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#515410 - 11/19/13 10:14 pm Largest rear tire for a Thunderbolt  
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Thunderman Offline
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What is the largest rear tire that I can put on my 1969 BSA Thunderbolt ???
What is the largest front tire I can put on the same bike ??
Thanks in Advance, Ride Safe !!!!

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#515415 - 11/19/13 11:00 pm Re: Largest rear tire for a Thunderbolt [Re: Thunderman]  
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Rickman Online content
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Ohio
Loaded question.

MANY threads about this conundrum....

First question asked would probably be "what size wheels? 18"? 19"? ... 16"??? WM1? WM2? WM3?

#515419 - 11/19/13 11:36 pm Re: Largest rear tire for a Thunderbolt [Re: Rickman]  
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Don't know for sure , just got the scooter. Every thing is stock and the book says its a 19 on the front and an 18 on the rear.

#515422 - 11/20/13 12:52 am Re: Largest rear tire for a Thunderbolt [Re: Thunderman]  
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leon bee Offline
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arkansas
The bike is stock? You want inch size tires to avoid a doofless looking gap between tire and fender, modern low profile tires look awkward on there. Traditional has always been Dunlop K70s- I and many others like these tires, some guys don't. 400-18 on back, 325-19 or 350-19 on front. Bike will look just right.

A development in the last few years has been modern tires in inch sizes. Avon Roadriders, they are wonderful tires based on my limited experience with just one set.

You say how big? Why? If you just want BIG for some reason, check out Firestone ANS or maybe Continental something like that. Remember you gotta clear the drive chain.

#515434 - 11/20/13 4:18 am Re: Largest rear tire for a Thunderbolt [Re: Thunderman]  
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Hi,

Originally Posted By: Thunderman
What is the largest rear tire that I can put on
What is the largest front tire I can put on

Bit of a vexed question, because going by the numbers moulded into the tyre walls doesn't tell you where the edges of a tyre are, but it's the edges that catch on other bits of bike ...

Originally, your bike would've been fitted with 4.00x18 rear and 3.25x19 front. However, that hides the fact that the original Dunlop K70's (or Avon SM's) are skinnier overall than the same sizes in Avon Roadriders, which are skinnier again overall than the 'same' sizes from, say, Michelin or Bridgestone (or possibly Firestone or Continental). frown

In theory, the 'next wider overall' tyres would be 120/90x18 rear and 100/90x19 front, but you're looking at the rear catching/rubbing on other bits of bike while I know the front will need to be deflated (or the mudguard/fender removed) just to get the wheel in and out.

Another problem you'll run into with wider tyres is the rim widths. Standard on your bike were WM3 rear and WM2 front; these equate to 2.15 and 1.85 respectively. As you can see, these are the minimum for 4.00x18 and 3.25x19; try and fit a wider tyre without widening the rim and the overall dimensions can go really strange ...

Finally, bear in mind that widening the tyres on a relatively light bike like an old Brit. and you risk some funny handling. It isn't guaranteed - handling depends on many factors - but the only Britbikes lots of testing determined could do with wider front tyres than 3.25 were the triples (at least 50 lbs. heavier than your twin), and when the Co-op started fitting wider tyres to the Triumph twins was when the stories about funny handling started. frown

Hth.

Regards,

#515540 - 11/20/13 8:18 pm Re: Largest rear tire for a Thunderbolt [Re: Thunderman]  
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Mark Z Online content
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Look where your chain runs with respect to the rear tire. Without a wheel offset modification, anything wider than four inches will rub on the chainguard.

You can get a 350 on the front, but it will be VERY close to the stock fender or center fender stays.

I had a 350 on the front of my '67 for a while, and then went back to a 325, and I thought it handled better with the 325.


Mark Z

'65(lower)/'66(upper, wheels, front end, controls)/'67(seat, exhaust, fuel tank, headlamp)/'70(frame) A65 Bitsa.
#515544 - 11/20/13 8:47 pm Re: Largest rear tire for a Thunderbolt [Re: Mark Z]  
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I still learn something most every day. Somehow I got it in my head that the rear on the dual sport models (Victor Special and Firebird Scrambler) were WM-3's and the rear on all the rest were WM-2's. I been running 3.50 x 18 on the back of my A65 for years. I'll check it out in the morning.

Mr Mike

#515562 - 11/21/13 8:16 am Re: Largest rear tire for a Thunderbolt [Re: Mark Z]  
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Stuart Online content
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Hi,

Originally Posted By: Mark Z
Look where your chain runs with respect to the rear tire. Without a wheel offset modification, anything wider than four inches will rub on the chainguard.

It's possible that even a 'modern' (e.g. Roadrider) 4.00 will rub on the chainguard, because it's a little bit wider overall than a K70. frown That said, if BSA were as generous as Triumph with space between the chain and 'guard, just cutting a chunk out of the inner side of the 'guard beside the widest part of the tyre stops it rubbing but without the tyre and chain coming into contact. bigt But it's close - BSA (including Triumph) designers don't appear to have been particularly generous with space around the tyres. frown

Originally Posted By: Mr Mike
I got it in my head that the rear on the dual sport models (Victor Special and Firebird Scrambler) were WM-3's and the rear on all the rest were WM-2's. I been running 3.50 x 18 on the back of my A65 for years.

If you've a mind to check A50 and A65 specs., you might find that 'UK & General Export' variants (especially A50's) might've been fitted with 3.50x18 rear on a WM2 rim as standard - Triumph certainly did that on the UK&GE 500's. Then UK&GE variants were certainly sometimes supplied to Canadian dealers ...

UK&GE Triumph 500's had an 18" rim on the front too.

Hth.

Regards,

#515595 - 11/21/13 1:13 pm Re: Largest rear tire for a Thunderbolt [Re: Stuart]  
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Rickman Online content
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I'm curious here; what about the 4.00 x 19" tires used originally on the front of Hornets???

Fenders or not?

[ Yeah, I know it's an off-road bike, not street as is being asked about... ]

{ I plan to use 3.25 x 19" on the fronts of all these street BSA's I'm building, except the two very early A65's. So, what size tire for 18" fronts?
3.50 x 19" rear tires, except where there are 18", where I'll try to find whatever is closest to 3.50 x 18", 110? }

Fat tires on skinny rims isn't a safe way to ride... I would think...

#515597 - 11/21/13 1:41 pm Re: Largest rear tire for a Thunderbolt [Re: Thunderman]  
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arkansas
All the mudguard stays on a West Coast Hornet are longer. And the 6 tab fender came with the tabs bent outward. Wheel is supposed to be a WM3-19.

A 400 tire runs fine on a WM2 rim, recommended or not.

My 67 Hornet is a pig with a 400 on front, handles far better with a 350.

#515603 - 11/21/13 2:03 pm Re: Largest rear tire for a Thunderbolt [Re: Thunderman]  
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A 350 handles fine, but a 325 handles better wink

don't know why BSA ever fitted a 400 tyre to an off roader, too big.


beerchug
#515646 - 11/21/13 7:29 pm Re: Largest rear tire for a Thunderbolt [Re: Allan Gill]  
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Allan,
I'd bet they were trying to "float" above soft sand washes and tracks?

Skinnier tires would just 'knife' into the soft sand...BT!DT!

#515650 - 11/21/13 7:35 pm Re: Largest rear tire for a Thunderbolt [Re: Thunderman]  
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I don't know Bret, I have travelled at over 30mph off road with a 3.50 trials tyre (on the canam) The easiest thing to do was try and keep the front wheel in the air (not something I can do unless I get lucky), hit anything like a rut with a wide tyre and rut controls you. But these bikes were designed for scrambles - which is what I haven't any experience in.

Mind you, you could have a point, didn't they fit a K70?


beerchug
#515655 - 11/21/13 7:55 pm Re: Largest rear tire for a Thunderbolt [Re: Allan Gill]  
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THAT I have no idea about!

Ask Gary?

#515662 - 11/21/13 9:22 pm Re: Largest rear tire for a Thunderbolt [Re: Stuart]  
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[quote=Stuart]Hi,
If you've a mind to check A50 and A65 specs., you might find that 'UK & General Export' variants (especially A50's) might've been fitted with 3.50x18 rear on a WM2 rim as standard - Triumph certainly did that on the UK&GE 500's. Then UK&GE variants were certainly sometimes supplied to Canadian dealers ...

UK&GE Triumph 500's had an 18" rim on the front too.

Stuart,
I checked my parts book and it indeed calls for a WM-3 on the rear of my 66 Lightning. I measured the front and rear rims and they appear to be identical. How much wider is a WM-3 than a WM-2? Is this just another case of BSA putting on the bikes what they had on hand.

Leon,
I too have run 4.00 tires on WM-2 rims but only on the back.

Mr Mike



Last edited by Mr Mike; 11/21/13 9:24 pm.
#515663 - 11/21/13 9:22 pm Re: Largest rear tire for a Thunderbolt [Re: Thunderman]  
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It's ok, I won't loose sleep over it.

Im more curious about the initial question.
Originally Posted By: Thunderman
What is the largest rear tire that I can put on my 1969 BSA Thunderbolt ???
What is the largest front tire I can put on the same bike ??


WHY. When I was changing my K70's on my A65 (3.50 front, 4.00 rear) to RoadRiders (3.25 front, 4.00 rear) I initially changed the back tyre, the difference in height was such that as the 3.50 K70 was taller than the 3.25 roadrider, the centre stand was merely a prop until I had put on the slightly lower tyre height roadrider.

Wide tyres don't make for a good handling bike, well matched tyres do.


beerchug
#515673 - 11/21/13 10:35 pm Re: Largest rear tire for a Thunderbolt [Re: Rickman]  
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Gary E Online content
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Originally Posted By: Allan Gill
A 350 handles fine, but a 325 handles better...

Not for me. I like the 3.50x19 on the Firebird better than the 3.25 on the Spitfire. The Spitfire is too squirrely on the front.

Originally Posted By: Allan Gill
...don't know why BSA ever fitted a 400 tyre to an off roader, too big.


Originally Posted By: Rickman
Allan,
I'd bet they were trying to "float" above soft sand washes and tracks?

Skinnier tires would just 'knife' into the soft sand...

Exactly. The reason BSA made the West Coast model Hornet was by request from west coast US desert riders. They wanted the big 4.00x19 tire on the WM3 rim and the larger oil tank in the hot western desert. Both Hornets and the Wasp were fitted with K70's front and back. 19" up front, 18" on the back. In the mud a narrow tire works. In the sand a wide tire works.


1967 BSA Wasp
1967 BSA Hornet (West Coast Model)
1967 BSA Hornet (East Coast Model)
1968 BSA Firebird Scrambler
1968 BSA Spitfire Mark IV
#515674 - 11/21/13 10:37 pm Re: Largest rear tire for a Thunderbolt [Re: Allan Gill]  
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Rickman Online content
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Mr. Mike,
WM2 is supposed to be @ 1.85 inches in width.

WM3 is supposed to be @ 2.15 inches in width.

And I think that's inside the rim, bead to bead?

#515676 - 11/21/13 10:43 pm Re: Largest rear tire for a Thunderbolt [Re: Thunderman]  
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Gary E Online content
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Yes with those numbers. Inside measurements. The WM3 is about 1/4" wider than the WM2.


1967 BSA Wasp
1967 BSA Hornet (West Coast Model)
1967 BSA Hornet (East Coast Model)
1968 BSA Firebird Scrambler
1968 BSA Spitfire Mark IV
#515687 - 11/22/13 12:26 am Re: Largest rear tire for a Thunderbolt [Re: Thunderman]  
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No Name Man Online content
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Avon Roadrider 120/90/18 (similar to 4.25) fits fine on my Tbolt. Slightly narrower than the same size Super Venom that I took off, which also fit. By fine I mean less than 1/4 inch from chain guard.
Seems to handle great to me.

Bill


69 A65T
71 B50T
85 K100RS
54/59 A10SR
69 B44VS
71 A65FS
Too much moderation is bad for you.

#515704 - 11/22/13 8:37 am Re: Largest rear tire for a Thunderbolt [Re: Thunderman]  
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Rickman,
I measured both rims with mounted tires (outside to outside)and the are identical. So Stuart is correct. BSA kinda did what they wanted to do. Next time I change tires I will put a 4.00 K-70 equivalent on the rear WM-2 rim. Thanks for the guidance.

Mr Mike

#515712 - 11/22/13 10:11 am Re: Largest rear tire for a Thunderbolt [Re: Rickman]  
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Stuart Online content
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Hi Brett,

Originally Posted By: Rickman
I'll try to find whatever is closest to 3.50 x 18",

When Dunlop created the K81/TT100, one of the things they were aiming at specifically was Avon's dominance of production racing and cafe 'racing' with the 3.50 GP. So the original 4.10 section is the same height as a 3.50 but wider. The metric equivalent of 4.10 is 100/90.

Originally Posted By: Rickman
what size tire for 18" fronts?

3.50 rears generally tend to have been matched with 3.25 or 3.00 fronts (Triumph used 3.25x18 front on the UK&GE 500's). Difficult widths to match anything like closely with a metric size but, if you're using 100/90 rear, 90/90 front is a widely-used combination by many bike makers.

Hth.

Regards,

#515713 - 11/22/13 10:18 am Re: Largest rear tire for a Thunderbolt [Re: Stuart]  
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Rickman Online content
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Thanks Stuart,
Your tire knowledge, once again, can be counted on.

I do have one 90/90-19 tire, but it looks WAY too thin to me, on this stock BSA wheel.... Almost like a rubber band on the wheel...

Sure, no side wall flex, but a VERY poor match for these large, wide, full valenced fenders...

#516146 - 11/25/13 4:28 pm Re: Largest rear tire for a Thunderbolt [Re: Rickman]  
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gunner Online content
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IM using a 120/90 18 inch Chen Sing C907 rear on my A65 which fits OK though is a little tight near the swinging arm. This is a modern size designed for tubeless rims, though IM using it with a tube.

IM quite happy with this tyre as it grips well and has good road holding.

This is probably about as big a tyre you could fit on the rear of an A65 without running into clearance issues.


1968 A65 Firebird
1967 B44 Shooting Star
1972 Norton Commando
#517452 - 12/04/13 9:46 pm Re: Largest rear tire for a Thunderbolt [Re: No Name Man]  
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Thanks "No Name Man" for the information. Looks like we have the same scooter--1969 BSA Thunderbolt-- So if that is the case, I really like the looks of the Avon RoadRider so it is goin on my "To Bob from Bob Christmas list".When you change out a rear tire should you also change the front with the same tread design ??? If you should, what size tire do you run on the front ????

Last edited by Thunderman; 12/04/13 9:47 pm.
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