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Targa Top

One of the biggest criticisms levelled at the Elise is it's roof. Both the soft-top and hard-top are good to look at but haven't reached a suitable compromise in terms of practicality and ease of fitting for a sports car that's driven in the UK. Since I like designing and building things I'm going to have a go at producing my own targa-top for the Elise.

Objectives

  • To build a three piece targa-top that can be fitted in less time than it takes to put a tonneau cover over the Elise (i.e. about 20 seconds).

  • To be fully waterproof, even when the car is parked facing uphill or downhill. My definition of waterproof is a lot more rigorous than that used by Lotus! It's funny how so many sunroofs leak when you park your car on a slope, they don't seem to test for this.

  • To weigh less than the current soft-top design. This should easily be acheived since the current cant rails are designed to take a stressed soft-top and are very heavy as a result.

  • To be able to store the panels behind the car seats such that they don't make any noise or do any damage to the cars interior, under any road conditions.

  • To be able to fit the roof to any Elise without making modifications to the car. I.e. you can always put the old roof back on if required. This means that it can be considered an accessory and not a modification.

  • To look 'good' and to be capable of being body coloured. Very subjective I know, but trust me I'm a designer.

  • To be quiet whilst underway (i.e. aerodynamic) and to generate negligable lift at high speed. Quiet means it doesn't rattle or generate excessive wind noise. There will be no sound deadening of engine or transmission noise, since weight is more important than noise.

  • To be easily produced on a production line such that it can be sold for less than £500. Assuming it makes it into production. Plan is to use simple construction techniques such as aluminium extrusions and GRP mouldings.

Issues

  • The seals will need careful design at the front and rear of the panel meeting point, to ensure there is no transmission path for water into the car.

  • The roll-bar cover may need a re-design to provide a suitable seal at the rear of the car. My first attempt is going be be with the existing roll-bar cover since I've visited the factory to see how these are made and it's going to be too costly to produce a new roll-bar cover. If I can't get the seal and look required then I may reconsider.

  • Some means of holding the roof down at the centre of the screen header rail is required and this may mean a permanent modification to the car if a suitable clamp or other type of fixing can't be found. I don't expect this to be a problem though especially if the roll-bar cover is modified.

Assumptions

  • That you either open the doors or wind the windows down to fit the roof since the seals will protrude slightly below the window line. This is no different than the existing soft-top and hard-top.

  • That it is made of a light-weight opaque material. Most likely candidate is GRP and glass is definately ruled out.If you want sunlight, then you take the roof off. I did briefly consider aluminium but it will dent too easily.

The Design

Originally I'd planned on a two piece design but the measurements are such that a three panel design is required in order for it to stow in the car. The tricky bit is getting the seals right along the top of the window and where it meets the windscreen. This is where the current roof has most of its problems. The roof must also be reasonably curved to allow more headroom than a flat roof would provide. I've even considered a twin bubble type style roof but I don't think this would fit in with the existing Elise lines.

The outer two panels have the equivalent of the cant rails (but lighter as they are not stressed) built into the outside edge. These panels will fit in the same way as the cant rails and using the same fixings except the rear edge which will rest on the fixing lug (not slide in sideways) and it will be held down using a sliding latch. Once in place the center panel will sit on top (but flush) with them and two clamps (inside the car) will compress the overlapping seals and hold the centre panel in place. These clamps will clamp under the rool bar at the rear and the header rail at the front. The whole process should take about 15 seconds.

With three panels there is plenty of space to fit the roof on the shelf behind the seats and a simple storage back with three pockets should do the job. This could be fixed down using existing mounting points (the current string bag) or velcro. It would still leave some storage space behind the seats and it also moves improves the cars weight balance with the roof stowed.
 

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