Triumph tyre size

[jroberts]

I am replacing the tyres on my 68 triumph bonneville, the front tire is a 3.75 x 19 and the rear is a 4.25 x 18. the front tyre is larger than than the rear. This seems to go against modern common practiceof having a hugh honking big tire on the rear. My Hhaynes manual is silent on this topic. is this common practice or did i get a larger rim from another bike in the past. any help would b appreciated.. any leads for a tyre source would be helpful also. thanks

[Cass]

Dear jroberts,
After checking 3 of my reference books,1 by Bacon,1 by Nelson & 1 by Gaylin the following info was in all 3 books the front is a
3.00x 19, the rear is 3.50x18. The only exception I found in Gaylin's book stating that after engine # DU75452 the front was changed to a 3.25x19 Dunlop K70. Also to note the Dunlop's are now made in Japan & not England. British Cycle Supply carries these tires. A Matching tread design can be found in the JC Whitney Catalogue, but a different Company. Hope this helps & good luck.

[jroberts]

Cass, thank you for the info, I will check with the source you provided. I am restoring the bike from back to front. remving the rust, repaiting , replacing as needed.

I meet a gentleman from cape may and his lovely wife at Oley . He was driving a motor home and his spouse told me that he rebuilt bikes on the picinic table while in flordia. He had called my son and I over as I had a Cape May logo on my sweat shirt. Are you one and the same?

[Cass]

Thanks

Thanks for the compliments & we are one & the same. I remember you from Oley. If you ever get down this way you should stop in & see our collection. I think I lost count but were now up to 16 bikes & still growing. Will you be at White Rose? I could bring that list you wanted.

[jroberts]

I am not familiar with White Rose. Is that the Jefferson Pa show south of York PA , 5 and 6 October?

I would like to see your collection, especially if it of the same quality of the bike you had with you at Oley. regards

[Cass]

AMCA Meet

White Rose is the name of the club grounds where the meet is held just south of York Pa. It is the last meet of the year for AMCA.
We won't be bringing a bike this time, but I will be Judgeing on Sat. Hope to see you there?

[AFJ]

Re: Triumph tyre size

Originally posted by jroberts
I am replacing the tyres on my 68 triumph bonneville, the front tire is a 3.75 x 19 and the rear is a 4.25 x 18. the front tyre is larger than than the rear. This seems to go against modern common practiceof having a hugh honking big tire on the rear. My Hhaynes manual is silent on this topic. is this common practice or did i get a larger rim from another bike in the past. any help would b appreciated.. any leads for a tyre source would be helpful also. thanks

The standard home (British) market tires on the 1968 Triumph Bonneville were a ribbed 3.00 x 19 Dunlop on the front and a 3.50 x 18 Dunlop K70 on the back. West Coast US used a 3.25 x 19 ribbed Dunlop on the front, East Coast versions had a 3.50 x 19 Dunlop K70 on the front. All US Bonnevilles used a 4.00 x 18 Dunlop K70 on the rear.

The Dunlop K70 pattern is still made in 3.25 x 19 and 4.00 x 18. Widely used on British bikes of this period. They are now made in Japan.

Other sources of inch size tires are the Avon Speedmaster ribbed tires in 3.25 x 19 and 3.50 x 19 for the front and the Avon SM block style rear in 4.00 x 18. These are still made in Britain and are a nicely made quality tire which I have used and found quite satisfactory on my older bikes. If you want to go a bit upmarket I can also recommend the Metzeler Lasertec which can be fitted with tubes although they are a modern tubeless tire with a newer style tread. They are available in 3.25 x 19 and 4.00 x 18.

AFJ

[Britluv]

Yes, beginning in 1968, the K-70 was standard on Triumph 650 and 500 road models. HOWEVER, the K-70 was never meant to be used as a front tire.
It was never made to be "pushed" by the front forks, and thus has a problem with a wear pattern called "cupping" when so used.
Although it is the "correct" tire for 1968-on Triumphs, the bikes handle MUCH better with the tires they were designed for, the Dunlop or Avon "ribbed"-pattern tires. If you switch from a K-70 to a ribbed tire you will notice the difference.
As an AMCA judge, I will not penalize any bike that has made this change, as I believe the rib tire is the SAFER of the two types for the intended use on the front of a British road bike.