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Thread: Sticking front wheel

  1. #46
    Liaison 20774's Avatar
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    I recall getting a new piston for my /7 from Ted Porter at the Beemershop.

    https://www.beemershop.com/product/f...0-not-r65.html
    Kurt -- Forum Liaison ---> Resources and Links Thread <---
    '78 R100/7 & '69 R69S & '52 R25/2
    mine-ineye-deatheah-pielayah-jooa-kalayus. oolah-minane-hay-meeriah-kal-oyus-algay-a-thaykin', buddy!

  2. #47
    John D'oh
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    I have uploaded a photo of good clean spare caliper and a photo of new dust cover, seal and new stainless steel caliper piston for ATE. Your first caliper image has debris left from the plastic/rubber dust cap in the outer most dust cap pocket (first largest inside diameter in the bore). There may also be deposits of dirt and oxides from the water which found it's way into the caliper. It should be cleaned so that it looks like the image I have loaded - (a silver 78 40 mm ATE). Solvents or 0000 steel wool or both, 600 grit wet and dry paper as a last resort because you do not want to enlarge the dust cover pocket making it too large to fit the dust cap tight enough to seal out water and dirt which will ruin the new square seal and piston. The surface that is present in the rest of the bore on that caliper looks OK. The second caliper appears to have a graphic screen pattern superimposed over the lower half of the image obscuring the bore in that area but the rest of the caliper looks fine. Neither of the caliper pistons are serviceable - both should be replaced. If you use steel wool, stay away from the bore immediately in front of and behind the square seal pocket - the smallest inside diameter in the caliper bore.
    It looks like there is a ring of dark material in the back of the caliper where the surface transitions from horizontal to vertical. I believe it is outside the area where the piston sits and should not be a problem if you cant get rid of it... That dark ring could be a shadow in the image I am looking at too. Can't tell.
    Attached Images Attached Images
    John D'oh

  3. #48
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    Another surprise.

    As I toyed with the idea of purchasing new pistons, I thought it wise to put a caliper on each of them. Low and behold the left-side piston was 38mm and the right-side was 40mm in diameter. Is this normal? Having no knowledge of this motorcycle, it seems rather odd. I was also planning on purchasing Brake caliper repair kits for both sides. So one for 38mm and one for 40mm?

    rickt

  4. #49
    John D'oh
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    Both calipers should be the same size to be correct. If I read the Snabbkatalog correctly prior to 9/75, R90S calipers were 38mm. After that date they were 40mm. I noted a price of $850.00 for a new complete 40 mm caliper on MAX BMWs parts list (!). I do not know the efficacy of two different sizes of calipers in a dual disc application. I should not consider it ideal. What is the VIN number of your machine?
    John D'oh

  5. #50
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    Vin

    VIN # is 4093562

    rickt

  6. #51
    Liaison 20774's Avatar
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    So, a late 1976 R90S. The parts listing for this shows 40mm calipers, but for some reason, it shows "repair calipers" as being 38 and 40mm.
    Kurt -- Forum Liaison ---> Resources and Links Thread <---
    '78 R100/7 & '69 R69S & '52 R25/2
    mine-ineye-deatheah-pielayah-jooa-kalayus. oolah-minane-hay-meeriah-kal-oyus-algay-a-thaykin', buddy!

  7. #52
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    Possibilities

    Does this mean that it's actually possible the bike came originally with two difference caliper sizes. I bounced this off a mechanical-guy friend of mine (not a BMW owner) and he felt this was most unlikely.

    rickt

  8. #53
    Liaison 20774's Avatar
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    I was also say this is unlikely. I could see this happening if someone where to do a conversion on a basic R90/6 to add the second disk. Then maybe they could only find a caliper for a 40mm piston. I'm thinking that the stock left side caliper would have been 38mm and a 40mm R90S leg was grafted to the bike. BTW...which side is 38mm and which is 40mm?
    Kurt -- Forum Liaison ---> Resources and Links Thread <---
    '78 R100/7 & '69 R69S & '52 R25/2
    mine-ineye-deatheah-pielayah-jooa-kalayus. oolah-minane-hay-meeriah-kal-oyus-algay-a-thaykin', buddy!

  9. #54
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    Piston size

    The left side is 38mm

    What is my most cost effective option at this point?

  10. #55
    Registered User beemeruss's Avatar
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    Calipers

    Quote Originally Posted by r65lsk75c View Post
    The left side is 38mm

    What is my most cost effective option at this point?
    You have determined that either a 38mm or 40mm caliper will interchange and I just confirmed that by calipering a leg from my '76 and '78. My '76 came with the single 38mm version but your R90S should be both 40mm as discussed. I believe that you should find another 40mm to rebuild for several reasons. They are commonly for sale at reasonable prices as people retrofit Brembo's or other brands. Sorry that I don't have any spares. The braking force would then be balanced. A 40mm would provide more braking force, and while it would take slightly more lever travel, the action would feel less wooden. They are interchangeable left to right, increasing the availability too. Having mismatched calipers will not go unnoticed by the cognoscenti. The single 40mm ATE on my '78 R80/7 provides barely adequate braking power whereas the twin 40's on my R100RS provide satisfying stoppage. The past president of our local club, the Black Gold Beemers, has a '74 R90S which has not run for almost 25 years. In the spirit of the Airheads Beemer Club canon #8 I shall help him to resurrect it this winter so this thread has been most useful sir. Good luck!

    Russ '76 R90/6 '78 R80/7 '78 R100RS
    Last edited by beemeruss; 07-16-2019 at 08:22 AM. Reason: year of manufacture

  11. #56
    John D'oh
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    Rick - this link takes you to Snowbums airhead brake technical discussion. I just spent over an hour looking for comments on using two different size calipers. https://bmwmotorcycletech.info/brakes.htm - bookmark the page and when you have a chance read all of it as it pertains to ATE brake system design and function as well as service. In the articles he describes how the area of the caliper piston determines the available braking force applied to the disc (pads being the same) which seems logical. Therefore It would appear that a mismatched caliper set (38/40) is fine albeit with slightly less braking force than a set of two 40mm.
    There are as many reasons for your machines current configuration as there have been previous owners over the last 43 years. Neither of the current set of calipers could even be original to the bike. The issue is making what you have work. Clean the calipers. Make sure the holes for bleed screws and hydraulic tubes have good threading. Buy the parts and fix them. Read the article first.
    Again a tip-o-the hat to beemeruss..,
    John D'oh

  12. #57
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    Options going forward

    John, I will definitely read the technical article on the Snowbum website.

    I would like to put together a list of options based primarily on price. I do realize when we are talking about brakes price may not be most important factor.

    Option 1
    Use the current calipers and pistons that came with the bike. Thoroughly clean all pieces and remove caliper (deposits?) with 0000 steel wool or 1000 grit emery paper. Do the same with the pistons. Order one 38mm caliper repair kit and one 40mm kit. Reassemble and flush the system.

    Option 2
    Source a used 40mm caliper and piston (any source suggestions would be appreciated). Purchase two caliper repair kits and complete as described in options 1.

    Option 3
    Purchase a new caliper and piston. This will likely be cost prohibitive.

    Please let me know if this makes sense.

    rickt

  13. #58
    '99 '03 '06 National Co-Rally Chair Friedle's Avatar
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    rickt,

    do not neglect to put a price consideration for option 4....your life and well being.

    brakes are not a place to cheap out. the ability to stop in a quick, short and safe manner is what often separates riders from crashers. be careful where you choose to be frugal.

    Friedle
    submitted with all due respect for your feelings. =8^)
    Ride fast safely

  14. #59
    John D'oh
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    Quote Originally Posted by r65lsk75c View Post
    John, I will definitely read the technical article on the Snowbum website.]I would like to put together a list of options based primarily on price. I do realize when we are talking about brakes price may not be most important factor.
    Read before you make your list of options - Your list and price notes are your budget.

    Price isn't the most important factor
    No it isn't but money _IS_ necessary to make your bike go and stop. Often - lots of it. You are going to have to spend some money to do the job right. If you don't do it right you'll have to spend more money to do it over. Repeat after me: "never replace junk with more junk".

    Option 1- Use the current calipers and pistons that came with the bike. Thoroughly clean all pieces and remove caliper (deposits?) with 0000 steel wool or 1000 grit emery paper. Do the same with the pistons. Order one 38mm caliper repair kit and one 40mm kit. Reassemble and flush the system.
    Your pistons are souvenirs now. they will make great paper weights on your desk. See above -

    Option 2- Source a used 40mm caliper and piston (any source suggestions would be appreciated). Purchase two caliper repair kits.
    Look on eBay - use the search term 'airhead' under motorcycle parts, brakes. Check the IBMWR web site 'marketplace'. Research on line for private BMW mechanics and shops advertising airhead parts. There are a bunch of them. Some listed on Snowbum's site. http://www.airheads.org/airmarket/for-sale-parts - Look through the listings on the BMW MOA flea market in the magazine or on line. post part wanted advertisements. Motobins in the UK has stainless pistons and factory seal kits for less.

    Option 3- Purchase a new caliper and piston. This will likely be cost prohibitive.
    Yes, most likely - at 850 bucks a side. By the way, you can turn the calipers over - you don't have to try finding a left or a right. They are 'ambidextrous'.

    Please let me know if this makes sense rickt
    Well, everyone who has added their two cents to this thread has said, "... buy new pistons". If you want to do it by the book find another 40mm caliper. You'll be able to get some of your money back selling the 38mm caliper. Just remember, these ATE calipers are 30 - 40 years old by now so used calipers might need to be rebuilt too.
    John D'oh

  15. #60
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    Hope no one was holding their breath

    Had a good conversation with a parts guy at Bobs this morning. New calipers are not available in US or in Germany. I did learn that the caliper repair kits include new pistons. I took the bold step and ordered two repair kits, one 38mm and one 40mm. So if I consider that my life may depend on the function of the front brakes, any failure will rest directly on me. Before I became burdened with a host of medical problems, I was riding my 74 Trident to a local repair shop. About to exit a roundabout and a driver failed to yield and broadsided me. That little incident produced a host of physical and mental issues. A long way of saying if I actually reach a point when the R90s is rideable, Ill worry more about being taken down by a 4-wheeled vehicle.

    I very much appreciate all the assistance provided. Being an eternal pessimist, Im expecting to find something undesirable in the MC.

    rickt

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