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Some help with tyres

Discussion in 'Technical and Troubleshooting Torque' at netrider.net.au started by Changa, Jun 28, 2010.

  1. G'day guys,

    I have a little 1995 Kawasaki EL250 that I'm learning on. Recently I found that my speedo was out by about 10 kph....meaning when doing 70, I was really doing 60.



    It appears it's due to the size of my front tyre - in that someone has put on a lower profile tyre and it's thrown the speedo out.

    Now....the handbook says the front is supposed to be 100/90 17. Trouble is, I've asked a few places and no-one can help. What I was hoping to find out is whether I could use a different profile tyre that would correct the speedo. I used to try a tyre calculator for my old cars which would help work our gearing etc, but there's nothing I can find for bikes.

    Can anyone help ?

    Peter
     
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  2. Well, a 110/80 would give you about the same rolling radius as a 100/90, but I've no idea whether it would be compatible with your rim as I don't know the rim width on the VL.

    Whilst an indicated 70 at a true 60 is a bit big, an awful lot of cars and bikes have about a 10% discrepancy, so you're probably not that far from standard. What size tyre is on there?
     
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  3. What size tyre is on the bike now?

    If you put the right size tyre on it should fix it. If it already is the right size tyre, then short os replacing the speedo, there's probably not much you can do...

    You might be able to get it calibrated...

    Most people would simply adjust their speed to suit.
     
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  4. Don't sweat it, just ride at what you know the speed should be. The stock speedo on my Kawasaki used to read 116 at a true 100. You adjust.

    Of course the other thing you could do, for not much more than a new tyre, could be to get an aftermarket digital dash, and tune it to whatever speed you want it to say.
     
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  5. or just whack a pushbike speedo somewhere so you can run both.
     
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  6. Thanks for the replies,

    I was in a hurry when I posted this and omitted the current tyre size on my bike. It's 90/80 17 at the moment. I tried riding with a GPS between my legs and found it was almost exactly 10 kays out across the board. The problem is that i have my P's test next month and really don't want speed OR go too slow.

    Peter
     
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  7. that tire has a lower profile because it dose'nt fit the rim... it is a lower profile tire but it's not supposed to be that low...it's splayed out to seal with the rim...you should replace it with the handbook specified size for your own safety... you"ll get a more accurate reading, still less than spot on, maybe 5k under (just guessing, no speedos are accurate from manufacturer)... but that tire is going to wear very badly and the more it wears, the worse it gets.
    replace with correct size and bike will handle a lot better... best check the rear size too.
    you can get speedo healers, but if the reading is under true speed then no big deal.
     
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  8. hmmm...
    • 90 / 80 is a wall height of 72mm
    • 100 / 90 is a wall height of 90mm
    So thats 18 mm wall height difference.
    Which means the diameter is 36mm less than handbook recommendations.

    Adding this to a 17 inch (432mm)rim....

    Distance travelled for one revolution of the wheel
    Handbook (100 / 90 tyre)
    Total diameter: 432 + (90 * 2) = 612
    Circumference: Pi x D = 1,922mm

    Actual (90 / 80 tyre)
    Total diameter: 432 + (72 * 2) = 576
    Circumference: Pi x D = 1,810mm

    Using proportions...
    1922 / 1810 = 1.062
    So you would theoretically gain 6% accuracy by changing to the recommended tyre size. It's still not the missing 10kph you want.
     
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  9. it's the width of the rim that's the concern.
    picture a cut out profile of the 90/80 tire when it's inflated so that it beads to a rim that takes a 100/90 or 100/80 tire....it's not supposed to be shaped like that...it's dangerous and the bike won't tip in safely when you enter a bend.
     
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  10. I agree it's not ideal, but it's a 250 cruiser and the guy's not dead yet so I think you may be overthinking it :wink:.

    I would, however, recommend going to either the correct size tyre or, if that's not available, at least something slightly less spectacularly undersized. Somewhere on the rim, I would expect to find a rim width stamped into the alloy. Given that info, it should be possible to pick a tyre size from this chart.

    17in Tyres Permitted Rim
    225*17 1.20/1.40/1.60
    60/100*17 1.20/1.40/1.60
    70/100*17 1.20/1.40/1.60

    250*17 1.40/1.60
    275*17 1.40/1.60/1.85

    300*17 1.60/1.85/2.15
    80/100*17

    325*17 1.85/2.15/2.50
    350*17
    90/80*17
    90/90*17
    90/100*17

    425*17 2.15/2.50/2.75
    450*17
    460*17
    100/100*17
    100/90*17
    100/80*17
    110/90*17

    120/80*17 2.50/2.75
    510*17

    100/70*17 2.50/2.75/3.00
    110/80*17
    120/90*17

    130/80*17 2.50/2.75/3.00/3.50
    130/90*17

    140/80*17 2.75/3.00/3.50
    140/70*17

    120/60*17 3.00/3.50
    120/70*17

    130/60*17 3.00/3.50/4.00
    130/70*17

    140/70*17 3.50/4.00/4.50
    150/60*17
    150/70*17

    160/60*17 4.00/4.50/5.00
    160/70*17

    170/60*17 4.00/4.50/5.00/5.50

    180/55*17 5.25/5.50/5.75

    Edit: I've just had a bit of a search and it appears that the front rim on the EL is a 2.15. If that's the case, a 90/80 is an acceptable fitment for the rim. Whether it's suitable for the bike is another matter.

    According to the chart, that would let you fit a 110/80-17 or a 3.50-17, either of which will put you within a smidge of the rolling radius of the OEM fitment. The 3.50 would be physically narrower which could be useful if clearance is tight anywhere, but is likely to be an older design of tyre. A quick Google shows a few manufacturers offering 110/80s so that would probably be your best bet, assuming it goes in physically.

    A 110/90 will also go on your rim but will be a touch larger diameter than original and may start to run into the mudguard. It would also make your speedo read about 3% slower than original, which may or may not be OK, dependent on how much leeway Kawasaki built into it.

    Something that hasn't been mentioned above, though, is that tyre rolling radius changes quite measurably with wear. For instance, the front semi-knobbly on my DR gives me a difference of about 2% on the speedo between new and bald. On that basis, it's really not worth chasing errors of <3%.
     
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  11. G'day guys,

    firstly, thanks a lot for the replies, especially the length of the replies - much appreciated.

    Well......I have found that if i want the correct 100/90-17 I have to wait a few weeks because they are on back order.

    Now....yesterday I went for a ride into the City from Campbelltown. I strapped the GPS on the fuel tank using duct tape to see exactly what kind of speeds I was doing. It looks like the following :

    to do 60 kph I must do 70 on the speedo.
    to do 70 kph I must do 80 - 85 on the speedo.
    to do 80 kph I must do around 95 kph.

    No wonder people hated me on the M5 a couple of weeks back. I feel it must be the ratio of the tyre i'm using. I know "spenze" said it shouldn't be that much, but it is. I figure I'll get the tyre changed, and test it again, and then it'll possibly be a case of changing the speedo.

    Peter
     
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  12. I've got a question on tyres. I've recently noticed that are some cracks / dry rot on my types that are about 3-4 cm in length but not deep. Its on the tyres and not on the side. Is this a major issue? Does it mean that I have to change my tyres ASAP? I bought a second-hand bike and wasnt really paying attention during inspection *learnt my lesson*
     
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  13. It sounds like the tyres might be fairly old. They get these cracks in them with age. It seems to signal a drop in elasticity which also has handling implications as the rubber will be harder.

    It doesn't sound like it's a dire issue in your case, depending on how cracked they are. But personally I'd change them for some newer ones. I did that on an old bike and the difference was astounding, especially in the wet. Made it feel like the old ones were made of wood. I had way more confidence in the wet and when cornering.

    What sort of bike you riding?
     
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  14. Yeah, they could be. However, I've noticed that the tires are stock ones. I think they're Bridgestone BT45. Anyone know how much they cost?

    an 04' Honda VTR250. I have yet to do alot of riding (50+ km so far?), yet to experience riding in wet as I dont think I have the confidence yet or go on the freeway. Just doing some inner suburbs and city riding to build up my confidence.
     
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  15. Okay, I would recommend changing them. The VTR would like it :) Depending on how many kays the VTR has on it, yes they may be the original ones and now over 6 years old. This would make sense given what you describe.

    How many kays on the VTR and how many mm tread depth is left?

    You could get an experienced NR to have a quick look over them if you attend any rides or learner days (or post up some pics) just to double check that we're on the same page and let you know how serious it's likely to be.
     
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  16. If they're cracked, regardless of age, I'd be looking at changing them. And that's from a rider who's currently got 10 year old boots on his daily ride and finds them perfectly OK, although they were a bit......erm..... stiff to get onto the rims :shock:.
     
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