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Thread: 110 or 120 rear tire on a R75/5?

  1. #16
    Rally Rat
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    Quote Originally Posted by lkchris View Post
    They're on Continental's website, of course.

    The front tire follows rains grooves really badly.
    Or is that really well? Conti's RB2 and K112 never were any good.

  2. #17
    Grammarian no, Rider yes ISAMEMON's Avatar
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    years back I wanted to get as much ground contact as I could for my 1978 R80/7
    so I ran a 120 on the rear. regardless of the issues of mounting and dismount in the frame ( on the rim) I discovered another intersting tidbit.

    I then bought a 85r80rt and on the back of that put the same tire, a 120 metzler
    then when the bikes were side by side I noticed that the 120 on the 80/7 looked smaller than that on the Rt.
    sure enough, pulling out a micrometer, the tire on the 80/7 was so curled to fit into the narrowewr rim, it actually made the ground contact smaller than the same tire on a wider (RT) rim

    so I dont think that going to a 120 on a norrow rim does any good

  3. #18
    Registered User j_withers's Avatar
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    Quote Originally Posted by 42906 View Post
    Your bike doesn't make enough power to justify a wide rear tire. Superbikes have wide rear tires so the rider can apply power while still leaned over at angles your bike will never attain. Early application of power, given sufficient traction, improves lap times and overall race results. If you are not spinning the rear tire under power when leaned over, your tire is sufficiently wide as it is.
    So based on this theory, a wide tire is just extra material or overkill?

    Even though you are making road contact about the width of a credit card, I would think that an extra inch or so of contact could only help the bike's stability.

    Compare those thin road-race bicycle tires to a thicker beach cruiser style tire. Of course they are meant for completely different styles of riding but I can say that a thin tire sure is squirrly.

    I like the idea that the bike will turn better with thinner tires but will the bike become less stable or 'looser' as a result?

    I know this bike will never reach superbike/crotch rocket leaning turns but it will have many performance upgrades (suspension, siebenrock piston kit, lots of weight reduction, etc) that will make this not your stock R75.

  4. #19
    Administrator 20774's Avatar
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    Not to mention that the bigger tire has more unsprung mass which is a component of handling and performance.
    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!

  5. #20
    Registered User j_withers's Avatar
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    For those still interested in this topic, I'm having another look at the available BT45 sizes and it seems that the options are:

    Front:
    3.25-19 H
    100/90-19 H or V

    Rear
    4.00-18 H
    110/80-18 H
    110/90-18 H
    120/80-18 H
    120/90-18 V

  6. #21
    Administrator 20774's Avatar
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    I think you should stay away from the 80 profile tire...that means that the height is 80% of the width...too low IMO. If you start dropping the bike down, the sidestand and centerstand function becomes compromised.
    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!

  7. #22
    Themason 42906's Avatar
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    Quote Originally Posted by j_withers View Post
    So based on this theory, a wide tire is just extra material or overkill?

    Even though you are making road contact about the width of a credit card, I would think that an extra inch or so of contact could only help the bike's stability.

    Compare those thin road-race bicycle tires to a thicker beach cruiser style tire. Of course they are meant for completely different styles of riding but I can say that a thin tire sure is squirrly.

    I like the idea that the bike will turn better with thinner tires but will the bike become less stable or 'looser' as a result?

    I know this bike will never reach superbike/crotch rocket leaning turns but it will have many performance upgrades (suspension, siebenrock piston kit, lots of weight reduction, etc) that will make this not your stock R75.
    I run the narrowest tire that won't slide under power. Wider rear tires make the bike respond slower and require more effort at the bars to turn in. That might be an acceptable trade for a race bike putting over 200 horses to the ground wile the rider's knee skims the ground, you need the big contact patch to control that power and the rider is an athlete, but on public roads, say playing in second gear on some tight little squiggle in the mountains, you never have the opportunity to use big power while cranked over. It is all about agility and the ability to transition quickly.

    Stability is determined by the relationship between rake and trail. More trail buys you stability, the rear tire contributes only insofar as the sidewall isn't flexing and moving you around. As you bank the bike over, the rear contact patch moves towards the inside of the turn, and the more it has to move the more pressure you have to apply to the bars. Narrow rear tires don't move the contact patch as far inward as with wide tires and they transition easier. That is a choice for street riding. On a track at track speeds, I might make a different choice, but consider that most race bikes take Turn 8 at Willow Springs big track at higher speeds than even my modified K-100Rs is capable of with miles of straight road ahead of it. What am I trying to prove with a mondo wide rear tire?

    On my 2007 Street Rod I ditched the stock wheels, 19X3.5 front and 18X5.5 rear for wheels from an XR-1200, 18X3.5 front and 17X5.5 rear to access a wider selection of tires. Even though both rear tires were 180/55 profile, the 17 incher on there now is about 6 or 7 mm narrower than the 18 and it turns in beautifully. It is also dead neutral, meaning once banked over in a sweeper I can put some friction on the thottle lock (stock HD throttle locks are a thing of beauty), take my hands off the bars and the bike will track a corner perfectly. My old K-100RS used to be equally neutral on the stock wheels with Dunlop D205 radials, but once I could no longer find decent radials for those narrow old wheels, I converted the bike to Paralever so I could ues later model wheels. Now, with a 160/60ZR18 on the rear the bike has lost agility it once had, but I can buy radials that fit those wheels. I can live with it I reckon, but the old set up with a 140/80ZR17 rear was more fun on tight roads. My BMW tech friend like to put a 130/70V17 tire on the rear of K-75s to maximize agility.

    I had even considered mounting a 160/60ZR18 on the Street Rods original rear rim (you have no idea the difficulty fitting a Sportster style wheel to a V-Rod, getting pulleys, gearing, spacing and belt lengths worked out was a minor engineering adventure as nothing swaps between V-Rods and any other bike made) but the 5.5 rim width is just too wide for a 160 tire.
    Experience is what you get when you don't get what you want.

  8. #23
    Grammarian no, Rider yes ISAMEMON's Avatar
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    narrow tires
    like a bicycle, are easier to turn and pedal
    so the theroy above should be good
    but im an old fart, I dont road race, my bike is not a crotch rocket, I dont pedal this old girl

    so for me, an old fart in a old mind set, I want as much ground contact as I can get
    shucks, I only got two wheels on the ground at 70mph
    right or wrong. I will stick with fatties

  9. #24
    Themason 42906's Avatar
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    Quote Originally Posted by Isamemon View Post
    I will stick with fatties

    Don't let it get around. Next thing will be scooters. lol. I hear they are a lot like fatties, fun but don't let your friends see you.
    Experience is what you get when you don't get what you want.

  10. #25
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    Quote Originally Posted by lostboy View Post
    Or is that really well? Conti's RB2 and K112 never were any good.
    +1! Horrid tires!

    As to original poster, if I had to go metric, I would go 110/90 rear, 100/90 front. Although that rear does make bike sit rather high on sidestand, tire change way easier, no rubbing on swingarm

  11. #26
    RPG
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    Avon RoadRider 3.25 x19 and 4.00 x 18 are all the tire you'll need for that bike. They will fit and the bike will handle wet and dry just fine.

    have them on my R90s. Did a 2000 mile Blue Ridge Parkway trip in the spring, followed up by a May Track Day. Three days on the BRP was in pouring rain and had zero issues. And at the track, the bike was pushed pretty hard with no issues either.

    Actually thought of wider tires too, but I was so impressed with the performance, that I'm leaving well enough alone.

    RPG

  12. #27
    Registered User rapz's Avatar
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    I put Metzler stock OEM size on my '79 and I look over at my brother's RT and wonder about it every now and then, but I have no problems with my riding style.
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    Current Bike 1979 BMW 100RT; 2013 BMW R1200RT 90th Anniversary Edition; 2008 Harley Davidson Ultra Classic 105th Anniversary Edition

  13. #28
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    Conti Twin tires

    Quote Originally Posted by j_withers View Post
    I've been real curious about those tires since i read the article on them. What is the tread pattern like? Do you have any photos?
    Althought they say that these tires were created for the BMW twins, you have to keep in mind that we're talking about 70s technology.
    So if you're doing a restoration and want period correct tires, then go ahead and get the Contis. If you're going to ride the bike, then I'd much rather have a modern tire that will last longer, stick to the road better, be much more competent in the rain, and not get as squirrelly on metal grate bridges and rain grooves.
    Just a thought!
    Boxerbruce

  14. #29
    beemerfield
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    Anyone had any experience with Heidenau? I have not really looked at them and assume they make a size that will work. If I can get my /5 to be reliable, I will start looking into some new tires.
    beemerfield

  15. #30
    Registered User kwb210's Avatar
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    Avon AM26

    I have a 1977 R100, ride it 10,000 miles a year, all touring and loaded. I have tried a few differnet brands. I come back to the Avon AM26. last spring I needed a tire sooner than I had planned, returning from Mexico to WA state, so I stopped at a BMW dealer who had a 18 inch tire in stock, you take what they have! It was a Metzler ME88 and I don't care for it, to hard a compound, I would be willing to bet it will last forever but if you like to run a bit hot in corners it is simply to hard and slips or slides a bit on corners, I don't like that! The Avon is probably softer, I still get about 7.500 on a rear if I watch the tire pressure, WAY less if it is under inflated. I check it cold each morning while on a ride, after a couple of days it needs a couple of pounds. But the Avon allows me to run a bit faster in corners. Sometimes a faster corner is planned and then there is the time that you are entering to fast, that is when a softer compound works for me. I run 32 in the front and 38 in the rear, remember, that is a loaded bike. I mistakenly ran a 90/90/19 on the front instead of the 3.25x19, still got about 10,000 miles out of it and never felt a difference. I just changed it out for a 3.25x19 (avon am26) since I am planning on one more 3,500 mile run before the riding season comes to an end, remember, I am just north of Seattle, WA. My comments are simply what works for me and my riding style on a bike that really gets out there. I also have a 1972 slash 5 Toaster Tank, it has 6 year old Metzlers, they work fine on the club rides the bike runs. Anything would be fine, the bike just happened to come with them and at maybe a 1,000 miles a year it does not matter to me.
    Kurt

    PS: I just realized I need to update my signature line to include the 72 Toaster
    1977 R100/7 1971.1972.1972.1973 R75/5
    1974 R90/6 multiple boxes
    Airhead Revival
    "Objects in the mirror appear to be losing" unk

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