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Thread: why 13 inch wheels?

  1. #1
    Join Date
    Jul 2018
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    9

    Default why 13 inch wheels?

    Forgive me if this is a topic that has been covered to death but...

    I have a 2015 Caterham 280 with 14 inch wheels. I have read that it would be better to track the car on 13 inch wheels. Why is this true? Lower center of gravity perhaps? And if I were to buy 13 inch wheels, what specs would be appropriate, and would I need to change other things in the car such as speedometer settings?

    Thanks,

    Rick Ratliff

  2. #2
    Join Date
    Feb 2009
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    You may not find it as as pronounced as me going from 15 inch wheels to 13 inch wheels but....

    The 13 inch wheels are lighter. The reduction in unsprung weight allows the car to handle better and on my cars seems to improve the acceleration slightly. I find the ride is better with a thicker sidewall/higher profile. The breakaway predictability at limit is improved/less sudden. I slightly improved gearing points

    The downside, is that the rubber selection in 13 inch wheel sizes is somewhat limited although probably about the same or better that you experience with 14 inch wheels.

    No impact on speedo unless the rolling diameter changes. For my example, my CSR came on 15 inch wheels and (I think...its before my morning coffee) a 45 tire profile. There are web calculators that allow you to calculate the tire diameter with wheel and rubber. Knowing that then you line up your proposed 13 inch wheel and tire combo and measure the tire diameter. Ideally they will match. However, in the real world, they will not. My fronts were a 1% smaller diameter but no impact on speedo. My rears were about 0.6% smaller. Given the Caterham speedometer is one of the most inaccurate known to the motoring world and consistently reads higher than GPS with an error rate of 3 to 8% depending on speed (thanks to the Hall effect sensor), my shift just made that slightly worse but by only 6/10ths of a mile over 100 miles. I felt that was an acceptable error rate especially as I improved the gearing points when on track. You can buy the kit to fix the speedo but it depends on if the error rate really warrants it.

    However, in your case, you are on 14 inch wheels. The change will not be as profound. If you are not going to track the car then I would question the need for it as you may not notice the difference on the road so much. One possible reason for you to evaluate 13 inch wheels is the range of tire options you have available to you in the 14 inch size. If the choices are not good then shifting to 13 inch does get you into a small range of very good tire options that suit the Caterham well.
    Mike
    2010 Caterham CSR with Cosworth 2.3 Duratec
    2018 Caterham 420R with 2L Duratec 210hp at Donington UK
    1975 BMW CSL Group 4 (restoration - engine and dry sump install time)
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    and another project car coming....

  3. #3
    Join Date
    Oct 2006
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    FWIW: The Dunlap Direzza ZIII (catchy name, no?) tire comes in a 14" size and is supposed to be a good street/track tire compromise. Toyo makes a pretty aggressive 14" tire as well, the Proxes R888, that is more track oriented. Not sure either will be suitable for your use.

    Also FWIW: I have tried 15" and 13" tires and haven't noticed a performance advantage, although I would defer to Croc who has done many more Caterham track day laps that I have. I am mostly making his point that the differences are subtle.

    Currently, I am running 15" on the back and 13" on the front as that is the way the suspension was set up for my track-only car.
    Kitcat:AKA, Sir Spinsalot
    '97 Caterham Super Sprint, 1700 Crossflow-sold
    '09 Birkin S3, Duratec-sold
    '03 Caterham Zetec track car
    '19 Honda Type R, sold
    '19 Miata Clubsport

  4. #4
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    I've gone from 14" to 13" and the biggest differences I noticed were identified by Croc: improved breakaway characteristics and ride. I read an article many years ago talking about why, aside from the rule book, F1 cars were on 13" wheels. According to the author, very light vehicles run into issues with short, stiff sidewalls due to the low air pressure required. As the car corners and the tire deforms, that deformation takes the path of least resistance. If the tire has short, stiff sidewalls and low air pressure, that deformation happens in the tread which buckles in the middle, decreasing the contact patch. If, however, the sidewalls are taller and softer, it happens in the sidewall as tire engineers intend.

    -John
    '95 Westfield SEiW w/2.0L Duratec
    '68 Lotus Elan FHC
    '91 Miata w/Flyin Miata suspension & brakes
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  5. #5
    Join Date
    Feb 2012
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    FWIW I put on a set of The Dunlap Direzza ZIII 14" recently. They are like glue compared to the 8 year old all season tires they replaced. I have no plans for track or autoX, so I can't give you any feedback on that subject. I don't expect I'll wear then out since it's a fair weather car and so light. The only item of note is they pick up more pebbles and stuff.
    Life happens while you're making other plans
    1995 Caterham Crossflow RHD
    1990 Miata
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    JOHN DEERE D130

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