Britbike forum Forums British Motorcycle specific Brands Triumph bulletin board Best tyres for nervous handling 1969 TR6C....

Hello
I have a 1969 TR6C fitted with new Dunlop K70 tyres (3.50-19 57P front & 4.00-18 64S rear). The bike is really nervous particularly on rough worn tarmac. I have fitted new swinging arm bushes, checked the alignment, balanced the wheels and made sure that they are running true, adjusted the head bearings and changed the fluid in the forks, checked wheel bearings, tried different tyre pressures and varied the steering damper but nothing seems to really make big difference. Running higher tyre pressures might have improved things a bit but the bike doesn't inspire confidence.
I was thinking of changing tyres (K81 TT100s) although I am a bit reluctant to do this as the K70 tyres only have about 600 miles on them so it could be just an expensive experiment. Several people think that the 3.50 section tyre on the front is a too big.
Does anyone have any ideas or suggestions as what to try next? Which tyres do you recommend and what pressures do you run?
Thanks CAB

Avon Road Riders. The bike falls into the corner and doesn't have to be pushed. And you have traction in the rain. Recommend highly.

Thanks
What pressure do you run?
I was thinking of putting Road Riders onto my T120R but I would like something a bit chunkier on the TR6C (as long as they work of course).
CAB

It's hard to tell from your post. Do you have K70's fitted, or K81's?

I don't know if it matters to you, but modern tires are a lot better than the new production vintage styles by a long shot. Especially in the wet.

I was curious if you have another bike to compare the Triumph handling to. The Triumph is nothing if not nimble and that goes for nearly any year. It was one of the characteristics that make them so appealing to a lot of riders. When I'm tossing a bike through the twisties, I'd rather be on a Triumph than nearly anything and that includes modern bikes, too. There's something so predictable and comforting about a Triumph that to me inspires confidence.

But, that's the opposite of what you're experiencing and that is a bit worrisome. I'm not sure that tires are responsible for that. I've ridden quite a few Triumphs with these tires (K81's) on them and yes, they do tend to turn in quickly and you need to get used to that, but twitchy? The K81 require a bit more concentration to keep going in a straight line.

But if you have K70's on there, that could affect your handling adversely. While they are just OK for cruising on dry roads, spirited riding and wet weather riding reveal their worst side.

Keep looking for problems before you throw money after it. Could be something you'd least expect, like a problem with the forks or frame.

As an aside, I struggled with the same twitchiness on my Norton for a while before I figured out that the previous owner had fitted Shocks one inch longer than stock. When I put the stock ones back on it, the bike was transformed.

Cheers,

Bill

+1 on the Avons. Do you want the "look" or the handling? Venoms were better but they are not made in our size any more.
Predictable handling. If you hit a bit of sand on a curve you can feel them slip. If you don't do anything stupid and pay attention you can feel them slipping and gripping. When you run out of sand you'll still be upright and on your way.

CAB,
I have some Bridgestone Battleax BT 45 tires on my 74 Norton Commando. Also have them on a later model Airhead BMW. Have had several sets over a few years.
They wear well, and handle very smoothly in wet or dry conditions. Very predictable.
Not sure if they make them in the sizes you'd need, but they make lots of sizes.

BT 45 Tires

Using Avon RoadRiders on my '69 TR6R, a world of difference over the K81s. For chunky, check out Avon Distanzia. But if you are riding on the road all the time, go with RoadRiders.

If you are really going off-pavement, I'd recommend Heidenau K60 Scout.

Sorry bike currently as nearly new K70s (I will edit the original post).

I'd like tyres that work but if they can look "right" as well then that would be a bonus.

CAB

My 68 had K70's on when I bought it. The bike was fairly original and those tires sure looked the part. My first ride and my first right hand, uphill turn and I was on the wrong side of the road. Avon Super Venoms were mounted before I rode it again and the bike was transformed.

Wonder if someone left the shuttle valves out of the forks, no damping is the result.

Hello
I don't think so, forks seem to work in a controlled way.
CAB

I am using Michelin Pilot Activ tires on my 69 TR6C and my 70 TR6R. Best tires I have used for these bikes so far. They handle great and wear well. K-70s look correct but when they wear they become nervous on anything but perfect pavement. The Michelins are available in the correct sizes for these bikes and are reasonably priced. Install new michelin tubes and new rim strips and you will be handling great !

Hello
Thanks for the reply.
What size Activs do you have and what pressures do you run? I'm not sure to go for the equivalent of the original K70s (3.50-19 front 4.00-18 rear) or change the front to the same as the home market bikes (3.25-19).
Have you tried K81/TT100 or Avon Roadriders?

Just found this on another forum:
I like the K70's other than on some new concrete surfaced roads we have here which have a series of small grooves running along the surface in the direction you are travelling. This causes a gentle, but very disconcerting, weaving sensation as the rear end in particular goes side to side, rather like a deflating rear tire. None of my other bikes even the one with knobbly tires does the same thing on these surfaces and every where else my Tiger tracks just fine. It's not so bad once you realize what it is, the first time surrounded by traffic that prevented me pulling over or slowing down from the 55-60 mph that everyone was travelling at, it was very unpleasant.

This exactly the same as my TR6C is doing, same tyres, same speed, same type of surface and same circumstances.

Regards
CAB

I use a 3.25x19 for the front instead of a 3.50 that the C models came with and a 4.00 x18 for the rear. I run about 26 in the front and 28 in the rear and this seems fine. I have used K-81s on other bikes and these handle very well until you get some wear on the outer edges and then they want to flop over in a turn.I have not used Road Riders before but I use Avon Super Venoms AM18s,19 inch front and rear on Our Norton Commando.

Thanks

Having read the other posts I'm pretty sure it's the K70s causing the problem. I think I'm going to go for the K81s so the question is now which size?

According to the Dunlop site the K70 front 3.50-19 is 675mm by 93. The K81 front is available in 3.60 or 4.10 -19 which are 641 by 93 mm or 667 by 108 mm. I'd like to go for the 4.10-19 and it looks as if there is sufficient clearance between the forks to accommodate the extra 15 mm as long as the dimensions are accurate (I'm slightly suspicious because I have seen different dimensions on different sites...).

On the rear the 4.00-18 K70 is 671 by 108 mm and the K81 choice is 4.10-18 or (641 by 108 mm) or 4.25/85-18 (659 by 112 mm). I'd like to go for the wider tyre to get near to the K70 diameter so that the gearing and speedo are not affected too much. Again it looks as if there is sufficient space to accommodate the extra 4 mm width but it would be nice to find someone who is running this combination to be 100% sure.

I might ask this question more directly in a new post.

Thanks again.

On my 67 650 I have k70s- 3.25 19 on the front and 4.00 18 on the back and it seems fine but I suppose it is a different bike although pretty similar. I was using 24 and 25 psi (book setting) but changed to 26 and 27 and tried 27 and 28. The handling is the same but was told on this site that the tyres might wear at the lower settings, especially the front.

There is no doubt that modern tyres are better but because mine seem pretty good I have stuck with k70s because I like the look of them. What is funny is that the made in Indonesia 4.00 18 K70 is smaller in both height above rim and width than the previous made in Japan ones!


Dave

CAB, although the K-70 was supplied as the "standard" front tire beginning in 1968 on Triumphs sold here in the USA, I found it to be a lousy choice for that use. It appears to be designed as a REAR tire, and the strange "cupping" wear pattern that happens when it is PUSHED along on the front of a bike proves it, at least to me. You do not see this wear pattern on a K-70 mounted on the REAR of a Triumph.

The handling of my Triumphs always improved greatly when I junked the K-7o and installed a Dunlop rib front tire I suppose for authenticity at bike shows a 1968 and later Triumph twin should have a K-70 on the front, but if you ride the bike a lot, try something else.

And YES, the proper front tire size for a K-70 is 3.25 - 19

Signs of cupping on a K70 is a symptom of under-inflation!! When properly inflated, and the pressure adjusted for the load when necessary, K70's do not cup.

Modern copies of classic designs (e.g. K70) typically require significantly higher pressures than the ones made 50 years ago. If you use the recommended values from old handbooks the tires will be underinflated and cupping, and poor handling, will be the result. For what it's worth, I cut/pasted the following from something I posted on this subject two years ago:

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Over 20 years ago an acquaintance who worked in a motorcycle shop, but who may or may not actually know anything about this, said he always ran his tires at 90% of the max. load rating that's on the sidewall. He probably gave me some reference for this advice, because it was plausible enough for me to try it. Anyway, for the last 20 years I've used this 90% value for every vehicle in my family, which means many hundreds of thousands of miles on many types of motorcycle, car, truck, and trailer tires.

It's claimed that an F1 driver can tell if the inflation of one of his tires is off by 1 psi. I'm not that kind of guy. Although the ride and/or handling might be better/worse at some other inflation, the wear on all over the years has been uniform.

Another nice feature of the "90% rule" is that since one value fits all, it's easy to remember as well as to apply. For example, if the max. pressure on a tire is 38 psi, 10% of that is 3.8 (call it 4), so subtract 4 from 38 and you have 34 psi. No calculator required.
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Another sign of under-inflation is pressure creep. When a tire is used the movement of the tire carcass creates heat. Normally with reasonably aggressive riding you can expect an increase of approx. 2 pounds over the cold inflated pressure. Any more than two pounds and the tire is not being supported properly.

Tire pressure does more than define the shape of the tire. It has a large influence on fuel mileage, handling, and directly effects the load capacity of the tire. Tire pressures also affects the carcass of the tire and its contact patch which can affect its ability to dampen natural oscillations or wobble. Sometimes raising the pressure above recommended levels can increase the chances a bike will wobble.

The maximum rated load/pressure on the side of the tire is a good place to start. From there you can calculate your load and adjust the pressure accordingly.

Hi MM,


Mmmm ... this is too much of a generalisation ime:-

. The last time Triumph published Dunlop-recommended tyre pressures for K70's on a twin was 1977, when iirc the recommendations were 26 psi front and 28 rear.

. A thread on the same subject ran about a year ago, when I 'phoned Dunlop UK's technical guys to check if the advice I was given years ago had changed. It hadn't.

. Certainly Dunlop UK do not recommend any higher pressures for Dunlop K70's (or K81's) on Triumph twins than the company did in 1977. As the tech. guy said, "There hasn't been any design change to the tyres since 1977, so why would the recommended pressures change?"

. What the tyre companies would like some of us to forget - because the advice gives their modern product-liability lawyers palpitations - is the published solo pressures are based upon a 10-stone (140-lb.) rider, and they can be increased to a formula - front by 1 psi for every 2-st. (28-lb.) increase in rider weight, rear by 1 psi for every 1-st. (14-lb.) increase in rider weight.

Hth.

Regards,

Ten years earlier, in my c1967 Triumph 500 shop manual, they recommended 24 psi front and rear for a K70. The 1977 value you quote for the rear tire is 17% higher so something changed between those two years.

Given that the Dunlop was sold to Sumitomo over 30 years ago I wonder how much, if any, technical work is still done in England. If the office in the UK is like that of many other such offices, the "tech guy" is most likely to be someone in the PR department without any engineering background.

While there hasn't been a "design change" to the tread it seems very unlikely there haven't been internal design changes including the use of different materials for the "rubber" and cords than were in the tires 40 years ago. That said, while it seems very plausible to me that things have changed, I can't point to documentation supporting this, so perhaps all aspects of the construction are identical to the tires produced in England in 1977. But, while that's possible, it sure seems unlikely.

Seemingly unable to throw anything out, I have a Made in Japan K70 in the garage that has the max. load information on the sidewall (I also have earlier Made in England tires that were produced before this information was required). Anyway, that tire dates from c1998 and has a rating of 36 psi at max load. For what it's worth, using the 26 psi value you quote for the front tire, that's only 72% of the max. load rating at 36 psi of a tire made 17 years ago. Perhaps someone who has purchased a K70 made more recently could see if the max. value is still 36 psi.

Addendum: A friend has a new Made in Indonesia K70 at his shop. It's also rated at 36 psi max.

One can always recommend a little experimentation with tire pressures. On the other had, the most often neglected maintenance task is just keeping some air in the tires. My neighbor crashed his big ass Harley last year breaking his ankle because he had maybe 10 pounds of pressure in his front tire. I know cuz I rode the pig home after the crash and it truly was a pig.

I think there is light years difference between the tires of the bad old days and today. I used to get flats on a regular basis. You carried a pump and tube patch kit in those days. I've mounted a few new production vintage tires on my bikes over the years, but for bikes I ride hard or on a daily basis, it's modern tires all the way.

Actually, I'm down to the one classic tire for front wheels and that's the Avon Speedmaster ribbed tire. I like the precise feel of the steering with it although I think that it has a lower limit of adhesion than a modern tire. It's for my Norton which is a dream in fast sweepers and that's where the tire shines.

I may try a Roadrunner on the back of my Honda when(perhaps if) that rock hard Shinko ever wears out. I'm not that impressed by that particular Chinese tire, but my Duro's are doing the trick.

Cheers,

Bill

Well, guys, my previous entry was based on what the TRIUMPH handbooks stated was the correct tire pressure to be used for the front tire, whether K-70 or rib-style.

Foolish me! I improperly believed that the wizards of Meridan actually knew what was required!
As my latest Triumph (still owned!) is a '72 500cc T100R Daytona, that is what I used for tire inflation specifications.

I still believe the RIB tire is the best-handling one for a Triumph, as the original Triumph fork geometry seems to have been calculated to use it.

In this sport, everyone has his or her own opinion based on experience. Do what you will.

They weren't stupid, but we're not in 1972 any more.