Getting enough clearance

Posted on May 30th, 2014 in Driving Emotion,Safety,Suspension,testing by Julian Edgar

When is enough clearance sufficient?

If you’ve modified a lot on cars, you’ll have come across this question. It might be the clearance between the exhaust the bodywork, clearance of a driveshaft at full suspension bump with a chassis member or subframe, or even clearance between a large turbo and bodywork.

Years ago I read an excellent book written by an automotive suspension engineer working in the 1950s. In it he made the (almost throwaway) line that there’s no need to provide tyre clearance at full suspension bump AND full steering lock – the idea being that this situation almost never occurs, and if contact did in fact occur in that situation, the car would be moving so slowly that it wouldn’t matter much anyway.

These thoughts are intruding because at the moment I am massaging a turbo dump pipe so that it clears a steering tie-rod, with the greatest potential conflict occurring at full suspension droop and with full right-hand steering lock.

At full droop but with the wheels pointing straightahead – no problem. And at full steering lock and with the wheels at normal ride height – again no problem.

It’s just at that particular combination – one that again is very unlikely to ever occur – that I have the issue.

I am concerned because if the car has to undergo full engineering approval, I can just imagine an engineer saying something along the lines that conflict should not be able to occur at ANY combination of lock and suspension movement….

And even if clearance is achieved, how much clearance is enough? If I were ornery enough to throw in maximum engine torque reaction movement at just that moment, perhaps another 10mm of clearance would be needed.

But hold on! How could the engine be developing maximum torque if the suspension is at full droop? After all, in that situation there’s very little – next to none in fact – of the car’s weight on the tyre… so how could it transmit the torque anyway? No torque transmission means no transverse engine rocking!

Hmm, what about if the car has an LSD, and a very stiff front anti-roll bar, and is cornering hard enough (at full lock!) to lift a wheel? Then I suppose one could imagine a situation where something like contact could occur.


5 Responses to 'Getting enough clearance'

Subscribe to comments with RSS

  1. Austenite said,

    on June 1st, 2014 at 10:01 am

    If you can’t engineer out the possibility, can you engineer the consequence? Would it be possible to use a flex joint of some kind at that location?

    Since it’s the dump pipe it will be subject to high exhaust temperatures and whatever exhaust peak backpressure there is, so might be difficult. What about a rotary joint (and mounting) so that the exhaust moves with the suspension? Perhaps you could use your beading tool from the recent story and make a triple-bead rotary joint which doesn’t leak. Fill the gaps with nickel anti-sieze (good for up to 1315 degrees C) and should rotate freely and not leak.

    Now that I think about it, attached to the engine and moving with the suspension is far too much movement and far too frequent. Maybe just have it move one way when tie-rod pushes it at full suspension droop, and pulled the other way then suspension and steering returns to more normal position. I’m thinking some kind of ring attached to the tie-rod. Clearance is enough so that this movement only happens at extreme conditions.

    Yes, this does seem over-complicated, but I reckon you’re one of the few people on the planet who could make it work. Even if just to experiment!

  2. Julian Edgar said,

    on June 1st, 2014 at 8:01 pm

    The flex joint suggestion is an interesting one.

    At the moment I have steam pipe bends coming that I hope will allow me to go from 2 inch round to 3 inch (equivalent diameter) oval, and then back to 2 inch round, all while doing two changes of direction. I can’t see it taking me less than a few days of work, but I’ll see how I go.

    Looking at the clearances, it should all be possible (without full engine torque reaction at the same time, anyway!).

  3. Ben Powell said,

    on June 4th, 2014 at 10:19 pm

    Another way to think of it, what *are* the consequences of contact? I mean will it change the steering angle slightly or will it break something? And the conditions it occurs under. I couldn’t see what side of the car has the clearance issue. But if the car needs to understeer to have a problem and the steering suddenly bumps it isn’t going to be as dramatic as if the car is oversteering when it does something unexpected.

  4. Julian Edgar said,

    on June 7th, 2014 at 7:51 am

    Steam pipe bends didn’t work – too thick a wall to provide sufficient internal cross-sectional area past the squeezy area.

    Now have a 2.5 inch mandrel exhaust bend that I will ovalise, and then place some small bends in the other plane to provide clearance (I hope). Will be tricky to get these things right: am looking to use sand-filling of tube and oxy, along with shaped wood blocks and my press.

    Have bought three of the bends for when I make mistakes!

    The consequences of contact will be zero. But I am just worried about an engineer insisting that no contact can occur under any circumstances.

  5. Julian Edgar said,

    on June 11th, 2014 at 6:55 am

    Managed to do it – 2.5 inch mandrel bent pipe, sand-filled and then ovalised in my press. Maintains cross-sectional area and clears everything at full droop, full right-hand lock.