Vintage Dirt Bike Q & A

Raced a few moto's yesterday on my 80' CR and due to the fast track conditions it became pretty obvious my rear brake is WAY too touchy. Its either on or off it seems, cause me to swap ends a few times coming into corners. My pads are good, i just cleaned the whole assymbly recently. Drum surface is fine. I was thinking of swapping to a cable system like the CR480's instead of the rod to give me some stretch to play with, also thought of shortening the lever to take away some mechanical advantage. If anyone has any good ideas i would appreciate it.

If putting a bit more bend in the rod doesn't help enough, one thing that might be worth trying is to replace your 95014-73100 brake rod spring, then cut a small section from the old one and put it between the adjusting nut and the rod joint in the end of the hub brake actuator arm. This would give you a softer brake engagement and the same max braking you have now. I'd keep an eye on the adjusting nut at first to make sure it doesn't want to unscrew and fall off, though. If you had sufficient adjustment range you might also try changing the actuator arm's angle relative to the brake rod; angle > or < 90 degrees = less braking leverage.

Ray

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'74 CR125M (175cc), '75 MR175, '82 RM250Z, '08 YZ250F, '14 Zero FX electric, '14 Zero MX electric, '18 Alta MXR electric

I did put a couple bends in the rod on saturday but to no avail! Now let me get this sraight, if i move my arm forward so it engages past 90*, i will have less braking force.. but the arm is still the same length? That actualy makes some sense cause i moved my arm 1 spline back before the race to get me back in the meat of the adjustment threads, which put the arm at about 6 oclock when the brakes came on, which according to your logic would make for the most braking force. I also like the idea of putting a spring between the adjuster and the hollow tube thingy in the brake arm, I will try that too.

Yes. I set up my MR's brake to have the shoes contact the drum right at 90°, and the braking improved noticeably. If the arm contacts the drum at less than 90° and then moves to 90° when the brakes are applied harder, you'll have at least some measure of 'progressive' braking.

Slotting the shoes may help drop braking power a bit, and there's another thing that (in my experience) will definitely reduce braking power; aftermarket brake shoes. I tried Vesrah slotted Kevlar shoes which were slightly weaker than stock, and also SBS shoes which I found to be scary-weak, almost unsafe. I allowed both these shoe sets to fully break in and went out of my way to give them the benefit of the doubt, as I really didn't want to discover I had wasted my money.

Ray

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'74 CR125M (175cc), '75 MR175, '82 RM250Z, '08 YZ250F, '14 Zero FX electric, '14 Zero MX electric, '18 Alta MXR electric

i think my pads are aftermarket cause they have the grooves in them. So if i move my arm back a spline(the arm on the CR points DOWN from the shaft) the brakes will come on before 90*(assuming 90* is straight down, zero is pointing back and 180 is pointing at the front tire). I like this idea.... cause its FREE!!! Although i still struggle with figuring out how it changes since the arm length is still the same????? On another note, does anyone know of an available brake arm that would fit the CR hub? My aluminum one is getting pretty sloppy at the adjuster hole, its very egg shaped. I would go for a steel one if i knew of one that would fit. Also, ray the physics expert can chime in here.... if i get a shorter arm, that would reduce the leverage and it would take more force to give the same braking power right? The trade-off is that the arm has to move a shorter distance to engage though. So maybe i could just find a non-sloppy short arm to fix my problem?

That sounds right.


Actually, the effective arm length (distance from the brake rod to the brake cam pivot) gets shorter as the angle decreases, and some of the brake pedal motion is wasted when the rod/actuator junction moves away from the cam pivot, in an arc.

Shortening the arm will work, but it will still be sloppy. I'd suggest getting the part number for the pivot cam, searching for other models that use that cam, then checking the trail or street models for steel actuator arms using the same 'thingy' pivot barrel as your bike. It takes a while, but at the end you know what your options are, if any. Who knows, there may be several steel actuator arms that will fit, possibly with different lengths.

Ray

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'74 CR125M (175cc), '75 MR175, '82 RM250Z, '08 YZ250F, '14 Zero FX electric, '14 Zero MX electric, '18 Alta MXR electric

Thanks again ray, i found a site once where i could type in the part# and the list of appicable models would show up. Does anyone know of this site?

I find the BikeBandit search engine to be the easiest to use (one-line entry accepting most OEM and aftermarket part numbers), the most complete, and the most up-to-date as far as discontinued parts go. Once it finds your part(s), click on the 'Fits machines' link in the 'Your Price Today' section to show all the models that part fits (way cool!). The only thing I don't like about it is that once you've found the part, BikeBandit assigns their own proprietary part number to it. It's not that big a deal.

http://www.bikebandit.com/oem-parts

If I need to find an OEM part number, though, I use Babbitt's:

http://www.babbittsonline.com/pages/par ... fault.aspx

Ray

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'74 CR125M (175cc), '75 MR175, '82 RM250Z, '08 YZ250F, '14 Zero FX electric, '14 Zero MX electric, '18 Alta MXR electric