup keeping the stock size rims. Re-lacing the front rim to a 19 is definitely the cheapest option. Keep in mind too the smaller that front rim gets, the less effective that stock brake rotor size will be.

Have you sorted out a charging system on that thing?
I'm surprised you say the brake gets less effective with a smaller wheel. I'd have thought that the smaller the wheel, the more leverage the brake has on it - similar to going with a larger rotor, although without the extra heat capacity.

Of course, if the smaller wheel comes with a tire that's more effective, you'll likely be braking harder and therefore need more brake.

I can't say that I've tried swapping between them, though.
 

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Yeah if just rotor size were the only factor I'd agree. Problem is that mx rotors hsve less braking surface, are thinner so they heat soak quicker and are generally paired with a fairly light rim and tire. As the wheel size drops to a sm setup the rim gets heavier. Tube and tire are much heavier too. So while the mass of the bike doesn't change you have a lot more inertia on the wheel.

I tried a 260, 275 & 320 on my drz. I couldn't get anywhere near the same braking with the dirt rotor.
 

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I agree that a larger rotor is more effective than a smaller rotor (at the expense of rotational inertia)

What I question is (for example) 21 vs 19, both with the same rotor. I'd expect the 19 to stop better. Is a 19" street tire that much heavier than a 21" street tire?

If you're talking about changing rim size AND changing rotor size, that isn't what I'd interpreted your post as saying - you said "the smaller that front rim gets, the less effective that stock brake rotor size will be" so I thought you meant the stock rotor in all cases.
 

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I agree that a larger rotor is more effective than a smaller rotor (at the expense of rotational inertia)

What I question is (for example) 21 vs 19, both with the same rotor. I'd expect the 19 to stop better. Is a 19" street tire that much heavier than a 21" street tire?

If you're talking about changing rim size AND changing rotor size, that isn't what I'd interpreted your post as saying - you said "the smaller that front rim gets, the less effective that stock brake rotor size will be" so I thought you meant the stock rotor in all cases.
Yeah my initial post was hurried but here is what i meant/mean.

If you keep the dirt rotor and go to a smaller diameter rim, which will end up with a heavier tire/rim/tube etc, that dirt brake rotor will mostly likely heat soak, have less braking surface for the pad and just generally not provide the braking you will need. I rolled around on a drz400 with an oversized offroad rotor for awhile and on the street it was ok, but not great. I have tried variations of different rotors, rims, calipers, masters etc on several bikes. I run a 310 berringer on a 16.5 front right now with a 6piston caliper. I have run 320s on 17s with stock mx brakes. with stock mx master and SM calipers, with aftermarket floating calipers and radial masters and my current setup. I have tried several on the street and all on the track. The demands on the track are obviously much higher, but even on the street just hard braking a few times from say 60mph is enough to heat soak an mx front brake rotor.

Like i said in my last post if we are comparing just the change in rotor diameter or tire diameter it is apples to apples. There are a half of dozen factors though besides just the diameters.

1. Rim mass(inertia)
2. Tire mass(inertia)
3. Tube mass(inertia)
4. Rotor thickness(heat soak)
5. Rotor braking surface (friction force)
6. Tire contact patch


And yes "street" tires are that much heavier. I don't have an exact figure but all dirt and dot knobbies are made to be used with a tube. All street radials have steel belts, thicker carcass and more rubber that meets the road. Bias ply slicks aren't light either. I run alpinas to go tubeless but the rims still aren't light(aluminum not carbon matrix ones). I have my SM rims off right now as well as a set of dirt rims off a bike. Ill try to weight them and see this weekend.


So in conclusion :) sorry about my first post. Yes from a pure leverage standpoint all other things being the same, the same diameter rotor will be more effective on a smaller diameter rim(17) than a larger one(21).
 

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I agree that a larger rotor is more effective than a smaller rotor (at the expense of rotational inertia)

What I question is (for example) 21 vs 19, both with the same rotor. I'd expect the 19 to stop better. Is a 19" street tire that much heavier than a 21" street tire?

If you're talking about changing rim size AND changing rotor size, that isn't what I'd interpreted your post as saying - you said "the smaller that front rim gets, the less effective that stock brake rotor size will be" so I thought you meant the stock rotor in all cases.
I just weighed my stock KTM 21in rim with tire/tube etc and my 16.5 alpina with dunlop slick, no tube.

KTM 21in - 18lbs
Alpina 16.5in - 25lbs

I imagine a 17 with a street tire and tube would be a bit more as well. I couldn't tell you exactly where all the weight is since they are fully assembled but i did a just get a set of tusk impact dirt rims and some new tires recently and know the rims weren't super light. My Alpinas run Kite hubs and the ktm wheel i weighed was off an XC so it was a machined hub, not cast, for whatever its worth.

I only have the one set of SM rims at the moment but have 3 sets of dirt rims. Stock, tusk impact and DNA. I have to pull the tusk this weekend to swap a tube and ill try to weigh it as well. That rim is running a tusk rotor which is solid but stock thickness and SS.
 
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