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How much weight is lost with a rear disc brake conversion?

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Hey All-

how much weight does one shave off when converting to a rear disc brake? I understand its not a "simple" process. Does anyone have a link to the process? I assume new wheel hub, swing arm modifications (TIG welding I assume), brake master cylinder etc.

I will say this- the simplicity of the drum is very nice. If a lot of weight isnt saved Im not sure it's a modification I'll consider. But i figured I'd throw this post out there.

T

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It's a wash.  No saving.  BTR did it because he had the parts in his shed.  He said it would be the last mod to do after everything else.  If you ride through a lot of water, that might be the reason.

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The weigh saving really depends on the wheel chosen. And the caliper, bracket, and MC add some weight. 

The drum brake torque is transferred to the swing arm via a tab on the center line of the SA, the Honda disc brake brackets transfer the torque via a tab on the top edge of the SA. But Suzuki disc brake brackets use a tab on the SA center line. 
I suspect the 230 rear wheel w/ brake is close to 30lbs, you should measure yours.

XR250R disc brake rear wheel w/ tire, al sprocket, rotor is about 28+lbs
CRF250X disc brake rear wheel w/ tire, al sprocket, rotor is about 24.75lbs
89 RM125 disc brake rear wheel w/ tire, al sprocket, rotor is about 24lbs

All of the above are with aluminum sprocket and 4.00R18 Trials tires (heavy). Add 2lbs for the caliper, bracket, hose, and MC.

 

 

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I would be much more interested in swapping drum out for disc, simply because drum brakes suck....

 

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Another advantage with disk is you get rid of braking issues due to non-floating drum.  Really good bikes had floating drums to combat reaction at the brake linkage during suspension movements.  While not so critical on a utility bike like CRF230 the problem still exists.

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I would be much more interested in swapping drum out for disc, simply because drum brakes suck....

Honda small bike drum brakes are very good for drum brakes. The 230 uses the same brake shoes as the old XR200R used on front and rear.  The shoes do become glazed with use and water/mud gets inside so a good cleaning and deglaze can really help. New shoes often take a while to bed into the drum so after a few hours I remove the shoes and file/sand down the high spot. Often need to repeat this several times until I see good contact on the shoes.  Cross cut grooves in the lining can help scrape water off the drum surface, so can sintered lining material. 

Quote

Another advantage with disk is you get rid of braking issues due to non-floating drum.  Really good bikes had floating drums to combat reaction at the brake linkage during suspension movements.  While not so critical on a utility bike like CRF230 the problem still exists.

I agree but some things can be done to help a drum setup:
81-83 XR200Rs used a cable operated rear drum that eliminated the linkage problem.
With rod linkage you can also eliminate the linkage effect by positioning the connection between the brake pedal arm and the pull rod as close as possible to the center of the swing arm pivot. Even with the linkage fix there still is the effect of the brake torque going thru the swing arm, even with a disc, and that can cause rear brake shatter if there is not enough rear sag. The early eighties RMs had a floating backing plate that eliminated that.

Quote

The disc makes it a little easier to R&R the wheel when fixing a flat tire.

I agree, and the XR system is very easy. Some food for your thoughts when considering modding the rear brake:
Most of the XRs used a quick remove/release system for the rear wheel whether drum or disc.
Rear brake arm that has a quick release for the pull rod. #9 & 10 on 2nd fiche, plus #6, 7, 8, &26 on the 3rd fiche.
The end of the SA slot is open at the rear with a clip that keeps the axle in place if the axle nut comes loose. #12 on 2nd fiche
The end of the SA slot has metal tabs that extend aft beyond the slot to hold the axle when sliding the wheel out of the SA.  #6 & 7 on first fiche
The Caliper bracket slots onto the axle so it can be lifted off. #10 on 4th fiche
The axle has rod welded to on its head so a wrench is not needed. #2 on 2nd fiche
Snail adjusters are used so the axle position can be quickly adjusted the same on each side. #1 on 2nd fiche

All in all a system that spoils you when you need to work the rear wheel of a regular bike. The following part fiche are from two different years of  XR250Rs.

KK14F2001B.gif

 

KK14F1100.gif

 

 

KK14F1800A.gif

KK14F1102A.gif

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I am sure this has been discussed before, but will a 230M/L swing arm with disc brake work on the 230F? I seem to remember some differences with geometry of the linkage issues or such. Probably too good to be true as a simple bolt on option.

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I can only repeat this. I have one disk brake 230 the other with stock drum brake. Repeated again I only did it because I have all parts in my garage. Ride them both no difference unless you where racing the bike. Who would race a 230 in the first place.The stock drum brake 230 works dam good. If your a nut case like me do it.

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21 hours ago, Chuck. said:

Honda small bike drum brakes are very good for drum brakes. The 230 uses the same brake shoes as the old XR200R used on front and rear.  The shoes do become glazed with use and water/mud gets inside so a good cleaning and deglaze can really help. New shoes often take a while to bed into the drum so after a few hours I remove the shoes and file/sand down the high spot. Often need to repeat this several times until I see good contact on the shoes.  Cross cut grooves in the lining can help scrape water off the drum surface, so can sintered lining material. 

I agree but some things can be done to help a drum setup:
81-83 XR200Rs used a cable operated rear drum that eliminated the linkage problem.
With rod linkage you can also eliminate the linkage effect by positioning the connection between the brake pedal arm and the pull rod as close as possible to the center of the swing arm pivot. Even with the linkage fix there still is the effect of the brake torque going thru the swing arm, even with a disc, and that can cause rear brake shatter if there is not enough rear sag. The early eighties RMs had a floating backing plate that eliminated that.

I agree, and the XR system is very easy. Some food for your thoughts when considering modding the rear brake:
Most of the XRs used a quick remove/release system for the rear wheel whether drum or disc.
Rear brake arm that has a quick release for the pull rod. #9 & 10 on 2nd fiche, plus #6, 7, 8, &26 on the 3rd fiche.
The end of the SA slot is open at the rear with a clip that keeps the axle in place if the axle nut comes loose. #12 on 2nd fiche
The end of the SA slot has metal tabs that extend aft beyond the slot to hold the axle when sliding the wheel out of the SA.  #6 & 7 on first fiche
The Caliper bracket slots onto the axle so it can be lifted off. #10 on 4th fiche
The axle has rod welded to on its head so a wrench is not needed. #2 on 2nd fiche
Snail adjusters are used so the axle position can be quickly adjusted the same on each side. #1 on 2nd fiche

All in all a system that spoils you when you need to work the rear wheel of a regular bike. The following part fiche are from two different years of  XR250Rs.

KK14F2001B.gif

 

KK14F1100.gif

 

 

KK14F1800A.gif

KK14F1102A.gif

I need to see the old style quick release arm #9 &10.  BTR said the 230 arm can be modded.  I vaguely remember this on my XL100.  Even in the shop, it's a bit of a pain to replace the rear wheel and get the brake rod threaded into #10.  Welding a piece of O1 drill rod onto the axle head is not a problem.

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One of the really nice features on the XRs was the little details that made pulling or field servicing the rear wheel easier, and I've tried to keep those when modding my XRs. I no longer have a rear drum XR so I can't provide pictures.

Six items make the quick release brake work:
A stop on the brake rod for the spring, see small lump on part fiche.
The spring that keeps pressure on the brake arm.
The half ferrel ("joint", #10) that fits into the brake arm and allows the brake rod to be lifted up.
The brake arm with an opening on the top so the brake rod can be lifted out.
And most important is the clip that keep the rod attached to the brake arm except when the spring is compressed.
The wing nut for quick adjusting.  
 

 

Edited by Chuck.
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The caliper bracket has a small axle hole so prob a sub 250cc MC. The caliper is early XR/CR because the banjo fitting in on the bottom, these were used on CRF250L.

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I did this conversion because I live in the UK and it's wet a lot and I got bored with going through a puddle and having no back brake for a while. Mine was a home brewed conversion using a CR125 wheel, CRF250 hydraulics and home made everything else. It was a fairly complex process which I found fun, cos I like doing stuff like that.

Weight saving ; nil. Weight gain ; nil. Braking ; slightly better in the dry, MUCH better in the wet. Big gain; I ended up with a really nice wheel change system with no loose spacers.

Bottom line IMHO, if you have a lathe and a TIG welder and you like projects then do it, otherwise, don't.

 

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2 hours ago, Fernacticus said:

Bottom line IMHO, if you have a lathe and a TIG welder and you like projects then do it, otherwise, don't.

 

Where does the tig welding come in? Which model caliper hanger did you use with your CR125 hub?

Edited by xplodee
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The TIG welding is to attach the new receiver for the caliper bracket to the swingarm. I don't have a milling machine and it took me a day to form one out of a block of ally from a metal merchant .

THE moment of commitment; taking an angle grinder to the drum receiver!

Wheel is from an '89 cr 125, uses the same sprocket I think. Caliper and carrier are CRF250, bought off Ebay. I turned a press in reducer for the caliper carrier to bring it down to the spindle size and turned up stepped spacers so I could stay with the original size wheel bearings. Removing the wheel is now ; nut off, wheel forward and chain to the side spindle out and slide the wheel out. No loose spacers. Wahoo! I used the stock brake pedal with the operating arm cut off and welded on at the back.

As I said, if you like an interesting project, then go ahead, but I don't think there's a bolt on kit available.

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Thank you Fernacticus! This helps a lot. Any pics you can provide of your setup would help me a ton. Hoping to avoid any welding but i can machine anything to help that goal (knee mill, cnc mill and lathe on hand).

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One other advantage of a rear disc is better control of braking near lock up, important to reduce engine stalls  when descending hills. These stalls are more likely on loose soil because I use a Trials tire, and they have less void space and shorter knobs than a knobby so pack up and slide. And the high compression of my engine makes bump starting difficult. Do I need it? NO and my riding partner has a near identical bike but with a drum rear brake and he does just fine on all of our rides.

From my previous post you can determine that there may, or may not, be any weight saving. It all depends on the selection of parts; most of my weight saving was from switching to an aluminum swing arm, not the disc brake. 

You also need to weld a bracket onto the frame for the MC and fab linkage to operate it.

Calipers come in two basic flavors relative to the location of the banjo fitting and attachment to the bracket.

Honda dirt bike disc brackets transfer brake torque to the SA via a tab on the upper edge of the SA, the drum does it via a tab on the center line of the SA.

MC come in two styles; remote reservoir and integral reservoir.

Two different caliper brackets and a master cylinder

 

598dcde303378_CR125RearCaliper01.jpg.198e082e1105abb875bd2bf7a1af9baf.jpg

 

598dcde277d54_CR85RearCaliper06.jpg.cf4e0635ee6f3507a65a7e9d88145edb.jpg

 

 

598dce0992eae_RearMasterCylinder.thumb.jpg.fba5927014d9a69a5e13d297f546f8db.jpg

 

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