71 T100 tire size / brand - Triumph Forum: Triumph Rat Motorcycle Forums
Classic, Vintage & Veteran For Coventry and Meriden Models. Anything pre-Hinckley goes.

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post #1 of 8 (permalink) Old 10-16-2019, 10:53 PM Thread Starter
Grand Prix 125
Main Motorcycle: 1971ish Triumph t100
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71 T100 tire size / brand

So yes. I am not a tire expert and I don’t think I ever will be. I’ve learned loads about my t100 but really only have scratched the surface.

I’ve read other posts (and searched) about the modern tire (tyre) being bigger than 50 years ago— but I just want to pull the trigger and order tires without thinking too much about them. (The easy way out? Maybe.. but that’s why I’m here to glean info!)

Presently on my 71 I have a very rotten rear:
110/90 - 18 Dunlop
And front
3.25/85 h19

What should I order?

I’ve seen Avon being a popular choice. Not married to the tire, just heard good things.

I'd get it one piece at a time
and it wouldn't cost me a dime
you know it's me when I come through your town
I'm gonna ride around in style
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post #2 of 8 (permalink) Old 10-17-2019, 07:59 AM
Grand Prix 125
Main Motorcycle: 1969 Daytona T100T
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My 69 Daytona had oldish Avon Roadriders on when I bought it. The front was a 3.25 x19 - which is just about OK, but the rear tyre was a 4.0 x18 and way oversize for the standard WM2 rim size fitted front and back to UK bikes. (Check recommended minimum rim size on the Avon website). This oversized rear tyre caused seriously skittish handling when leant over. As your bike is a US export model it will probably have a WM2 rim on the front and may have the larger WM3 rear rim, so could take up to a 4.0 x18 rear tyre which gives you a greater choice of makes of tyre.

I have settled on a happy combination of Dunlop K82 3.5 x18 rear and Dunlop TT100 3.6 x 19 front as Dunlop don't do a K82 tyre in 19" diameter. Fitting a matching TT100 in 4.1 x18 is too wide for the rear rim, but the tread patterns are the same on both types of tyre and Dunlop technical service confirm that both are OK fitted on WM2 rims. Although the 3.6 x 19 sounds too wide it's actually narrower than the 3.25 Avon that came off. Diameters are very close to the original tyres but slightly smaller than the Avons. The rubber hardness with the Dunlops is a much softer compound than the Avons. The handling is really solid now.

Last edited by BrianG; 10-17-2019 at 08:04 AM.
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post #3 of 8 (permalink) Old 10-17-2019, 01:33 PM
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Hi,

Quote:
Originally Posted by copentx View Post
Iíve read other posts (and searched) about the modern tire (tyre) being bigger than 50 years ago
Depends ...

Overall diameter
... any 'modern' tyre should not be a greater overall diameter than the same 'size' fifty years ago; e.g.:-

. pre-'71 for many years and '71-'77, US-market twins were fitted with 4.00x18 Dunlop K70 rear and the front 3.25x19 was also either a K70 or a good match;

. if you go to US Dunlop's K70 webpage and scroll down to "SPEC CHART", it shows the 4.00x18 K70 is 26.73" OD and the 3.25x19 is 26.58" OD;

. love the precision to hundredths of an inch, but, in reality, they're conversions from millimetres - 679 mm. and 675 mm. respectively;

. then go to Avon's Roadrider webpage, click on "Show available sizes >", it shows the 4.00x18 Roadrider is 671 mm. OD and the 3.25x19 is 664 mm. OD;

. so hardly a "bigger" OD ...

Otoh ...

Overall width
. US Dunlop's K70 webpage shows the 4.00x18 K70 is 4.17" (106 mm.) wide and the 3.25x19 is 3.3" (84 mm.) wide;

. Avon's Roadrider webpage shows the 4.00x18 Roadrider is 116 mm. wide and the 3.25x19 is 98 mm. wide ...

Quote:
Originally Posted by copentx View Post
I just want to pull the trigger and order tires without thinking too much about them.
Presently on my 71 I have a very rotten rear:
110/90 - 18 Dunlop
And front
3.25/85 h19
One problem here is you haven't given enough information about one tyre ("110/90 - 18 Dunlop" what?) and I've never heard of a "3.25/85" - what make and range is it?

The other problem is Britbikes don't have lot of spare space around the wheels - you could order 110/90x18 or 4.00x18 and 3.25x19 Roadriders but find they don't fit because of the greater overall width. You could order the sizes in a different make, but modern tyres' overall dimensions tend to be much of a muchness, if another maker's overall width isn't even greater ... Not so much of a problem if your tyre supplier is local and doesn't mind returning tyres that don't fit to his supplier? More of a problem if you order 'em over the www?

Quote:
Originally Posted by copentx View Post
I just want to pull the trigger and order tires without thinking too much about them.
Then order K70's?

Quote:
Originally Posted by BrianG View Post
have settled on a happy combination of Dunlop K82 3.5 x18 rear and Dunlop TT100 3.6 x 19 front
Mmmm ... unfortunately, afaict US Dunlop don't sell either K82's or the 3.60x19 K81/TT100 ...

Hth.

Regards,

Stuart
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post #4 of 8 (permalink) Old 10-17-2019, 03:20 PM Thread Starter
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The front is a Shinko if I remember correctly. (Sorry for the brace blocking the “2” of the 3.25)

Rear is the Dunlop.
I can look closer at the tire type of the Dunlop.
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I'd get it one piece at a time
and it wouldn't cost me a dime
you know it's me when I come through your town
I'm gonna ride around in style
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post #5 of 8 (permalink) Old 10-18-2019, 05:17 AM
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Hi,

Quote:
Originally Posted by copentx View Post
The front is a Shinko if I remember correctly. (Sorry for the brace blocking the ď2Ē of the 3.25)
No worries. That's a weird one ... To explain ...

The first number moulded on a tyre sidewall is known as the "section width" (note, not the overall width). By-and-large, a section width divisible exactly by 0.25 is "100% aspect ratio" = the "section height" is the same as the "section width".

When Dunlop developed the K81 (aka TT100) in the late 1960's, they wanted to make a wider rear tyre but without the sidewall control problems of a taller tyre. Hence, if not today, certainly for a long time, 4.10-width K81's had "REPLACES 3.50" moulded on the sidewall - i.e. they had the same section height as a 3.50 (so the tyre's overall diameter wasn't any greater) but they had a greater section width ("4.10" vs. "3.50") and therefore greater overall width.

However, as Dunlop expanded the K81/TT100 range, there was one section width divisible exactly by 0.25 that had a lower section height; to differentiate them, Dunlop moulded "4.25/85" on the sidewalls = like most modern tyres (motorcycle, car, truck) the figure after the "/" is the section height percentage (85% in this case) of the section width.

So, whereas the 3.25x19 fitted originally to the front of your Daytona had both a section width and section height of "3.25" (inches), while the Shinko theoretically has the same section width, its section height is only 85% - 2.76"? - making for a tyre/wheel with a smaller overall diameter.

Quote:
Originally Posted by copentx View Post
Rear is the Dunlop.
I can look closer at the tire type of the Dunlop.
An educated guess says the 110/90 likely has a slightly smaller OD than the original 4.00, it could be a PO wanted a slightly smaller-OD front too? Or it could be nothing more than, when the PO wanted new tyres, that's what the tyre supplier had that looked about right ...

Hth.

Regards,
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Stuart
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post #6 of 8 (permalink) Old 10-18-2019, 03:40 PM
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The best tire set up I had on my '79 was Dunlop k81's with a 4.10 19 front and 4.25/85 18 rear. Worked really well. Dunlop still makes them. If memory serves I believe I had to cut a small relief in the chain guard but no problem with chain clearance.
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post #7 of 8 (permalink) Old 10-20-2019, 01:38 PM
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Quote:
Originally Posted by copentx View Post
So yes. I am not a tire expert and I donít think I ever will be. Iíve learned loads about my t100 but really only have scratched the surface.

Iíve read other posts (and searched) about the modern tire (tyre) being bigger than 50 years agoó but I just want to pull the trigger and order tires without thinking too much about them. (The easy way out? Maybe.. but thatís why Iím here to glean info!)

Presently on my 71 I have a very rotten rear:
110/90 - 18 Dunlop
And front
3.25/85 h19

What should I order?

Iíve seen Avon being a popular choice. Not married to the tire, just heard good things.

Hello Copentx
I used to have a 1971 Daytona 500 and you might like installing the K70 tires that came originally with the bike if you want. They are available from Dennis Kirk here is a link.

Juan

https://www.denniskirk.com/dunlop/k70-tire.pfp6510.prdf


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post #8 of 8 (permalink) Old 10-20-2019, 02:36 PM
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Quite happy with my K70's on my T100C......little xtra grip for going in my fields too.......

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'72 Bonneville T120V (above), '71 Trophy 500 T100C, '74 Trident T150V, '73 T150V (sold)
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