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Thread: What rear tire should I have for my R90/6?

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    Curmudgeon nrpetersen's Avatar
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    What rear tire should I have for my R90/6?

    I have a 1975 R90/6, just a bare bike, no fairing or bags, small gas tank, with a new Avon 100/90-18 rear tire, which has a tube in it because of the spoked rim. My BMW owners manual specifies 400 H-18 tires.

    It feels very squirrely above 60 mph. It is marginally convergent at ~ 4 Hz in what feels like a weave mode. I assume this is from poor lateral stability in the rear swing arm/tire system, but the tire pressure is the recommended 30 psi and there doesn't seem to be any clearance in the swing arm bearings.

    Somehow the rear tire has a smaller profile than I recall. What tire should I have?
    Retired w 2005 K1200LT, 2000 R1100RT, & 1975 R90/6

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    Superkraut typ181r90's Avatar
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    Quote Originally Posted by nrpetersen View Post
    I have a 1975 R90/6, just a bare bike, no fairing or bags, small gas tank, with a new Avon 100/90-18 rear tire, which has a tube in it because of the spoked rim. My BMW owners manual specifies 400 H-18 tires.

    It feels very squirrely above 60 mph. It is marginally convergent at ~ 4 Hz in what feels like a weave mode. I assume this is from poor lateral stability in the rear swing arm/tire system, but the tire pressure is the recommended 30 psi and there doesn't seem to be any clearance in the swing arm bearings.

    Somehow the rear tire has a smaller profile than I recall. What tire should I have?
    4.00x18

    I'm running Heidenau k36 rear which is significantly outperforming the Metzeler I previously used. Tire pressure should really be around 36 or so psi
    // 1975 BMW R90/6 (cafe'd) // 1957 BMW R60 (in pieces) // 1967 Aermacchi/H-D Sprint 250 SS (custom special) // 1973 VW Type 181 Custom SOLD )

    http://symphonyofshrapnel.blogspot.com

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    Quote Originally Posted by nrpetersen View Post
    I have a 1975 R90/6, just a bare bike, no fairing or bags, small gas tank, with a new Avon 100/90-18 rear tire, which has a tube in it because of the spoked rim. My BMW owners manual specifies 400 H-18 tires.

    It feels very squirrely above 60 mph. It is marginally convergent at ~ 4 Hz in what feels like a weave mode. I assume this is from poor lateral stability in the rear swing arm/tire system, but the tire pressure is the recommended 30 psi and there doesn't seem to be any clearance in the swing arm bearings.

    Somehow the rear tire has a smaller profile than I recall. What tire should I have?
    100/90 - 18 is roughly close to the recommended 4.00 x 18 original, but not exact. My personal opinion is to purchase the originally recommended size (4.000 x 18) to maintain the original sizing in relation to frame and stands. But, since you have a new Avon, I think, based upon a thread I saw within the last few weeks, that you can't go anymore by the recommended pressures suggested by BMW on those stickers. The newer tires are better, and need higher pressures - take the recommendations from the new tire, not the frame. However, do not go over the recommended high limit and be sure to allow for temperatures which raise the pressure and if already high (even if under the recommended high) could go higher than recommended high when hot.

    Usually rear in the mid 30's to upper 30's and the front lower 30's.

    I run my a little on the higher side (but not over upper limits) because I tend to like a stiffer ride, and less mushiness on corners.

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    Administrator 20774's Avatar
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    Are you new to the bike? Or have you have tires in the past and you didn't have this kind of squirreliness? I agree that 30 is too low for the rear...might be OK for a front tire, or maybe a little higher. Correct the air pressures and see how things are. Weaves can be caused by a number of things, but first get the pressures settled in. Please update us and there are other things that you can look at if the weaving continues.
    Kurt -- Forum Administrator ---> 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!

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    Curmudgeon nrpetersen's Avatar
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    Quote Originally Posted by 20774 View Post
    Are you new to the bike? Or have you have tires in the past and you didn't have this kind of squirreliness? I agree that 30 is too low for the rear...might be OK for a front tire, or maybe a little higher. Correct the air pressures and see how things are. Weaves can be caused by a number of things, but first get the pressures settled in. Please update us and there are other things that you can look at if the weaving continues.
    I bought the bike new (!). This handling issue is a new one for me though. I'll try the higher pressures - I agree that makes sense.

    It may be spring before I'll have an answer though.............
    Retired w 2005 K1200LT, 2000 R1100RT, & 1975 R90/6

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    Administrator 20774's Avatar
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    Quote Originally Posted by nrpetersen View Post
    It may be spring before I'll have an answer though.............
    Ouch...Minnetonka...I understand!
    Kurt -- Forum Administrator ---> 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!

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    Old Salt MEAirhd's Avatar
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    Quote Originally Posted by typ181r90 View Post
    4.00x18

    I'm running Heidenau k36 rear which is significantly outperforming the Metzeler I previously used. Tire pressure should really be around 36 or so psi
    Happy with my K36 also after 3600 mi. Not sure it perfectly matches up correctly with the K70 I have on the front.
    Superkraut; what are you running for a front match up?
    T. Waits.."Got to get behind the mule.. in the mornin and plow"

    R100T'79

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    Superkraut typ181r90's Avatar
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    Quote Originally Posted by MEAirhd View Post
    Happy with my K36 also after 3600 mi. Not sure it perfectly matches up correctly with the K70 I have on the front.
    Superkraut; what are you running for a front match up?
    Still running the front Metzeler Lasertec, even though the Lasertec rear was shot the front still has about 50% more life. The combination is working really well so far.

    I'm running Heidenau K34s on my Aermacchi, but I don't think I'd trust them with the higher speed of the BMW, they have a more aggressive tread and seem to track rain grooves and metal grating pretty badly - they might be good for a GS though
    // 1975 BMW R90/6 (cafe'd) // 1957 BMW R60 (in pieces) // 1967 Aermacchi/H-D Sprint 250 SS (custom special) // 1973 VW Type 181 Custom SOLD )

    http://symphonyofshrapnel.blogspot.com

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    The 100/90 is too small. Even a 110 sits substantially lower than the stock 400. Then you are running very low pressure. No wonder it is squirrely.
    You don't say which Avon you are running; I run the Roadriders at 40-42 psi. I would run more air.
    Also, it is safe to assume the steering head bearings have never been serviced - get in there and clean them; relube. Use red grease, not Moly. I clean with alcohol so it dries quick. Adjust a tad towards the tight side - the fresh grease will set in a bit.

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    please please please tell us you're not still running the original shocks!

    are you?


    and 30 psi is too low, front or rear.
    Ride Safe, Ride Lots

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    Minnesota Nice! braddog's Avatar
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    Quote Originally Posted by bikerfish1100 View Post
    please please please tell us you're not still running the original shocks!

    are you?


    and 30 psi is too low, front or rear.
    I've said it on these boards many times. New rear shocks, new tires, both front and rear, proper air pressure, my R100RS rides like it's on rails. Previous to those upgrades, let's just say I had some "wobbles" here and there. I only put a couple of hundred miles on it after I purchased it before I made those purchases.

    IMHO, I'll agree with what the others have said. 30 is too low. I'd say 38 as a minimum, I usually run 40-42 on my Metzler ME880 rear.

    You'll also just need to figure out what really works for you.

    Oh, it's sunny today here in Minne-snow-ta. It is November-cool, but definitely a rideable day today.
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    Brad D. - Member #105766
    '77 R100RS - Black Beauty (big pipe, baby!)
    '94 R1100RS - Sylvia

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    I run Dunlop K70 front and rear 1975 R60/6 running R75/5 engine
    Usually 28-30psi never more than 30
    Well worn old somewhat neglected high mileage bike orig fork springs and rear shocks bit of a rocking horse
    Rock solid stable up to 90 mph hands/no hands -- I don't care to go faster

    Suspect instability may not be tire related

    Check that steering head bearings allow free movement but no ability to shake fore/aft when front wheel off ground

    Maybe wandering because of crusty old fork head grease

    Confirm both shocks on rear set to same load notch

    Confirm no wheel problems with loose spokes

    Confirm by spinning wheel and sighting along edge and again at right angles that your new tire mount is really true and
    all the way out on the bead area

    If you are running an old school tire - use the factory tire pressure <= 30psi or just suffer a harsher ride and quick tire wear in center

    If you continue to run a modern tire use whatever the majority may post here

  13. #13
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    Quote Originally Posted by 44006 View Post
    I run Dunlop K70 front and rear 1975 R60/6 running R75/5 engine
    Usually 28-30psi never more than 30
    Well worn old somewhat neglected high mileage bike orig fork springs and rear shocks bit of a rocking horse
    Rock solid stable up to 90 mph hands/no hands -- I don't care to go faster

    Suspect instability may not be tire related

    Check that steering head bearings allow free movement but no ability to shake fore/aft when front wheel off ground

    Maybe wandering because of crusty old fork head grease

    Confirm both shocks on rear set to same load notch

    Confirm no wheel problems with loose spokes

    Confirm by spinning wheel and sighting along edge and again at right angles that your new tire mount is really true and
    all the way out on the bead area

    If you are running an old school tire - use the factory tire pressure <= 30psi or just suffer a harsher ride and quick tire wear in center

    If you continue to run a modern tire use whatever the majority may post here
    What sort of tire life are you getting out of those K70's? How are they in the wet?

  14. #14
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    I run the 4.00x18 K70 on the rear of:
    R50/2
    R60/2
    R60/2/900cc Conversion
    R60/6 w 750 engine
    R75/5
    R65 1980

    I run the 3.25x19 K70 or rib tread on the front of:
    R60/6
    R75/5

    I am looking for a source for this tread style in 3.50x18 size for the front of the /2 bikes and for both wheels on /2 bikes with the higher rear end (25x8) rear ends. If anyone knows of a source please let me know - I used to be able to get this tread style in Pirelli and Cheng Shin and both now seem to be no longer available

    I ride SOLO - usually run the speed limit + maybe 10 on pavement and never over 90mph - not an aggressive rider other than I like to get into the curves a bit (not like the sport bike kids) but more so than most folks on a tour bike (Wing or Hog)

    Off pavement mileage between 5 and 10% performance fine unless clay mud

    Typical mileage combined 35-40k miles/year full year ride almost every day.
    I usually have 4 bikes up and running and licensed etc at any given time and one of the advantages of the K70 is that my spares on hand fit all bikes and mount/dismount easily with hand spoons and heal stomp bead break technique

    Have not bought anything other than the K70 or Cheng Shin same style for years except for using up tires that came on a new(used)bike purchase and I am convinced that the metric sized half worn continental tires on a /2 conversion nearly killed me in a 75mph interstate wobbleout crash a few years ago - for me - never again anything other than the same style tire the bike was designed and tested with.

    Typical mileage at usually 28psi is 7k-9k miles to weardown of center groove to twi marks.
    I sometimes run beyond that (not recommended) in Florida winter never rain and course blacktop. I never noticed any improvement by increased air pressure and just got harsher ride and faster tire wear in the center.

    I would describe these as rather soft rubber for vintage tires - good grip -not the hard rubber like used to be on the notorious "touring elite" tires for heavier bikes that slid in the rain almost like trying to ride in snow slush.

    The "right" tires for your bike is really a personal thing - what is convenient, affordable and feels good to YOU

  15. #15
    Curmudgeon nrpetersen's Avatar
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    OP report (so far):

    The rear shocks (sorry they are original) were set to different heights. Fixed.

    The tire pressure has been raised to 36 psi. Tonight we are supposed to get 4 " of snow so I may not have an answer until next spring, dang it. THX guys!
    Retired w 2005 K1200LT, 2000 R1100RT, & 1975 R90/6

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