Ian Foster

M It's been covered previously on this forum. Ian

Bob I decided to add the additional leaf in the centre of the spring, so calculated the length accordingly and estimated the curve required based on the original leaves. Thickness also to match the original.

Bob On my GT6 Mk2, I always felt the rear springing was too soft (see chart above) and allowed big bumps to get through and too much roll. I tried SPAX adjustable shocks, but this just made the ride more uncomfortable. When we rebuilt the car in 2010/11 I reused my original spring, but with Koni 80-1717s dampers fitted with a bespoke bracket, which also corrects the fore/aft alignment (as posted previously). I wasn't happy with the ride height so bought a new replacement spring, which I initially fitted with a lowering block, but it made very little difference and quite quickly relaxed. I then reverted to my original spring and added an additional leaf in the centre (now 7 leaves) and the ride is actually really good now. Owen Springs made the additional leaf and actually made a pair (what cars have just one leaf spring eh?) and the second one now resides in Nick Jones's GT6 Mk3. The roll stiffness is much improved and the car has the stance that you see in period shots of the car where the wheels sit close to vertical. I think it OK to modify springs to get what you need, rather than looking for modern replacements, which may not deliver. Ian

Paul If you've already gone to the trouble of removing the tunnel, I would change the cable as a matter of course. Ian

John I think we can safely say 'that has been slammed'!! Ian

That's call thread drift. Triumph forums are well known for it. Fun eh?! Uncle I like your design and it a further refinement of where Triumph managed to get to in the mid 1970s. I don't have enough space to take the feed pipes across the face of the box as I have 175CD carbs, hence the bulbous affair on the front of my mock-up. John Aluminium honeycomb is the most efficient way of producing sandwich construction and is probably best suited to reasonably flat panels. It can suffer from delamination if in high performance yachts where they experience severe slamming. The marine industry typically use end grain balsa in deck mouldings and foam in sandwich construction hulls. Ian

Taggart You just might be opening a can of worms here, but for what it's worth here's my 2d's worth:- Since 1977, I have had a pattern OEM Mk2 box, a Mk3 box and three twin silencer systems on my Mk2 (the first two twins were TT offerings in mild steel and eventually rotted out) and I am currently using twin stainless silencers and link pipe of unknown origin, with a mild steel centre silencer. The silencer system was second hand and was 'thrown on' after a rebuild in 2010/11 and somewhat surprisingly is still going strong. The silencers mount onto to the inside face of the rear valence. The tail pipes are 2" diameter and have a perforated acoustic liner. I think the centre silencer is quite important and on mine, the sound is really quite nice and doesn't drone. Back in the late 70s, a friend had a semi-sports silencer on a Mk3 and it was quite loud. I would suggest that you try and experience the twin Bell system in conjunction with a centre box before you commit. Ian

John Yes blue rigid closed cell foam as used in the construction industry. Epoxy resin will bond to it, but does not soak in. For nice parts creating a good finish on the pattern is required, as is multi layers of special release polish. I have used polish on brown parcel tape surfaced hardboard for one-off flat components. If you are really interested, this is a link to part 1 in a tutorial series by Easy Composites, showing pattern making. https://www.youtube.com/watch?v=XeeSsmsm9h0 Subsequent parts in the series cover mould making, laying up the part, vacuum bagging and curing. For home produced parts it is possible to omit the vacuum bagging, but the risk of bridging and air bubbles increases. Not all epoxies need to be post cured (with heat), but this is usually required for structural components. Aerospace and other high tech industries use autoclaves which provide heat and pressure. It's a fascinating subject, but for one off parts quite a lot of faff if you go the whole hog. Ian

John You need to be careful with the choice of foam. I used a blue rigid foam on the rudder build, gluing blocks of foam into a thin ply structure a bit like a model aircraft wing. The foam was then shaped to the aero foil ribs and then sheathed in epoxy/E -glass laminate. Carbon reinforcement would have been classed as an exotic material under the rating rule and attract a rating penalty. It was extremely light and worked really well (hydrodynamically) replacing heavy, old fashioned glassfibre shell construction. I might progress along the lines of making the box fit a standard backplate, so that might be of use to others currently using a standard airbox arrangement. Business opportunity...I'm not sure I want to be knocking out multiple units, but you never know. Ian

Carbon/epoxy composite is just a stiffer lighter version of glassfibre. Carbon airboxes are used in all sorts of performance and competition situations and I cannot see that resonance would be any sort of issue. Ian

Looks as though we have thread resurrection...well done someone! The design will require a split mould, ie built in sections and joined with flanges, to prevent it locking onto the moulded part. If I do follow the composites route, the mould could be used multiple times. The really time consuming bit will be creating the plug (the foam and filler bit) with a good enough surface finish so I'll be satisfied with the final mouldings. Being a retired Structural Engineer with well developed OCD, does not help in this regard. I've done a fair bit in composites, from early teenage days building canoes, an E glass and epoxy sheathed foam cored yacht rudder, to various carbon fibre elements, including a mast instrument bracket, cleat bases and the heat shields for my GT6 carbs. Carbon and epoxy are much nicer to work with than regular glassfibre. Previous research suggested that the airbox volume should be equal or greater than the engine capacity. My current design has an internal volume of approximately 3400 cc, so should be big enough for a 6 cylinder car. The opening to the box and the filter are designed around a 70mm diameter pipe. My Honda S2000 has a 80mm pipe from the airbox to the throttle body and that produces nearly 240 bhp. I have pondered on the issue of getting sufficient air to the rear carb and have refined the latest, latest design to incorporate a ramped base as shown by this photo from the rear. Still work in progress and it will probably a next winter project. I would like to do it before getting the car RR tuned. Ian

Ooops!

My current thinking regarding the airbox as described above. The large diameter (circa 70mm) single pipe is achieved by flaring out the box at the front end where the wheel arch allows. If I make it the right depth it can use either my existing K&N filter elements, or a remote filter at the side of the radiator. Ian

Late to the party on this one, but there was a similar thread in late 2022. My contribution at the time:- I think Triumph struggled with their twin carb airbox design, as there are many iterations from the early cars to later Spitfire and Dolomite boxes. The late saloon box has an enlarged box with a centre bottom feed. My GT6 Mk2 already had 175 CD2s on it when I bought it in 1977, but with an empty original airbox. Clearly a case of someone just slapping bigger carbs on. In my ignorant youth I put on a pair of wire gauze pancakes, which probably didn't filter or flow well, but made a lot of sucky noise. I subsequently bought a pair of K&Ns which I still have to this day which actually work well giving decent power and economy, but I am troubled by the fact that they are probably drawing hot air when the car is not moving. The early GT6 box had a pair of open tubes in the front and by the time they got to the MK3, Triumph had added the feed pipes. This arrangement seems to favour the front carb and there is a rectangular hole in the bottom of the box under the rear carb which is perhaps intended to compensate for this (or to let water out). The later Spitfire and Dolomite boxes have indentations in the cover (and presumably unequal length tubes internally) to distribute more air to the rear carb. There isn't space on my GT6 to do this. The TR(250) boxes seem to have various slots on the sides of the box, perhaps to address this issue, but surely this then also allows in hot air in when stationary. I have made a mock up of a new airbox to enclose my K&N filter elements, which flares out forward of the front filter (making use of the shape of the wheelarch) in order to accept a larger diameter single air duct into the radiator cowl. I have yet to commit the design to metal/glass fibre/carbon fibre, but it might just happen this winter. Ian

Slightly worrying if they are on the drivers door! Definitely no pimples on mine. Ian