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Old 04-Jun-2003, 02:27 AM
RiggsyB RiggsyB is offline
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748 front tyre size

Ok... As most of us know, 748's (not sure whether they changed at all) come with 120/60 front tyres fitted... Wheras the 916/996/998 came with 120/70 tyres fitted, despite often coming with the same 180/55 rear tyre...

So my question is, should I keep with the original 120/60 size on my 97 748SP, or go up to a 120/70???

It seems to me that there is no difference in any of the cycle gear between my 748SP and its 916 equivalent (my mate has a 96 916SP for me to compare to)...

But is there different gearing on the speedo drive between the 2 bikes? And if not, is it the 748 speedo that is over reading, or the 916 speedo under reading?

I'm completely flummoxed by the whole situation, as it seems like nonsense to me that one bike should come with a different sized front tyre to the other, despite being almost identical (engine size being the only real difference)...

The main reason why I'm asking is that all the tyres I'm after seem to be much more readily available (and easier to get a deal on) as a pair when bought with a 70 profile front...

Hence, should I buy a 70 profile front tyre? Or will it mess my handling and speedo up?

And if it doesn;t mess my speedo up, could I just drop the forks through the Yokes by 5mm to negate any height gained by the extra tyre sidewall height, or is it a little more complicated than that?

PLEASE HELP!!! (rear tyre is almost bald, so I need to order them today really!)

cheers
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  #2  
Old 04-Jun-2003, 11:20 AM
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DSC Member Guido Guido is offline
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Riggsy,

Not up to speed with the susp' bits you're asking about but my 2001 748R came with 120/70 as standard
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Old 04-Jun-2003, 01:35 PM
RiggsyB RiggsyB is offline
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Cheers Guido...

Anybody else with any more advice??? It'd be greatly appreciated...
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Old 04-Jun-2003, 01:50 PM
sevenfourate sevenfourate is offline
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RiggsyB,
Im possibly the least technical guy on here - so dont ask me why,but if you do a search (old board i think) you will see a post regarding front tyre profiles and i think the general consensus was that a 70 profile was the one to go for.
There were explanations and more info amongst the various threads.I think 90%+ of the riders on here on 748/9*6 + 998's use the 70 profile.

Take a look using the search facility.

Regards Syer.
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Old 04-Jun-2003, 09:01 PM
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Jon Jon is offline
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I changed from the 60 to a 70 profile on the advice from Monty. Did'nt care for it much at first, both tyres front and rear where changed together. Got the rear right over with no chicken strips, wher the front still had 10-15 mm. Once I was let loose on the track though managed to get the front well onto and beyond the edge. My mind is set now on a 70.

Jon
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Old 05-Jun-2003, 01:34 PM
RiggsyB RiggsyB is offline
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Decided to go for a 70 profile in the end upon the advice from the guys at John Hackett...

Anyway, I've been going off the edge on my 60 profile front tyre, yet have 5mm wide chicken strips on the back... Apparently the 70 profile front tyre will enable me to lean further as the profile is steeper... So I'll have no excuses for rear chicken strips soon!

Oh, and the 70 profile helps sharpen the steering too apparently...
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Old 06-Jun-2003, 06:53 PM
adrian748 adrian748 is offline
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Don't forget to set the ride height, the 70 profile tyre is 10% taller so will increase the front ride height by 12mm.
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Old 08-Jul-2003, 06:17 PM
rhubarb rhubarb is offline
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Sorry to bring an old thread back to life but can you tell me whether or not fitting the 70 series in place of a 60 affects the speedo or not. If so can you do anything about it?
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Old 08-Jul-2003, 06:26 PM
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DSC Member Shazaam! Shazaam! is offline
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Try it and see if you like it. It's really a question of handling preference, but there are some side issues.

Keep in mind is that (unlike the back tyre), the front tyre wears most quickly at it's edges. So over time, the profile gets progressively steeper and the steering gets more tippy; that is, it falls into the corner more readily. Starting with a 120/60 or a 120/65 section tyre reduces this wear-induced effect.

Further, a lower 60-profile tyre has a shorter height so it doesn't flex as easily. This increases the severity of the loads transmitted to the wheel rim, suspension and handlebars. This in turn, increases the likelihood of alloy wheel damage, so a 70-series tyre will be kinder to magnesium wheels.

Your tyre s are an integral part of your suspension so when you go from a 60 to a less-stiff 70 tyre, the resultant handling change will probably benefit from front suspension setting modifications. Increased front compression damping may be in order to offset the less-stiff 70-series tyre. Also, turn-in is slower with the 120/70 but this is easily compensated for by adjusting rear ride height

Also, the increase in the outside diameter of the tyre will cause your already inaccurately high-reading speedometer to read more accurately and will change your ride height so you may want to compensate by repositioning the front forks.

The decreased loads to the handlebars and increased sidewall flex combine to give you less (better?) feedback on the state of the front suspension and the traction condition at the front tyre.

All this may better suit your riding style. Or not.

Some, like myself, find a less-nervous front end to be more appealing for the road so we run a 70-series tyre. On the track, the better road surfaces make the benefits of a lower profile 60-series tyre more obvious.
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Old 08-Jul-2003, 11:13 PM
Gannet Gannet is offline
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I've tried both profiles and I prefer the 60. I guess that the tyre manufacturer will also affect the way the bike handles, as different manufactures will lay-up the tyre differently. I use Michelin Pilot sports, basically because I can get them dirt cheap (80 for a pair)

Regards,

Gannet
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