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tyres sizes #315494
05/27/10 2:30 pm
05/27/10 2:30 pm
Joined: Feb 2010
Posts: 24
Belgium
T
thundebird64 Offline OP
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Joined: Feb 2010
Posts: 24
Belgium
Hello,

i'm looking for a new set of tyres .
I now have dunlop K81 tyres .
the rear tyre is marked 4.25/85 H18(110/90 H18)
on the front tyre it is marked 4.10 H18.
Reading on this forum I have decided to replace them by avon AM26s

can i just replace them by

AVON ROADRIDER AM26
100/90 -18 TL 56 V (front wheel)
110/90 -18 61 V (rear wheel)
?

the strange thing is when i look in the parts catalogue of my triumph the standard wheel sizes are :
3.25x18 (front).
3.5 x18 (rear).
whats the metric equivalent ?
should i go for the standard sizes or stick with the oversized ones ?
I have to be completly sure they will fit because i'm ordering them on the net..
thanks for your collective wisdom bigt




thunderbird64
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Re: tyres sizes [Re: thundebird64] #315496
05/27/10 2:45 pm
05/27/10 2:45 pm
Joined: Dec 2006
Posts: 3,895
Massachusetts, U.S.A.
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btour Offline
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btour  Offline
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What is wrong with those?


Bob, Lifetime bike: '71 T120R, bought in '71 at Ken Heanes, England.
Re: tyres sizes [Re: btour] #315497
05/27/10 2:53 pm
05/27/10 2:53 pm
Joined: Dec 2002
Posts: 3,252
arkansas
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leon bee Offline
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arkansas
Buy the inch size AM26. 325-19 and 400-18.

Re: tyres sizes [Re: leon bee] #315503
05/27/10 3:17 pm
05/27/10 3:17 pm
Joined: Feb 2010
Posts: 24
Belgium
T
thundebird64 Offline OP
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thundebird64  Offline OP
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Joined: Feb 2010
Posts: 24
Belgium
@btour :
these have been the same tyres since 1980.
the rubber has become hard and show signs of cracks .
I'm afraid the'll burst when i'm driving them on the highway at 90 miles /hour
@leon bee.
i agree on the 400-18.
but isn't it a problem that the front tyre is 19 inch (325-19) instead of 18 " (325-18) ?


thunderbird64
Re: tyres sizes [Re: thundebird64] #315506
05/27/10 3:27 pm
05/27/10 3:27 pm
Joined: Dec 2006
Posts: 3,895
Massachusetts, U.S.A.
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btour Offline
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Massachusetts, U.S.A.
I understand your concern. smile They have tubes, right? I do see cracks where the larger tread rubber attaches. None is the sidewalls, though. Newer, softer rubber will undoubtly give you better grip.

They just do not make tires like they used to though. The rubber is not as good, and the side walls softer, depending upon tire pressure to do its job.

I just had a conversation with a guy, who bought some new sidewall design that stands up under a blow out. And he had a blow out, and was glad he bought those. I can not remember what he called it. I wonder if they are available in our sizes.

A blowout with the new wimpy sidewall tires, I think, would result in dire consequences.

Just saying.


Bob, Lifetime bike: '71 T120R, bought in '71 at Ken Heanes, England.
Re: tyres sizes [Re: btour] #315527
05/27/10 5:22 pm
05/27/10 5:22 pm
Joined: Oct 2003
Posts: 11,221
North Georgia, USA
RF Whatley Offline
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RF Whatley  Offline
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North Georgia, USA
There's the standard (old school, full height) tire sizes you have listed in your manual.

There's the metric tire sizes, which are usually 90% of the full height tires.

Then there is the K81 size. K81 4.10 equated to a standard ~3.50, K81 4.25 equates to a ~4.00 size.


You have several issues before you....
18" front tires are hard to find. You may have to take what you can find. Start with the Avon catalog. They simply offer more in that size.
Low profile "metric" tires are tiny compared to the standard old K70 size. This can throw off your ground clearance, and speedo readings. Take particular care to measure the circumference and inflated height of your tire choices.

bigt


Don't hide 'em, Ride 'em !!

RF Whatley
Cornelia, GA
Re: tyres sizes [Re: RF Whatley] #315583
05/27/10 11:12 pm
05/27/10 11:12 pm
Joined: Feb 2010
Posts: 24
Belgium
T
thundebird64 Offline OP
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Joined: Feb 2010
Posts: 24
Belgium
found this :
avon tyre sizes

according to this conversion table (combined with the info out of my parts catalog ):
I should be able to fit

Roadrider AM26 (front) 90/90-18 51V

on my front wheel
and

Roadrider AM26 (universal) 110/90-18 61V
on my rear wheel

right ?

does these work with my old inner tubes ?


thunderbird64
Re: tyres sizes [Re: thundebird64] #315594
05/28/10 12:09 am
05/28/10 12:09 am
Joined: Jun 2002
Posts: 9,957
Scotland
S
Stuart Online content
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Stuart  Online Content
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Posts: 9,957
Scotland
Hi,

Originally Posted By: thundebird64
the strange thing is when i look in the parts catalogue of my triumph the standard wheel sizes are :
3.25x18 (front).
3.5 x18 (rear).

Not strange at all, these were the standard sizes on most non-US-market Triumph twins.

Originally Posted By: thundebird64
whats the metric equivalent ?

3.50x18 is relatively easy - 100/90x18 is close.

3.25 is difficult - 90/90 will have a similar width but a noticeably-smaller overall diameter, 100/90 will have a similar overall diameter but be noticeably wider. On balance, I'd probably go for 100/90 but the choice is yours.

Originally Posted By: leon bee
Buy the inch size AM26. 325-19 and 400-18.

Only if you rebuild the wheels.

Originally Posted By: thundebird64
i agree on the 400-18.

Don't. The 4.00x18 has both a much larger overall diameter and width than either the original 3.50x18, a 100/90x18 or the existing 4.25/85x18. If you go for 4.00x18 on the rear, you also need the 3.25x19 at the front because they are similar overall diameters.

Originally Posted By: btour
They just do not make tires like they used to though. The rubber is not as good, and the side walls softer, depending upon tire pressure to do its job.

On the contrary, the rubber is much better. What you're talking about here is fitting 'V'-rated (up to 150mph/240kph) tyres to bikes that would've been fitted originally with 'S'-rated (up to 112mph/180kph) tyres.

As for softer sidewalls, they give you a better ride. If you want stiffer sidewalls, use Dunlops; they still recommend 1970's tyre pressures.

Originally Posted By: btour
some new sidewall design that stands up under a blow out.

Hardly new - iirc, Dunlop Denovo(?) car tyres came out in the 1970's. Might be ok after a blow-out but you'd have endured tens of thousands of miles of pi$$-poor ride - more comfortable in a tank! frown

Originally Posted By: btour
A blowout with the new wimpy sidewall tires, I think,

Having actually experienced a small number of blowouts on bikes (including one front), the stiffness or otherwise of standard bike tyre sidewalls makes not one iota of difference, they are still one of the best natural laxatives known to Man. If you're really that concerned about what might happen after a blowout, fit a couple of security bolts to each wheel.

Originally Posted By: RF Whatley
There's the metric tire sizes, which are usually 90% of the full height tires.

Too much of an over-simplification, imho; as I say, the 3.25-section (in either 18" or 19") does not have any easy metric equivalent.

Originally Posted By: RF Whatley
Then there is the K81 size. K81 4.10 equated to a standard ~3.50, K81 4.25 equates to a ~4.00 size.

K81 are simply Imperial-sized 90%-aspect-ratio tyres, original Avon Roadrunners are the same. K81 4.10(x19) is a good equivalent of 100%-aspect-ratio 3.50 because Dunlop had the late 1960's production racers' favourite, the Avon GP 3.50x19, in mind when they designed it.

Otoh, the metric equivalent of 4.25(/85x18) is 110/90x18. 4.00x18 has a similar overall diameter to 120/90x18 but is thinner overall, but not as thin as 110/90x18 - more like '115-width'. wink

Originally Posted By: RF Whatley
18" front tires are hard to find.

True.

Originally Posted By: RF Whatley
Low profile "metric" tires are tiny compared to the standard old K70 size. This can throw off your ground clearance,

As I say, 90/90x18 is a noticeably-smaller overall diameter than 3.25x18 (think daylight between mudguard and tyre) but on a twin doesn't affect the cornering clearance unless you're regularly scraping the bottom curve of the exhausts. Otoh, 100/90x18 is very similar overall diameter to 3.50x18.

Originally Posted By: RF Whatley
and speedo readings.

Avon 4.00x18 is 795 turns per mile, 100/90x18 is 834 turns per mile (about 5% difference) - given Triumph fitted the same speedo. drive to both US- and non-US-market bikes ... wink

Hth.

Regards,

Re: tyres sizes [Re: Stuart] #315618
05/28/10 2:34 am
05/28/10 2:34 am
Joined: Dec 2002
Posts: 3,252
arkansas
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leon bee Offline
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arkansas
I didn't pay close enough attention before I posted.

Re: tyres sizes [Re: leon bee] #315636
05/28/10 7:03 am
05/28/10 7:03 am
Joined: Oct 2007
Posts: 1,311
Nanaimo, B.C. Canada
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Wilfred Offline
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Wilfred  Offline
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Posts: 1,311
Nanaimo, B.C. Canada
I recently bought a pair of roadriders for my T100. It was a general export T100S when it started out life and subsequently has a WM2 18 on the front and rear. I got 90/90 for the front and 100/90 for the rear. Both tyres are a little narrower than the same size numbers roadrunners that came off the bike and are larger in diameter. I think they should be fine. While the front is indeed a front tyre, the rear is able to be mounted on front or rear with appropriate reference to the directional arrows. The extra diameter will, of course, affect the centre stand so I might need to add a bit to the feet to get the rear wheel off the ground to facilitate easy chain lubricating. All the tyres I have bought recently have been at the dealer so I can be sure that the date is recent. Mine were only a few months old and the dealer had a 20 % off sale on all tyres so they ended up costing a lot less than the roadrunners were years ago. Your original thoughts on sizes seems to make sense to me. I think it's best that you buy new tubes as well. Old tubes are only good for making big rubber bands and slingshots. Cheers, Wilf.


"It's about the ride..."

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