Okrasa Special

[Barry]

And the new coach builders (Mouland & Yates) visit next Saturday - a long story but ultimately the best choice. They did the Auto Union and Mercedes Silver Arrows so know what they're doing, are enthusiastic about the car, 'get' what we're trying to build, and have a slot late in the year. All good progress…..

Good progress indeed . You're certainly playing with the big boys in the coach-building world, and their work will be some of the best on the planet (literally). I visited them in 2014, and Vic was kind enough to give me a quick tour. The overall shapes and wheeling were exemplary, you'd expect that, but one detail really stood out as it just happened to catch my eye. It was, I think, a tiny recess for a bonnet stop. It looked as though it had been CNC machined from solid, it was that perfect. In fact it was simply a small fabrication TIG welded together and into place, before being lightly linished. A detail not much bigger than a large acorn, but one that I remember with a grin. Lovely.

I'd say that if they confirm your place, we're all in for a bit of a treat .

[steve wright]

Indeed Barry. When I was there late last year they had two wonderful projects in train and the work was a joy to behold. I looked through their portfolio of photos (pre-war Auto Unions, Ferrari Sharknose, Bentley Blue Train Special) so the Special should prove to be no problem for them

Gary and Vic visited last Saturday to look at the car and discuss details, and confirmed they are taking the car towards the end of the year. We spent a good chunk of time chatting about design details and possible solutions to some of the thornier issues which they had good ideas on. They suggested building it using 20 gauge aircraft grade ally would suit a race car - besides saving weight it's stronger than usual grades. We also agreed to build it like the Lotus of the period, quickly detachable panels with the rigidity created by the panel shape, so we should get an end product that's light and strong. Now all we have to do is get a completed car to them to clothe!

[Barry]

That's excellent news, and music to your ears I'm sure.

Roll on later in year then ....

[Pandora]

Indeed!

Al

[steve wright]

Good progress this month. a few giggles mocking the mounting of the twin superchargers for motor number 2…


Then more serious mocking up to ensure the Solex P11's don't foul the roll cage support for motor number 1…


Then motor out (thanks to Ian and Alex for the heavy lifting…) to see what an NOS Okrasa engine looks like…


And yes, it needs to come out the back window as the flat floor and chassis rails mean it can't come out underneath!

Pretty uninspiring as it turns out but look at the flywheel lightening, beautifully done and again evidence of the racer/engineering intent of the original builder…


Here's the motor we're chasing. Darrell Vittone's 170hp fixed pedestal Okrasa motor - more technology than we're allowed for historic racing but an inspirational motor nonetheless…


Speaking of inspiration, a recent visit to Prototyp museum provided an idea for cooling vents from the Auto Union cars, fortunately the replica's were built by the same guys doing the bodywork on the Special, so they still have the tooling from the first job!


Because of the Karmann Ghia roof we're able to use the original KG glass as templates for the Plexiglas except for the front windscreen which is a new one, which makes it a whole lot easier to draw up the shape of the windows. It's about the only time the roof has provided an advantage in the build - in every other element of the design it's been a complete pain in the arse. Here's original glass being used to mock up the vent holes…


I noticed the wheels on the RS61 when we visited the Porsche museum last month had an additional set of ventilation holes drilled in the hub (apologies for the rubbish photo, snapped in haste as it was being loaded on a transporter …


So… following the factory I did the same thing to the drums…


We've now taken almost a kilo from each brake drum assembly, which is a massive amount for rotating unsprung weight.

This weekend we have an 'Okrasa Special workshop day' with a host of mates and partners coming over, more soon….

[KS]

Cool as f...

[Pandora]

Steve 'Hole saw' Wright. If it's standing still, he'll drill it!

Looking good. The twin superchargers look fun!

Al

[neilbardsley]

Looks good thank you for sharing but having two super chargers is just greedy!

[steve wright]

yes nothing is safe Then again, obsessive attention to weight reduction never did any harm! Neil we're having to go with two blowers as one will only suit standard engines up to 1300cc. the larger 142 Shorrock will take up to 2 litres of capacity, but only revs to 5k. The smaller blower (the C75) runs to 6,500rpm. So we're trying to have our cake and it it too by getting a higher rev limit with the better breathing capability of two blowers. Otherwise using either of the single blowers means we run out of puff pretty quickly without realising the potential of the motor Ian is now building.

Think I've located a ZF-style diff which would be a major step forward in improving the handling, but need to chase this down before I can celebrate. What we have managed to conclude on is lightening the gear set in the gearbox. New ratios to suit the motor, but will be significantly lightened to reduce hp loss in gearbox, throttle response translated to the wheels, and again, reduce weight.

Will update next week on progress over the weekend….

[Winston Teague]

Wow 6500 is a sh*tload of revs for any vane blower, and 1 c75 is quite a big unit for an 800cc twin, assuming you're building a 1600cc motor. What you propose may never turn out to need 6500 as it should have monster torque low down.
Please ask yourself when drilling-for-lightness becomes drilling-for-weakness from time to time. It was quite a shock to have to steer around a wheel on the racing line in Brooklands at vintage Silverstone in April. It had been drilled to save unsprung weight, and failed through the drillings. The nearer to the centre of the hub/drum you drill, the greater the weakening effect, and that weight becomes more static than dynamic in rotary terms. The standard 356 hub/drum is demonstrably strong enough but I don't know whether it is a casting or a forging. If a forging, surely its strength is, to an extent concentrated at its surface. If a casting, it could perhaps be lightened by thinning, which wouldn't look as cool, or let the rain into the brakes, but it also wouldn't provide a set of postage stamp tear here holes either. It might be interesting to drill an otherwise scrap drum, calculate the cornering load, express it in lever length and try to load up the drum (in a press?) and measure distortion and failure load. Have you got lateral g info from your in car kit?
All the best, W

[steve wright]

Thanks Winston, great questions and advice. The Shorrock will stand higher revs than virtually any of the other blowers of the period except the Pepco apparently. The period brochures say 5,000 rpm for the 142 and 6,000 for the C75, but Derek Chinn who is rebuilding them and a guru in blower rebuilding and development has made tweaks that will allow the C75's we're running to spin to 6,500rpm for short blasts. The motor will be 1,500cc but highly developed from standard. We're planning something pretty special for the car hence the reason for two blowers, but I'm not going to give any more away until the build is further down the line and we've got some confidence that the twin-blower set up is even going to work. It might turn out to be a very expensive fantasy!

Your comments on the drilling is obviously based on experience. I can't answer your (good) questions on cornering load nor do I have any ability to measure distortion and failure load. I'm simply going on what the factory did with it's race cars which was typically quite conservative and well tested prior to racing. It might be folly of me to take this route. Interestingly the RSK and RS61 used both the thinning method and drilled. There doesn't appear to be any rhyme or reason other than both were being tried. It looks like the Carrera drums were only drilled, and usually around the outer edge, but there are some with holes closer in as I've drilled mine. The later race cars like the RSK & then RS61 had both.

I've held off drilling a bunch of things (pedals for instance) - even though I'm tempted I've seen too many broken brake pedals to do this. Anything structural I'm leaving (e.g. trailing arms) which I've seen drilled in plenty of 1950's race cars including a bunch at the Prototyp museum.

Great comments and input. What's driving me is being acutely aware that the car is going to have a tiny motor - our aim is 150hp from 1,500cc which is hugely ambitious for a naturally aspirated motor and one as weak and ill-suited to being developed as this. So the more weight we can take out the better obviously. I would love your insight into where you think I can save weight (or not as the case may be)!

[Winston Teague]

If Derek says it'll be ok that's good enough for me! The rest were idle lunchtime musings. As far as weight saving goes I think it was summed up by Arthur Mallock (?) "nothing weighs less than nothing". So if you don't absolutely need it leave it off. It should go very well with 150 bhp, especially if the blowers work, you'll be able to pull a tall cwp. Great stuff. Keep it coming.
W

[steve wright]

Thanks Winston. The 150 target is for the normally aspirated motor. We're getting 176hp from 1600cc from the Porsche motor, so 150 from 1500cc should be doable. If we can keep it from going bang the blower motor should be substantially higher The latter motor we're building will be a grenade motor - only needs to last for a short period to do it's job….

In terms of weight, the 1958 RSK was 530kg's so with the roof I'm hoping the target for the car is 550-600 kg's. Any more than this and we've done something wrong.

Cheers,
Steve

[steve wright]

Carrera GTL with drilled drums...

http://www.google.co.uk/url?sa=i&rct=j& ... 3531316090

[Pandora]

So the more weight we can take out the better obviously. I would love your insight into where you think I can save weight (or not as the case may be)!

I'm sure this doesn't apply to you Steve, but I do remember someone (can't remember who) was quoted as saying "The easiest and cheapest place to save weight is in the cockpit." I always remember it whenever I see a car with lots of holes drilled in it.

So no puddings in the Wright household



Al