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Old 22 Aug 2017, 16:27 (Ref:3760978)   #1
johnfm
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Formula junior - tyre sizes

Chaps

Current FJ tyre sizes are L15 450 front and L15 500 rear.

Given that the HP tyres fitment guide suggests a range of 'permitted' rim widths, are there any obvious reasons why I shouldn't look to put wider tyres front (say to 500) and back (say to 550) to increase contact patch and grip?

I expect I am missing something obvious!
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Old 23 Aug 2017, 10:13 (Ref:3761136)   #2
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Originally Posted by johnfm View Post
Chaps

Current FJ tyre sizes are L15 450 front and L15 500 rear.

Given that the HP tyres fitment guide suggests a range of 'permitted' rim widths, are there any obvious reasons why I shouldn't look to put wider tyres front (say to 500) and back (say to 550) to increase contact patch and grip?

I expect I am missing something obvious!
Originally they would probably have been 450 x 15 & 525 x 15.
The 450s aren't a problem but you can't get 525s so it seems to be accepted that you use either 500s or 550s which are available.

Fitting wider tyres has a downside in aerodynamic and rolling resistance, so it might be that they don't offer an advantage over a whole lap.
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Old 23 Aug 2017, 10:58 (Ref:3761149)   #3
Mike Bell
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Permitted rim widths isn't same as 'optimum' rim width. The fitment guide usually shows this along with the 'fitted' dimensions and revs per kilometre or whatever. As well as the extra drag and rolling resistance mentioned by Peter, the gearing could be affected by larger tyres, and if the tyre is much wider than the optimum for the rim, you'll likely not get to use all the extra tread as it won't be flat to the tarmac as intended.....

I would guess there is a reason the majority of competitors use particular sizes- it could be that experience has proved them to give the best results!
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Old 23 Aug 2017, 16:15 (Ref:3761216)   #4
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If you are lucky enough to achieve more grip there is then the extra stress on the (often fragile) steering and suspension to consider. Besides, isn't it more fun to slide?
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Old 23 Aug 2017, 18:40 (Ref:3761241)   #5
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All very good points - I suppose this is part of the fun! Will stick with the 'optimum' size for the rims.
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Old 24 Aug 2017, 12:34 (Ref:3761365)   #6
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ahem

apart from technical aspects of what works how and is quicker..

how about a look at the regulations ?

to get an HTP, which is requirement for at least some Junior races, such as the Lurani and Goodwood
you need to adhere to the original specification
OF THE CAR IN QUESTION
or justify the changes (difficult)

you cant just simply find the quickest wheel / tire combo and fit it to the car
on paper at least....

what is done in Juniors and how this gets policed is a different matter
and how long this free for all attitude will last is even more of a different matter

fascinating, that this was not worth mentioning in this thread before

RuE
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Old 24 Aug 2017, 15:05 (Ref:3761380)   #7
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Originally Posted by Rudernst View Post
how about a look at the regulations ?

to get an HTP, which is requirement for at least some Junior races, such as the Lurani and Goodwood
you need to adhere to the original specification
OF THE CAR IN QUESTION
or justify the changes (difficult)

you cant just simply find the quickest wheel / tire combo and fit it to the car
on paper at least....

fascinating, that this was not worth mentioning in this thread before

RuE
Mmm, I looked at both the FJHRA and Lurani technical regs before replying, and neither states that particular tyre sizes are required. Wheel dimensions are obviously an individual car HTP (or whatever other papers are required) question, and the original post only talks about alternative tyre sizes, not wheels.....
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Old 24 Aug 2017, 16:06 (Ref:3761390)   #8
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I think wheel sizes and tyre types are controlled in FJ but not, aside from general safety provisions, tyre sizes. In any event, I ran tyres that were one size up from most of the other Lotus 18s when I raced in FJ a few seasons back, largely because that was how the car came to me. I am not aware that it made a significant difference either way
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Old 25 Aug 2017, 10:20 (Ref:3761498)   #9
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Originally Posted by Rudernst View Post
ahem

apart from technical aspects of what works how and is quicker..

how about a look at the regulations ?

to get an HTP, which is requirement for at least some Junior races, such as the Lurani and Goodwood
you need to adhere to the original specification
OF THE CAR IN QUESTION
or justify the changes (difficult)

you cant just simply find the quickest wheel / tire combo and fit it to the car
on paper at least....

what is done in Juniors and how this gets policed is a different matter
and how long this free for all attitude will last is even more of a different matter

fascinating, that this was not worth mentioning in this thread before

RuE
Being a lawyer, that was the first place I looked.

The tyre regs are only strict as to the need for CR65 (or CR48 in the case of the L15 450).

If you can point out where the rules restrict tyre size selection I'd appreciate it.

Thanks.
<B>
8.1. General

</B>
8.1.1. Formula Junior cars must use either Dunlop Vintage range tyre R5 pattern or older, or ‘L’ section tyres which have 204 compound and tread pattern CR65 or in the case of size 450 L 15, the CR48 (R6) tread pattern.


8.1.2. Cars which have wheels with rim widths of 3.5 inch (88,9mm) or less and for which there is no Dunlop Vintage range tyre of a suitable specification available, may use tyres offered for sale as cross ply or radial road tyres, with an aspect ratio of 75% or more, having a speed rating of ‘S’ or above and which have ‘E’ or ‘DOT’ approval. The manufacturer should be consulted as to the suitability of the tyre for the event.
Note:
This only applies to some Stanguellini, Volpini and other early Italian cars.

8.1.3. Rim widths must not be increased but may be decreased in order to accommodate available tyres. Due regardmust be paid to the impact upon the resulting vehicle track, refer to 6.18.1.

8.1.4. The interior of the tyre (space between the rim and the internal part of the tyre)must be filled only with air or nitrogen. This does not preclude the use of inner tubes.

Prohibitions:
8.1.5. It is not permitted to use any tyre heating/heat retention devices.

8.1.6. It is not permitted to use tyre or compound treatments.

8.1.7. It is not permitted to use Dunlop M section tyres
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Old 25 Aug 2017, 12:27 (Ref:3761522)   #10
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Rudernst should be qualifying in the top 5 on the gridRudernst should be qualifying in the top 5 on the gridRudernst should be qualifying in the top 5 on the grid
There are layers of regulation

foremost :
Appendix K and the rules / principles behind the HTP system
the idea behind the HTP is that the cars have to be technically identical to what the were in period.

consequence is that no regulation regarding tire sizes iand wheel size is needed, they have to be the same as per original period spec RACED CONTEMPORARY IN INTERNATIONAL RACING, end of story unless AN EXCEPTION is allowed,
this is why you wont find "allowed tire size regs", they are just not necessary and would be redundant
An allowed exception from the principle going down in wheel width is allowed (a deviation from period spec )as this will only make your car slower, to help cars with hard to get tires and wheels

another notable deviation allowed in Appendix K are ally rads instead of brass or whatever was used in period,
apparently is gets more and more difficult to source brass rads as the skill making them is dying

there are many other details that are not spelt out either,
a HTP specification is checked against available info on period cars
and if is found to differ, you get a query and no HTP

I had a missing bonnet opening rejected once on a sports racer
we had to cut the hole in to get the HTP

in my opinion the HTP system is workable on homologated cars
because is easy to check HTP application against the original homologation document
it gets difficult on badly documented one off obscure single seaters, for instance, because the question is: what was the original spec ?

now Junior regulations are fascinating because they expressly allow several modifications that are agains the principles behind the HTP system such as 1100 cc Ford engines in Lotus 18 in class C, because this combination can not have existed in period as 1.100 Ford were not raced on those years.

This means that currently Class C 1100 cc Lotus 18 Ford cars are legal to race in Formula Junior, but strictly speaking they should not get an HTP, at least not for that period.
This has been noted and the discussion is ongoing, expect some surprises.

Now currently an HTP is not needed in the UK Formula Junior Championship, there You can race with an HSCC VIF,
but for the Lurani Championship an HTP is required.


interesting, isnt it ?

RuE
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