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#378351 - 06/11/11 6:47 pm Tires for BSA Thunderbolt??  
Joined: Jul 2007
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Norton47_74 Offline
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Norton47_74  Offline
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Graham, WA
Hi
Need some help finding replacement tires for a 1964 BSA Thunderbolt. Currently has Dunlop K70 3.25X 18 on the front and a 3.50 X 18 on rear.
Trying to find a good replacement.
Most desired is Dunlop K70, seems to be non-existent in this size. So, we have read that Cheng Shin makes a repo and we are having a hard time locating that.
What else is out there in the 18" size that BSA riders are using?
We are not down to the threads yet but need some new ones soon.
Thanks in advance for any help.


Norton47_74
74 Commando
47 M18
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#378387 - 06/11/11 11:27 pm Re: Tires for BSA Thunderbolt?? [Re: Norton47_74]  
Joined: Jun 2002
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Stuart Online content
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Stuart  Online Content
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Scotland
Hi,

Originally Posted By: 1968BSA
You can get K82 in both sizes,

From a thread on this subject a while ago, K82's are available in GB but not in the US. frown

Originally Posted By: Norton47_74
What else is out there in the 18" size

Have you tried typing "3.25 18" into something like Google and following the links that comes up?

Originally Posted By: Norton47_74
3.25X 18 on the front and a 3.50 X 18 on rear.

A 100/90x18 will be pretty much the same diameter as the rear 3.50 but will be a little wider.

The 3.25 front width is always a pain to match; imho, the best compromise would be a 90/90x18 - it'll be a little smaller on the diameter but a very similar width.

Hth.

Regards,

#378395 - 06/12/11 12:17 am Re: Tires for BSA Thunderbolt?? [Re: Stuart]  
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Rickman Online content
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Ohio
I agree with the above statement about getting onto a search engine.
And it seems to me that 18" rubber is a lot easier to find than the 19" rubber I need for my A10's.

Here, try searching for iron pony and dennis kirk, they are large companies here in the Ohio area, and I remember seeing a number of 18" k70 rubber, and other brands. MUCH more than any 19" sizes...
I'm not advocating them, they are just local to me possibilities, that may help you in your tire search.

Then, after you've looked over those two, see what other suggestions your search engine offers, and go look at some of them.
Be aware that for some reason, some of those "other" possibilities aren't even motorcycle related...

There was one place I found the listed price was $16 LESS than iron pony.
Research and networking can be your friend!

A couple years ago, walridge in Canada sold me a set of Avon Speedmaster and Speedmaster MkII, new, at Mid-Ohio. There has been some discussion about how long wearing and hard {?} these tires are.... They are tall like the original tires were....

Last edited by Rickman; 06/12/11 12:25 am.
#378509 - 06/12/11 3:36 pm Re: Tires for BSA Thunderbolt?? [Re: Rickman]  
Joined: Jul 2007
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Norton47_74 Offline
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Norton47_74  Offline
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Graham, WA
Thanks for all the suggestions.
I have used the search engines and as we are trying to find tires to retain the period effect, it's limits our choices. British Cycle supply has a listing but they are not available.

The 18" seems to be the dooming factor. 19" possibilities seem to abound, especially if you want to go to a more modern tire pattern.

We did see that Dunlop and other's have a 100/90 and a 4.00 in 18" and that would work for the rear. The front is more problematic.
It has to be the narrow 3.25 to clear. If we could find a 90/90 that might work.
I had read that Cheng Shin had some replica's but I haven't be able to actually locate any or even find a listing on the web.
That is why I turned here. I was hoping someone might have been down this road already.
Thanks and perhaps someone will chime in.


Norton47_74
74 Commando
47 M18
#378520 - 06/12/11 4:13 pm Re: Tires for BSA Thunderbolt?? [Re: Norton47_74]  
Joined: Jun 2002
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Stuart Online content
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Stuart  Online Content
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Scotland
Hi,

Originally Posted By: Norton47_74
Dunlop and other's have a 100/90 and a 4.00 in 18" and that would work for the rear.

Don't fall into the trap of thinking 100 and 4.00 are similar; the numbers are section widths, not overall widths; moreover, 100/90 and 4.00 are different cross-sections, they are not directly comparable, you need the overall dimensions.

Originally Posted By: Norton47_74
The front is more problematic. It has to be the narrow 3.25 to clear.

Because 4.00x18 is larger all round than 3.50x18, when BSA and Triumph fitted 4.00x18 to the rear, they fitted 3.25x19 to the front.

Originally Posted By: Norton47_74
If we could find a 90/90 that might work.

90/90, whether x18 or x19, is a similar overall width but a smaller overall diameter to the equivalent 3.25.

Originally Posted By: Norton47_74
perhaps someone will chime in.

confused
Originally Posted By: Stuart
Originally Posted By: Norton47_74
3.25X 18 on the front and a 3.50 X 18 on rear.

A 100/90x18 will be pretty much the same diameter as the rear 3.50 but will be a little wider.

The 3.25 front width is always a pain to match; imho, the best compromise would be a 90/90x18 - it'll be a little smaller on the diameter but a very similar width.


Hth.

Regards,

#378521 - 06/12/11 4:21 pm Re: Tires for BSA Thunderbolt?? [Re: Norton47_74]  
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Rickman Online content
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Rickman  Online Content
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Ohio
Norton47_74
I was chasing tires again last night. Most of the websites don't make it easy to find the exact size you really want.

Still, once I managed to get into the various tire brands, I was too often lead to sizes I didn't want, and often saw those sizes you are looking for.

Are you saying now, you want 'modern' tread patterns now?

I saw a few MC supply stores in Denver, which oughta be closer to you than here in cow-town...

Dunlop, Bridgestone, and Avon, seem to be the best brands to look at.
But Shinko, Kenda, Pirelli, michelin, and a few other have hit-and-miss possibilities, if you are OK with mixing tires and brands. I don't ride very aggressive, so it won't bother me much... Although I will not try to mis-match TOO badly, like some crotch rocket race front, with a vintage rear set of tires... Or some other silliness...
You just have to go to the various websites, and start scrollin' and lookin'...

Myself, I will endevor to try to stay away from a ribbed front tire....
The Dunlop K70 3.25 x 19" fronts I've found, will suit me just great. A matching K70 rear has become difficult to find, the pony has ONE, they also have a K81 4.10 x 19", and there has been discussion about it actually being more like a 3.60 x 19 in actual usage { IF I understood THAT discussion correctly }, but it is half again as expensive as the K70!

There has also shown up, a couple universal tires, indicating they can be fitted front or rear. They do not have a 'modern' tread pattern, so I could use those in a rear fitment...

I remember some discussion about using a front tire on the rear, and while not recomended, it is a do-able solution: I was considering using a front 3.50 x 19" K70 front tire...

I've also considered using a dual sport rear, but nobody makes a 19" version, that I have found, yet.... LOTS of 18" sizes in those!!! I think many of them would make a great rear tire, just not the really aggressive off-road versions.

If no one has mantioned this to you yet,
90/90 is approximate to 3.25
100/90 is approximate to 3.50
110/90 is approximate to 3.75-4.00

Or so I've been told.... This applies when you cannot find the inch sized tires... I have been looking at both types of sizes. It's just that the metric sizing will get you a profile that is NOT as tall as the inch way of making tires. What I've seen, this is reflected in the pricing, often enough. The inch 'matching size' tires are slightly more expensive than the metric sizes, where there is a price difference....

If you choose, send me your email, I'll ping you when I find your sizes, but I don't think repeating suppliers names here, without them helping the site, is a good practice, or looked upon well...
Brett

#378534 - 06/12/11 5:25 pm Re: Tires for BSA Thunderbolt?? [Re: Rickman]  
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Stuart Online content
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Stuart  Online Content
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Scotland
Hi Brett,

Originally Posted By: Rickman
Dunlop K70 3.25 x 19" fronts
A matching K70 rear has become difficult to find,

The matching rear K70 is 4.00x18. That's exactly what BSA and Triumph fitted o.e. 'til BSA closed down and Triumph twins started being made by the Co-op.

Originally Posted By: Rickman
K81 4.10 x 19", and there has been discussion about it actually being more like a 3.60 x 19 in actual usage { IF I understood THAT discussion correctly },

No. 4.10 is a section width, not an overall width. Its section height is near-as-dammit the same as the section height of 3.50 section width.

This is because 4.10 and 3.50 are different cross-sections.

3.50, like any cross-section number divisible exactly by 0.25, has a 100% aspect ratio; i.e. the section width and section height are the same = the section height is 100% of the section width.

Otoh, 3.60 and 4.10 are cross-section numbers not divisible exactly by 0.25, which means they are low-profile cross-sections, in that the section height is less than the section width; in this case the section height is 90% of the section width, which is why their metric designations are 90/90 and 100/90 respectively.

There is an exception - 4.25; you can have a 4.25 100% aspect ratio cross-section and a 4.25 low-profile cross-section. The former is considerably bigger overall than the latter, so no-one got confused, a 4.25 low-profile cross-section has the aspect ratio like a metric designation - 4.25/85x18.

Originally Posted By: Rickman
it is half again as expensive as the K70!

That's cheap - a K81 is far more than "half again" better than a K70. wink

Originally Posted By: Rickman
universal tires, indicating they can be fitted front or rear.
using a front tire on the rear, and while not recomended,

'Front', 'Rear' and 'Universal' are specific recommendations; 'Front' and 'Rear' are self-explanatory, only 'Universal' means it's recommended for both front and rear fitment (usually with directional arrows on the sidewalls).

Originally Posted By: Rickman
90/90 is approximate to 3.25
100/90 is approximate to 3.50

Define "approximate"? As I've said:-

90/90 is a very similar overall width, but a smaller overall diameter, to a 3.25.
100/90 is a greater overall width, but a very similar overall diameter, to a 3.50.

Originally Posted By: Rickman
110/90 is approximate to 3.75-4.00

Depends on the maker. Specifically Dunlop K70, Avon SM Mk.2 and Roadrider have relatively slim overall widths for their section widths; otoh, Bridgestone, Michelin, Pirelli, etc. have larger overall widths for the same section widths; this means that the latter makes will be difficult or impossible to fit in a British bike, because British bike makers weren't generous with the space they left around wheels.

That is why I bang on and on and on in these threads that you need to know the overall dimensions of tyres.

Hth.

Regards,

#378558 - 06/12/11 7:47 pm Re: Tires for BSA Thunderbolt?? [Re: Stuart]  
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Rickman Online content
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Rickman  Online Content
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Posts: 2,884
Ohio
Stuart,
You go on bangin' away, please! Eventually, it may soak into my head the correct way things are....
I do think I've got it that the metric aspect ratio way of designing a tire means they are never going to be as tall as the old inch way of making a tire.
This'll save them some rubber in making each tire...

>" - a K81 is far more than "half again" better than a K70. "

I think I need to ask why you have this opinion.
Better traction?
Longer lasting?
Better rainy road holding???
Better muddy trail into the campground traction?

Your opinion may sway me even more towards another type of tire maker entirely, for the rear. There were certainly better looking for-the-road rear tires available from other makers....

>" The matching rear K70 is 4.00x18. "

Well, I suppose I'd better ask, first, how long has Dunlop been making the K70, and second, even when the bikes [ like my A10 ] used 19" front and rear?

I REALLY do NOT want to be spending $150+ for each tire, I have about six [6] I need to buy....

These bikes I'm buying tires for, will be used. I'm not after authenticity, or restoration purposes. But it seems the only matching tires from one company, that I can find, is Dunlop, or Avon.
Bridgestone? I have to buy tires from two different numbered lines/styles of tires, but at least the company is the same...
The other possibilities, as I said, hit-and-miss, or rather mix-and-mash-together....

I have two old Pirelli 4.00 x 19" tires, with what I'd call a universal tread pattern... So old, I cannot remember where I got them from... Under the tire size, is "353" on both, and the DOT code is 065M7778, and 964U4101. Do you know what these numbers signify? Does anyone know how to read these old DOT codes?

One is mounted on an A10 rear wheel, and is well away from touching the swing arm or frame. The closest it gets, is like 5/16-3/8"? And that is one of the chain guard mounting bars.
This tire seems to be showing a little age, as the sidewalls are feeling a tiny bit rough, showing the.....start? of weather checking...

The second one has had someone cutting chunks out of the tread, on one half of the tire, with what look like an ice racing or flat track tire preparation.... But it's sidewall still has that powdered rubber look and feel, that new tires had, and maybe some still have?
Other than the cut treads, this tire looks near new.... The nubbies are all wiped off of it though... Y'know, the new tire mould whiskers?
Still, I'm hesitant about using them...
Brett

P.S. Sorry, I'm not trying to hijack this thread... But even though the sizes are different, slightly, the hunt is nearly the same....

#379340 - 06/16/11 11:05 pm Re: Tires for BSA Thunderbolt?? [Re: Rickman]  
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Stuart Online content
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Stuart  Online Content
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Hi Brett,

My apologies for the pause in service ...

Originally Posted By: Rickman
I've got it that the metric aspect ratio way of designing a tire means they are never going to be as tall as the old inch way of making a tire.
This'll save them some rubber in making each tire...

Uh-uh. Most of the "metric aspect ratios" that fit these old clunkers are simply metricated Imperial low aspect ratios - e.g. 90/90 and 100/90 are 3.60 and 4.10 respectively - and the 4.10 started out as a widened 3.50 but without the increase in height that would accompany an increase in the width of a 100% aspect ratio. So, if anything, "metric aspect ratio" tyres use more rubber than the 100%-aspect-ratio sizes they were intended to succeed.

Originally Posted By: Rickman
Originally Posted By: Stuart
a K81 is far more than "half again" better than a K70.

I think I need to ask why you have this opinion.
Better traction?

Definitely.

Originally Posted By: Rickman
Longer lasting?

whistle wink

Originally Posted By: Rickman
Better rainy road holding?

Definitely.

Originally Posted By: Rickman
Better muddy trail into the campground traction?

Is there anything better than a motocross tyre for this? wink

Originally Posted By: Stuart
The matching rear K70 is 4.00x18.

Originally Posted By: Rickman
first, how long has Dunlop been making the K70, and second, even when the bikes [ like my A10 ] used 19" front and rear?

You're quite right, the o.e. pairing of 4.00x18 with 3.25x19 seems to started in the mid-1960's(?). Depending where on the grip/longevity scale you want to be:-

. the K70 3.50x19 is 'Universal' so could go on the rear with a 3.25x19 front;

. the K81 4.10x19 or any 100/90x19, although a little wider, should still be a similar overall height to 3.50x19, the K81 and most (any?) 100/90x19 are 'Universal';

. the Avon AM26 Roadrider 3.25x19 is 'Universal'.

Originally Posted By: Rickman
old Pirelli 4.00 x 19" tires,
"353" on both, and the DOT code is 065M7778, and 964U4101. Do you know what these numbers signify? Does anyone know how to read these old DOT codes?

'Fraid not to both those questions. frown

Originally Posted By: Rickman
is well away from touching the swing arm or frame.

At some time around the late 1970's or early 1980's, bodies like the ETRTO in Europe and the US DOT seem to have mandated a general increase in overall widths for every section width. This led to tyres like the Avon AM20 3.25x19 and AM21 4.00x18 Roadrunners becoming difficult or impossible to actually fit in Brit. bikes. mad Avon seemed to have tried to address the problem with the AM26 but, even there, about two years ago, some sizes (including the two above) suddenly increased in overall width, although they aren't up to those of the previous Roadrunners, and a lot of the current offerings from other makers. frown So, I suspect a modern 4.00x19 might be too wide ... cry

Hth.

Regards,


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