Author Topic: TYRE SIZES/PRESSURES FRONT AND REAR  (Read 872 times)

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MALCOLM.JOHNSON

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TYRE SIZES/PRESSURES FRONT AND REAR
« on: 27 March, 2017, 21:29:02 »
Been having discussions/arguments recently regarding tyre fitments and pressures used on typical post war motorcycles and would welcome opinions from the forum. What is the point in having different tyres front and rear? A typical post war machine with 19 inch WM2 rims may often specify 3.00 front and say 3.50 rear. Why not have them both the same? Early A7 with Q/D hubs front and rear were intended to be interchanged. I can see that modern sports bikes need them different, but our relatively low powered classics - I am not so sure. Also early specification for tyre pressures, to me seem on the low side for modern tyre equivalents. Love to hear your views.

STAR TWIN

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Re: TYRE SIZES/PRESSURES FRONT AND REAR
« Reply #1 on: 28 March, 2017, 07:24:54 »
Perhaps it's because the rear carries more weight, especially with a pillion passenger. Narrow section front tyres also impart greater stability to the bike. Putting a 4.10 on the front of a Norton Commando makes it more prone to weaves and shakes than when it has a 3.60 for instance. The tyre contact patch is an oval. With a narrow front tyre, the long axis is in the direction of travel. With a wide tyre, the long axis is at 90 degrees to the direction of travel. OK on the back, not so good on the front. One of my pet hates is bobbers. Minimal mudguards (in Britain, really?) and fat front tyres. Bet the handling isn't what is should be.
Grump over. 

STAR TWIN

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Re: TYRE SIZES/PRESSURES FRONT AND REAR
« Reply #2 on: 28 March, 2017, 07:28:09 »
P.S. Low tyre pressures meant that the tyre was able to flex and that the tyre were acting as part of the suspension. Should you ever ride a veteran with a rigid frame and skinny beaded edge tyres (45 to 50 psi or they come off the rim) you will realise that tyres at low pressures do add a bit of comfort to the ride.

JulianS

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Re: TYRE SIZES/PRESSURES FRONT AND REAR
« Reply #3 on: 28 March, 2017, 09:41:04 »
It is not always possible to fit a wider tyre at the front - the design of the mudguard may limit the width of the tyre fitted.

The type and make are also considerations - on my A10 I use a 100/90 Avon Roadrider rear and 3.25 x 19 Roadrider front.

The front Roadrider is wider than the Avon 3.25 x 19 ribbed tyre. It is about the widest that would fit the mudguard.  The Roadrider  gives better grip but more ponderous steering than the ribbed.

The rear Roadrider is significantly better for ride and grip than the old Avon Safety Mileage but only does about 2500 miles before it needs replacing.

I could not fit a 100/90 on the front (no space) and would not want the 3.25 x 19 on the rear. I would not want to go back to the Avon Safety Mileage.

So as ever it is a matter of personal choice.

DWS

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Re: TYRE SIZES/PRESSURES FRONT AND REAR
« Reply #4 on: 28 March, 2017, 13:07:39 »
Of course this question of "Narrow tyre on front - Wide tyre on rear" doesn't add up if you look at a Sunbeam S7...   ;)

STAR TWIN

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Re: TYRE SIZES/PRESSURES FRONT AND REAR
« Reply #5 on: 28 March, 2017, 17:19:12 »
Damn! I was hoping no-one would notice that. But then again, it was designed that way.

Editor

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Re: TYRE SIZES/PRESSURES FRONT AND REAR
« Reply #6 on: 29 March, 2017, 17:06:45 »
The above posts have just answered a couple of questions about tyres that were bothering me. Firstly, the low pressures on Vintage bikes. Members will see a feature in the forthcoming edition of The Star about a 1927/28 BSA Combination and the rear tyre looks decidedly low in pressure to me. I now know the reason why- Comfort!
Next was the Roadrider on the front as per my Thunderbolt and the 'ponderous' steering. This doesnt happen when the rear tyre (Roadrider aswell) is new but as the crown wears flat, the problem becomes more apparent until it gets to the stage when the ride becomes, lets say, less enjoyable. As Julian says this starts about 2500 miles and at 4500-5000, although legal treadwise, its time to renew. At 80 a pop this may be considered by some as uneconomic, but when the new one is fitted, the bike handles brilliantly. TT100's give the same effect.
Also, be aware, if considering to fitting a Road rider or TT100 to the front of an OIF B25SS or B50SS as its likely to rub against the wire mudguard stays.
Chris

MALCOLM.JOHNSON

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Re: TYRE SIZES/PRESSURES FRONT AND REAR
« Reply #7 on: 29 March, 2017, 21:30:50 »
Thank you for your replies. What prompted my initial post is that I have a s/a A10 which is a bit of a special, fitted with WM2 rims and shod with 19X4.10 (100/90)  Avon Roadrunners front and rear. Although I have only done about 400 miles on these tyres, all seems to handle well with plenty of clearance where required, but having two tyres the same has come in for some banter! If the tyres are a bad fitment, I do have an economical way out, as I am coming to the end of a plunger A7 rebuild which is now needing tyres for its WM2 rims. Do I buy a pair of smaller profile tyres to use on the fronts of the two bikes and use the partially used Roadrunners on the rears?  What tyre pressures should I adopt?  Rightly or wrongly I usually put in 2 bar, just to ease the fading memory cells!  :-\ 

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Re: TYRE SIZES/PRESSURES FRONT AND REAR
« Reply #8 on: 30 March, 2017, 07:13:52 »
Were it up to me (having a swinging arm A10 and a plunger A7) I would put the 4.10 on the back of the A7 and 3.25s on the front of both.
Both of mine have 3.35 x 19 Avon AM26 on the front and 3.50 x 19 Avon SM MkII on the back. The SMs come in for some stick but they aren't at all bad. Much better than part-worn TT100s in the wet as I discovered.

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Re: TYRE SIZES/PRESSURES FRONT AND REAR
« Reply #9 on: 30 March, 2017, 14:56:08 »
Malcolm,
Re the tyre pressures, I was advised by Avon Tech dept to pump up to 32psi rear and 28-30 front. This works weel on the Thunderbolt, anything less and the tyre wears in a strange way with the tread 'platforming' which I  think means, the chunks of tread wear down more at the front than the rear. However, the tyres are good to 44psi so its best to try what suits you best, i.e what you feel is comfortable.
Chris

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Re: TYRE SIZES/PRESSURES FRONT AND REAR
« Reply #10 on: 30 March, 2017, 18:52:45 »
It's interesting how recommended pressures have gone up so much over the years. The Pitman's book states 18 front and 20 rear, 28 psi if carrying a pillion. Has crossply tyre construction changed radically in the meantime? I can understand that you need higher pressures in the 28-32 psi range with radial tyre construction with very flexible sidewalls.
Does over-inflation reduce the contact patch significantly? I rather suspect high tyre pressures would adversely affect wet roadholding.  I compromise and go for 22 front and 25 rear. No odd wear patterns noted.