Twitchy handling caused by oversize tyre? - Triumph Forum: Triumph Rat Motorcycle Forums
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post #1 of 11 (permalink) Old 06-03-2019, 11:58 AM Thread Starter
Grand Prix 125
Main Motorcycle: 1969 Daytona T100T
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Twitchy handling caused by oversize tyre?

My '69 Daytona came with Avon Roadrider tyres (3.25 x 19 front, 4.0 x 18 rear), which are supposed to be reasonable tyres. There's plenty of tread on them, but now I'm using it more I'm getting a "twitchy" feel from the back when negotiating roundabouts, leant over at medium speeds, as though it's going to 'step out' - had to dab a foot down on one occasion when I felt it go.

I have checked swinging arm bushes for play in case this was the problem but I think these are OK. Avon say minimum for this 4.0 tyre is WM3 and I always felt that the rear tyre overhangs the rim by a rather large amount. The rim is not the original (no security bolt holes) and WM3 is specified in the parts list, so I've measured it at the rim and it is only a WM2 (same width as the front).

Do you guys think this tyre on this size rim is the problem?

Previous bikes: '58 Tiger Cub; '61 Bonnie; '67 Bonnie; '69 Bonnie '70 BSA Lightning; '71 OIF Bonnie; Honda 500-4
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post #2 of 11 (permalink) Old 06-03-2019, 01:12 PM Thread Starter
Grand Prix 125
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Quote:
Originally Posted by Tritn Thrashr View Post
You wonít improve on the correct tyre sizes. It seems unlikely that a 4.00 was original on a Daytona.
Hi Tritn Thrasher,
I'm not looking to improve on the specified tyre size. 4.00 was specified for the Daytona T100R, but 'original' profile 4.00s don't seem to be made any more: the Avon 4.00 is quoted as being 4.6" wide and expects a WM3 rim as a minimum. My bike should have had a WM3 but it's been changed at some point to a narrower rim (I can tell it's not original as there's no security bolt holes).

I reckon this tyre is way too wide for a WM2 rim, so unless I rebuild it with a WM3 rim I'm stuck with a Roadrider 90/90 metric tyre as the only fit on a WM2, but this tyre is 2" smaller in diameter. Rebuild with a WM3 is a bit problematic as it's a QD hub which has rather worn splines, so maybe not worth doing.

Previous bikes: '58 Tiger Cub; '61 Bonnie; '67 Bonnie; '69 Bonnie '70 BSA Lightning; '71 OIF Bonnie; Honda 500-4
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post #3 of 11 (permalink) Old 06-03-2019, 03:21 PM
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Hi Brian,

Quote:
Originally Posted by BrianG View Post
'69 Daytona
Avon say minimum for this 4.0 tyre is WM3
I've measured it at the rim and it is only a WM2
Depends where exactly "at the rim" you've measured and what result you got? "WM" is only the rim cross-section, the measurement should be across the rim where the tyre seats, WM2 is 1.85", WM3 is 2.15".

Quote:
Originally Posted by BrianG View Post
Avon Roadrider tyres
which are supposed to be reasonable
Roadriders are

Quote:
Originally Posted by BrianG View Post
Do you guys think this tyre on this size rim is the problem?
A 4.00-width Roadrider really on a WM2 rim - when Avon say minimum width is 2.15 (WM3 equivalent) for the tyre - it's certainly a problem.

Any 'modern' "3.25" or "4.00" - like Roadriders - are not "the correct tyre sizes" for a half-century-old bike. "3.25", "4.00", the first number of any string moulded on any tyre's sidewall, is a section width, not an overall width and, for any given section width, overall widths have increased in the intervening half-century:-

. in '69, Triumph fitted Dunlop K70 certainly to the rear of your bike, 4.00x18 K70 is 4.17" (106 mm.) wide overall, 4.00x18 Roadrider is 10 mm. wider;

. similarly, 3.25x19 K70 is 3.3" (84 mm.) wide, 3.25x19 Roadrider is 14 mm. wider ...

. fwiw, 3.25x19 Roadrider is what I'd use on the front of my T160's, because the overall width is very similar to that of the original 4.10x19 TT100; you're riding a 100-lb.-lighter bike on fat tyres and wondering why it handles like a Hardly ...

Finally here, please confirm you were using the modern tyre pressures Avon recommends for the Roadriders, not the 24 psi front/25 psi rear in the '69 T100 Owner's Handbook ...

Quote:
Originally Posted by BrianG View Post
unless I rebuild it with a WM3 rim I'm stuck with a Roadrider 90/90 metric tyre as the only fit on a WM2,
If you really do have a WM2 rim, as you're in GB, I suggest Dunlop K82 3.50x18 - WM2 and 3.50-width tyre were what pre-'71 'UK & General Export' 500's were fitted with as standard, K82 looks like the more-famous K81 but is in smaller sizes for less-powerful bikes.

And fit a skinnier front - 90/90x19?

Quote:
Originally Posted by Tritn Thrashr View Post
It seems unlikely that a 4.00 was original on a Daytona.
US-market 500's had 4.00x18 rear tyres certainly from the early 1960's, possibly earlier. By the late 1960's, Triumph and BSA fitted 4.00x18 rear tyres to everything from 250's to 650's, the only models spared were the triples.

Hth.

Regards,

Stuart

Last edited by StuartMac; 06-03-2019 at 03:44 PM.
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post #4 of 11 (permalink) Old 06-03-2019, 03:27 PM
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Quote:
Originally Posted by BrianG View Post
Hi Tritn Thrasher,
I'm not looking to improve on the specified tyre size. 4.00 was specified for the Daytona T100R, but 'original' profile 4.00s don't seem to be made any more: the Avon 4.00 is quoted as being 4.6" wide and expects a WM3 rim as a minimum. My bike should have had a WM3 but it's been changed at some point to a narrower rim (I can tell it's not original as there's no security bolt holes).

I reckon this tyre is way too wide for a WM2 rim, so unless I rebuild it with a WM3 rim I'm stuck with a Roadrider 90/90 metric tyre as the only fit on a WM2, but this tyre is 2" smaller in diameter. Rebuild with a WM3 is a bit problematic as it's a QD hub which has rather worn splines, so maybe not worth doing.
Hi Brian G,
It sounds like it is time to bite the bullet and replace the hub and rim at the same time, not a pleasant experience for your wallet.

On a cheaper note, have you checked wheel alignment, and experimented with tyre pressures to see if you can alleviate some of the unpleasantness on bends.

As this appears to be a progressive problem, how has this Avon worn when pinched in by the smaller than spec rim? I doubt that the unusual curvature of the tread caused by this smaller rim, has done any favours for the wear pattern.

Regards
Peg.
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post #5 of 11 (permalink) Old 06-03-2019, 04:35 PM
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Still think it’s a dodgy combination though.
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post #6 of 11 (permalink) Old 06-03-2019, 05:04 PM
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Quote:
Originally Posted by StuartMac View Post
Depends where exactly "at the rim" you've measured and what result you got? "WM" is only the rim cross-section, the measurement should be across the rim where the tyre seats, WM2 is 1.85", WM3 is 2.15".
To supplement this info I've measured my wheels across the outside of the rim and found the front stamped WM2-19 to be 2.7 inches (68.6mm) wide and the rear stamped WM3-18 to be 3.25 inches (82.4mm) wide. I'm with RPW, get the rims right first and take tyre choice from there.
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post #7 of 11 (permalink) Old 06-04-2019, 12:51 AM
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Hi Rusty,

Quote:
Originally Posted by Rusty1 View Post
To supplement this info I've measured my wheels across the outside of the rim and found the front stamped WM2-19 to be 2.7 inches (68.6mm) wide and the rear stamped WM3-18 to be 3.25 inches (82.4mm) wide. I'm with RPW, get the rims right first and take tyre choice from there.
Mmmm ... note I posted "the measurement should be across the rim where the tyre seats".

Overall rim width depends not only on seat width but also rim cross-section profile, material, etc. - Brian posted the rims he's using aren't stamped; if they're WM cross-section, steel and the maker used the same material thickness as Dunlop or Jones, they might be the same overall width as the ones on your bike.

Otoh, the measurements I quoted are an international standard, independent of rim cross section - e.g. the same numbers appear in the tyre specifications on both the Avon and US Dunlop websites, but are modern MT cross-section rims.

Brian hasn't clarified where he measured the rims, what measurements he got to conclude the rim is what he's posted so far or even the tyre pressures he's using, any or all of which could be wrong ime.

Hth.

Regards,

Stuart
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post #8 of 11 (permalink) Old 06-04-2019, 02:58 AM Thread Starter
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Hi Folks,
Many thanks for the replies. Answers as follows:

Stuart:
1) I am aware that tyres have got bigger. I had K70 tyres on my OIF Bonnie which were OK, but again these would need a WM3 for the rear to fit properly on the Daytona.
2) I looked at changing to K82s but they don't do anything in 19" sizes so a non-matched pair would be yet another problem. The nearest in tread pattern would be a K81/TT100 front, but there's no guarantee the compounds are similar to the K82. A 90/90 metric rear would fit the existing rim but would seriously under gear the bike as it's so much smaller (2") in diameter. I was already going to change to a 43t rear sprocket and/or a larger gearbox sprocket as I feel the standard gearing is too buzzy.
3) I'm afraid I am using the standard 24/25 psi tyre pressures. The tyres seem Ok, not soggy. What would you recommend instead?
4)I have not measured the rims with the tyres removed but with calipers across the widest part I get 2.7" This concurs with what Rusty has measured in his post, so that confirms, for me at least, that it IS a WM2 rim.

Peg:

Yes, I have checked the wheel alignment using the Mason's Line method. It was good to within a millimetre or two. I have not tried changing the tyre pressures as they seemed about right at 24/25 psi - certainly not soft and soggy. Perhaps they should be much harder? The shape of the tyre looks OK, no flats but a smooth curve similar to the front tyre, and the wear goes evenly across and continues into the unworn 1/2 inch 'chicken strips' at each edge.

Rusty:
Thanks for measuring the rims. Your US import already has the proper rims so a 4.00 rear tyre would be OK for you. Mine has been messed with at some point by a previous owner: they are definitely NOT original rims. They are not in wonderful condition chrome-wise, so changing for new rims could be on the cards except for the rear hub having worn splines. Looks like I need a wider rear rim whatever I do!

Regards to all,
Brian

Previous bikes: '58 Tiger Cub; '61 Bonnie; '67 Bonnie; '69 Bonnie '70 BSA Lightning; '71 OIF Bonnie; Honda 500-4
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post #9 of 11 (permalink) Old 06-04-2019, 06:07 AM
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Hi Brian,

Quote:
Originally Posted by BrianG View Post
I have not measured the rims with the tyres removed but with calipers across the widest part I get 2.7" This concurs with what Rusty has measured in his post, so that confirms, for me at least, that it IS a WM2 rim.
Which then also concurs with the "T100T" in the details beside all your posts; also, the qd hub would've been more-likely fitted to a T100T than a T100R. Pre-'71, it's only the US-market T100R and T100C that originally had 4.00-width rear tyres on WM3 rims, T100T (and T100S) had 3.50-width rear tyres on the WM2 rims.

Quote:
Originally Posted by BrianG View Post
I am aware that tyres have got bigger.
It wasn't in your earlier posts. You asked for help, I'm telling you what I know - irrespective that the numbers moulded on Roadrider sidewalls are the same as printed in '69 Triumph 500 manuals, Roadriders in those sizes are the wrong tyres for your bike, particularly as you've now confirmed the rear rim is narrower than the minimum 2.15/WM3 for a 4.00-width tyre.

Quote:
Originally Posted by BrianG View Post
I looked at changing to K82s but they don't do anything in 19" sizes so a non-matched pair would be yet another problem. The nearest in tread pattern would be a K81/TT100 front, but there's no guarantee the compounds are similar to the K82.
You are over-thinking.

Any '69 T100 will struggle to exceed 25 bhp at the rear wheel, it isn't the latest mega-bhp plastic rocket. If there'd been anything wrong with K82 on the rear and another model or make of tyre on the front, I wouldn't have advised it. Otoh, I have advised that you reduce the width of the front tyre with the K82.

Avon/Dunlop combinations on more-powerful triples pre-date Roadriders by decades - 'front-only' Avon tyre on the front, TT100 on the rear. My experience of those tread combinations is precisely why I recommended it.

However, if you really must have a similar tread pattern on the front, 3.60x19 TT100 is available in GB, 3.60 is the nearest Imperial low-profile 'equivalent' of 90/90. "compounds" will not be something that affects your bike, trust me.

You posted , "I'm stuck with a Roadrider 90/90 metric tyre as the only fit on a WM2". Roadriders are not primarily intended for low-powered bikes; if you insist on unsuitable tyres and other bizarre arbitrary restrictions, yes, you're "stuck".

Quote:
Originally Posted by BrianG View Post
A 90/90 metric rear would fit the existing rim but would seriously under gear the bike as it's so much smaller (2") in diameter.
Which is why you should use a more-suitably-sized rear tyre. A 3.50-width tyre is a similar height to a 100/90.

Quote:
Originally Posted by BrianG View Post
I was already going to change to a 43t rear sprocket and/or a larger gearbox sprocket as I feel the standard gearing is too buzzy.
Depends what you change the rear hub to. 43-tooth pre-dates 46-tooth; non-qd drum takes a bolt-on sprocket so 43-tooth might be possible, qd 43-tooth could be harder to find unless you have one already.

My '69 T100 is 46-tooth qd rear and 19-tooth front. Having discovered some earlier T100's had 20-tooth, I'm debating the change ... I'd only make it if I had the primary apart for another reason, but there's then the obvious possibility I might have to pull the primary apart again to change it back.

Quote:
Originally Posted by BrianG View Post
I'm afraid I am using the standard 24/25 psi tyre pressures. The tyres seem Ok, not soggy.
Nevertheless, they're at least contributing to the problem you're experiencing. You should be using at least the Avon-recommended pressures: 2 bar (29 psi) front, 2.2/2.3 bar (32/33 psi) rear.

Quote:
Originally Posted by BrianG View Post
Looks like I need a wider rear rim whatever I do
If you increase to WM3-equivalent 2.15, 100/90 rear becomes an another option; personally, I wouldn't bother with a modern 4.00-width.

Hth.

Regards,

Stuart
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post #10 of 11 (permalink) Old 06-09-2019, 05:15 AM Thread Starter
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Hi Stuart,
I've not had a chance to try the suggested tyre pressure changes until now (been changing oil seals on the kick start and gear shift shafts in the gearbox cover the last couple of days). I raised the tyre pressures to 28 psi front and 34 psi rear as per your suggestion. Wow! what a difference: it's so much better and has given me confidence again for my modest riding style. It's only a temporary solution of course until I decide what to do about possibly changing the rim size, or not (it turns out other folks think my QD hub splines are not as worn as I thought).

Thanks again,

Brian

Previous bikes: '58 Tiger Cub; '61 Bonnie; '67 Bonnie; '69 Bonnie '70 BSA Lightning; '71 OIF Bonnie; Honda 500-4
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