Upgrade your kit car with a freewheel and a range of easily changeable sprockets, allowing fine tuning of your gear ratios.

The standard kit car drive train is robust yet simplistic, however it will not win many races. Changing your motor sprocket is simple enough, but what about your axle sprocket? Do you have time to pull the rear axle during race day?

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The GPMods split sprocket and freewheel solution is built around a sprag clutch (one way bearing), and a common chain ring carrier part. Split sprockets are then bolted to the carrier as needed, and are easily changed between practice and the race, or between races.

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Many teams change their motor sprocket as needed, but this is a very coarse change to the final drive ratio. Using a range of axle sprocket sizes combined with a variety of motor sprockets allows a fine tuning option, giving the effect of changing the motor sprocket by a fraction of a tooth rather than by a whole tooth.

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For example, changing from a standard 72 axle 12 motor ratio to a 72 axle 13 motor changes the final drive ratio (FDR) from 6.00 to 5.54, which is a change of 8%.

Using the GPMods split sprocket set would allow a smaller percentage change to fine tune your ratio. For example, the 81 axle 15 motor FDR of 5.40 could change to the 80 axle 15 motor FDR of 5.33. This step change is just 1%. Finer incremental changes to your ratio will allow your team to tune your car to a circuit, which could be the difference between finishing with energy to spare or crawling to a halt in the final 10 minutes.

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The standard kit car drivetrain has no freewheel, which means that any time the throttle is lifted the car slows significantly as the axle turns the motor. It is better to freely coast where possible rather than decelerate the car, and this is the purpose of the sprag clutch freewheel.

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Standard kit car chain is over engineered for the job. The GPMods split sprocket and freewheel solution uses 8mm pitch chain and sprockets. This chain is able to deliver in excess of 2000W at the motor rpm values produced by a Greenpower motor. Greenpower motors produce a lot less than 2000W! Smaller chain produces a reduction of mass, and also allows more compact sprockets to be used for a given number of teeth, or more teeth for a given space available.

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A further advantage to the GPMods split sprocket and freewheel set is lower mass. The standard Greenpower axle platewheel is a solid steel 1.5kg component! The GPMods setup comes in at under 700g including the freewheel.

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First developed by team RLR on their car RLR 2 (2nd overall 2015 championships), this system has proven to be both reliable and useful. The same system was then fitted to RLR K (3rd in kit car final 2015 and 2016, 2nd in 2017 kit car final). Now very similar designs are used on the quadruple world championship winner RLR 3, and on world championship winner RLR 4.

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The GPmods split sprocket and freewheel set includes:

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• Steel axle adapter sleeve (CNC machined)

• Sprag clutch

• Steel sprocket carrier assembly (Lasercut)

• Steel split chain rings (Lasercut)

• Range of motor sprockets

• 8mm chain and split link

• Fittings (nuts, bolts, circlips etc...)

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Customers have so far requested quite a range of sprocket combinations! For 2023/24 the GPMods split sprocket and freewheel set is available with a choice of motor sprockets ranging from 15 teeth and up (you decide!), and axle sprockets from 77-81 teeth, without taking up any more space than the standard setup. A 14 tooth motor sprocket is available at additional cost, but a well set up car shouldn't need to use it unless running on a very tight circuit.

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The range of motor sprockets suitable for your car depends on your motor type (old spec vs new spec), wheel size and race track. The new spec motor has a higher RPM sweet spot than the old spec motor; a 20 inch wheel needs to turn more slowly than a 16 inch wheel for the same vehicle speed; the Hull Street Race will need a much lower FDR than Goodwood! In addition, your car needs to be well set up - tyres, tracking, bearings and aerodynamics - will all affect the performance of the car, and the correct FDR is going to be dependent on all of the above. Please email to discuss your setup and I will be able to advise you. Please be aware that a GPMods sprocket set will not by itself be able to cure a poorly set up car or a poor ratio choice! It will give you the benefit of a freewheel, lower mass, faster setup changes and fine tuning of your final drive ratio. The rest is up to you!

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Pricing:

Kit comprises parts listed above plus your choice of motor sprockets:

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Kit plus 3 motor sprockets £425

Kit plus 4 motor sprockets £450

Kit plus 5 motor sprockets £475

Any additional sprockets are £25 each.

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For further info email Ben at info@gpmods.co.uk

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