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Author: Subject: Shorter wishbones

posted on 18/5/20 at 02:38 PM Reply With Quote
Shorter wishbones

Please forgive if this is mentioned before. I've now got some time to seriously start bulding my replica 1936 BMW328 and with careful drawings and measurements I think I can use the Haynes Roadster chassis design with a few simple modifications.

The track of the car will be less but I can get away with the same bottom chassis rails and dimensions by shortening the lower front wishbones by about 20%. But the upper front wishbones will need to be shorter in about the same proportion which means the inner pivot points need to move inboard. I think I can angle U1 and U2 further inwards and modify FF1 and FF2 with the knee facing inwards instead of outwards to allow the pivot points to move inboard.

I'm using a 1.6 2005 MX5 as a donor (and am looking at the Saturn drawings for necesary chassis mods). Can anyone see problems getting the engine/gearbox in and out and how far does the engine, when installed, sit behind BR7, U1 and U2? If it's too close, or actually clashes, any suggestions welcome!

My wheelbase is a little longer than the Haynes Roadster - 2400mm so not a lot, and if I move the engine/gearbox back I worry about the dimensions of the transmission tunnel.


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posted on 19/5/20 at 11:49 AM Reply With Quote
If you're moving wishbone mounting points about have you thought about the impact on bump steer - and the appropriate dimension steering rack you'll need? Just a thought - although you may have it well in hand.

If you tailor the front section of the transmission tunnel accordingly (I believe some widen it to fit a Ford MT75 gearbox) you could put the engine as far back as TR8 allows, in mine theres about a 25mm gap from TR8 to the back of the engine block (and I could probably stand infront of the block in the gap between the engine and BR7). The other factor to consider is where your gear lever is then going to come up.

Getting the engine in and out is not a problem, if needs be split the engine and box, engine out first then pull the 'box forward and up.

Worth you Googling MK Indy engine bay (I understand the dimensions are similar to a Haynes). The pic below speaks volumes, and shows how you can bring the engine back even further if you split the front bulkhead

[Edited on 19/5/20 by jps]

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posted on 19/5/20 at 03:39 PM Reply With Quote
This is hugely helpful, many thanks indeed.

Thanks for the reminder, I will indeed need to shorten the steering rack and the tie rods to get this all to work properly. I have to find someone who can do this properly, don't trust my welding skills here. Maybe using a rack from another car which is shorter. Don't laugh, but the Smart car rack and rods are pretty close dimensionally, but they do look a bit flimsy. To get the proportions right I'll be using 16" wheels with high profile tyres - this will not be a racer! Overall 15% bigger tyre circumference but I figure with the reduced weight the standard gears will cope. Figuring out how to calibrate the speedo is another brainteaser - don't know yet if it's electronic or cable.

Since posting my question I've discovered I will need to narrow the chassis a little, (shorter BR11, BR12 etc.) so widening the transmission tunnel would make the leg room even more cramped. And I'm hoping to use the drivetrain unchanged with the standard prop shaft.

So I'm pleased to see the MK Indy photo which doesn't have TR5. Whilst I can see the merits (going some way to triangulation) of this rail it could make it harder to get the engine in and out and I thought I could add something similar on both sides bolted to suitable welded anchor points.

Really appreciate the opportunity to learn from those who have gone before


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