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[Mike Wood]

Hi

Aerodynamic problems on Olympic - diagnosis & fixes?

I see mention of well-known aerodynamic problems with the rear of the Olympic. and potential fixes, such as new venturis (see: http://www.vord.net/cars/rochdale/rochdale-2003.html).

My questions:

1. Aerodynamic tests?
Has as anyone done a wool tuft test - sticking lots of short lengths of wool over the cars bodywork looking at airflow while driving along an airfield? Has anyone ever put an Olympic, or model of one, in a windtunnel and carried out other aerodynamic tests? (unlikely I know, either in the 1960s or now, unless a racing team/academic had one and the ideas and connections to do this. If a Lotus or Tyrell engineer had one, who knows!)

Sounds like a fun project to investigate the aerodynamics of an Olympic for one of the many motorsport courses out there these days...


2. Aerodynamic solutions.
If it is rear end lift casuing instability (not rear suspension, weight distribution etc) - which looks quite likely from the top shape of the car combined with a smooth flat underside - has anyone tried fitting a small lip spoiler on the rear of the car, or even reshaped the rear end with something akin to a Kamm tail?

I'd have thought disrupting the lift on top of the car (while not inducing too much drag) would have been the first thing to try, before fitting venturis to an already flat smooth floor to try to speed up the airflow under the car and slightly reduce the pressure difference and hence lift. My guess is the top body top shape is more of an issue - close to an aeroplane wing profile (and smooth with it and flat underside), so generating lift.


I wonder what Lotus' aerodynamics guru at the time, Frank Costin, would have made of the Olympic, or even the designer of shape of the first Lotus Elite - Peter Kirwan? I wonder when the Olympic's designer moved to Lotus whether the Olympic's aerodynamics were ever discussed, as well as the weight/thickness of fibreglass compared to the Elite. (As well as how Rochdale could mould grp monocoques and Bristol Aeroplane Company had problems - which may have been more of an issue the less over-engineered lotus moulding, as well as Bristol's lack of grp experience).

Any thoughts?


Thanks
Mike Wood

[Mike Wood]

[Paul Gething]

My Rochdale used to get front end lift and go all weavy at anything over 70mph. So I fitted a VW Golf Mk1 spoiler on the front. It cured any instabilty and was fine at well over 100mph.

You can see it fitted in the attached photo. The slot above it allows air flow into the oil cooler.

[Mike Wood]

[keith hamer]

Some years ago Roger Coupe made a bib spoiler to fit the Olympic, and as far as I am aware he still has the mould. I have fitted several of these over the years and on my car it solved high speed stability.

I am currently working on a developement of this spoiler to be fitted to my lightweight which is now starting to take shape ready for next years race season.

[robh]

Hi Mike,

The only aerodynamic tests that I'm aware of were undertaken by Richard Parker on his “Phase 3”. The results, which were reproduced in ROC75 (Autumn 1998) suggest that the Olympic's “aerodynamic problems” are anything but well understood! The report is well worth reading – doubtless Alan (F) will be able to provide a copy.

As a very brief summary:
Front axle, at 100mph steady speed- ~25lbs downforce (yes, downforce!)
Front axle, max acceleration through 100mph- ~140lbs lift

Rear axle, at 100mph steady speed- ~30lbs lift
Rear axle, max acceleration through 100mph- ~40lbs downforce

Notes:

i)The forces quoted during max acceleration include the weight transfer due to the acceleration, i.e. the tendency of the car to “squat”.

ii)The lift at the front axle with max acceleration was more than expected. This suggests that the aero force on the front axle is quite sensitive to the pitch of the body. A nose-up attitude (when static) or soft rear springs (leading to the same at speed) would therefore be more likely to result in lift at the front.

iii)The behavior of the Phase 1 might be slightly different as the air intake feeds the whole of the under-bonnet area directly rather than just the rad duct/wheelarches as on the Phase 2 & 3.

iv)It looks mighty unlikely that your, or any other, car is going to take off!!!

My view is that the aerodynamic characteristics of the Olympic are such that they are likely to add to the effect of other sources of perceived instability but are, themselves, unlikely to be the most important factor. The real gremlins are more likely to be:

1.On the Phase 1, having the loads from the semi-Panhard rod reacted via one of the rubber doughnuts at the head of the radius arm (this doesn't seem like the right way to load this kind of bush) and/or knackered bushes at the axle end of the radius arms.

2.Having wheel camber/castor/tracking all over the place – sometimes a feature which goes unchecked...

3.Crap rear dampers (either worn originals or poor quality aftermarket items). A lack of proper damping at the rear will make any car feel awful.

This can be backed up, to some extent, by looking at cars which have a similar form to that of the Olympic. In particular, Porsche 356 and 911's from the same period spring to mind. These have the addition disadvantage of having the engine slung out behind the rear axle but owners still seem prepared to compete in them (let alone just take to the public highway!) without fitting the whale-tails developed for later cars. They all suffer from a combination of rear end lift and instability in yaw (an inevitable effect of the low and rounded rear) and, probably, to a similar degree. This is bad news for high speed handling (the re-call of the original Audi TT for the same reason is another, more recent, example) but a sense of proportion is required – aero forces vary with the square of speed (so the forces at 140mph are 4*those at 70mph) and so, if your car is wobbling around on the dual carriageway, don't blame the shape - fix the other bits!

Cheers,

Robert

PS. the original intention of the diffuser referenced in your message was to reduce/eliminate rear end lift and, hopefully, improve stability in yaw via a “fin” like effect of the end plates without altering the basic shape of the car. However, it's unlikely that the benefit will outweigh the effort involved in fitting it – one lives and learns....!

[leselliott]

Richard actually said during his tests that rear spoilers were inneffective at speeds below 150mph, hence why it was never a permanent fixture on the PH3 and probably why Tony S offered itb for sale.