AMMO Posted January 19, 2001 Share Posted January 19, 2001 I think that the late cars have two alternative positions on the chassis for the de dion link arm. If fitted to the lower position does this introduce a certain amount of anti-squat? Can you then run slightly lighter springs than you would in the normal position? I want to do some mods to my '89 Supersprint and was thinking of making new link arms with a larger chassis bolt in double shear. Also any thoughts on mods for the A frame to improve the suspension and safety. Thanks in advance. AMMO Link to comment Share on other sites More sharing options...
Graham Perry Posted January 19, 2001 Share Posted January 19, 2001 Ammo, Before you go any further, have you checked that your car has the lower mounting position ? My early 1990 car has only the higher position welded in the chassis, and so the lower position is out of the question for my car. I have never fully understood why some cars have both positions available and some just the one. Link to comment Share on other sites More sharing options...
AMMO Posted January 19, 2001 Author Share Posted January 19, 2001 Graham, my car has only got the upper position too. I was thinking of putting my own beefed up version lower down. AMMO Link to comment Share on other sites More sharing options...
Mike Bees Posted January 19, 2001 Share Posted January 19, 2001 The lower position reduces roll-oversteer. Or if you set the suspension low enough it induces roll-understeer. For roll-neutrality you want to set the ride height so that the radius arms are more or less horizontal. My simple brain tells me that if anything the upper position might give you a degree of anti-squat that the lower position doesn't. Think of the DD tube trying to drive forwards when you put the power on - if the radius arms are higher at the front end than at the DD tube end then the tube pushing forward will try to lift the rear of the car. Well that's how it appears to me... Mike Link to comment Share on other sites More sharing options...
Chris Faulbaum Posted January 19, 2001 Share Posted January 19, 2001 You can draw a diagram of you car and draw a line through the axis of the pivot points to determine what anti-dive anti-squat charachteristics were built into it. You can find help on this in Carroll Smith's book Tune to Win, on pages 34 and 35. Link to comment Share on other sites More sharing options...
Graham Perry Posted January 19, 2001 Share Posted January 19, 2001 Ammo, What Mike says is true, to get round this I just did like he suggested and lowered the rear suspension until the radius arms were level. It seemed a quick simple solution. I have access to an engineering workshop and we contemplated fabricating lower mountings like yourself. However, after much headscratching and removing of panels etc we concluded it wasn't easily achievable without quite a bit of butchery of the outer skin,to allow access for the welder, so we didn't bother, and took the above route instead. Link to comment Share on other sites More sharing options...
Mike Bees Posted January 19, 2001 Share Posted January 19, 2001 Oh so *that's* why your car is almost scraping along the ground smile.gif (or is that only when Keith is driving?). Mike Link to comment Share on other sites More sharing options...
AMMO Posted January 19, 2001 Author Share Posted January 19, 2001 Thanks guys for your replies. What triggered a this off for me was accelerating over a series of small bumps and getting that wah,wah,wah noise from the engine as the revs go up and down as the wheels lose traction. I thought soft springs with the correct pre-load, wheel travel, damping and a little anti-squat should cure that. I never thought of the implications of going round corners! After what you have said the neutral route seems to be best. Can't work out if the link arms is in tension or compression though, been driving me nuts. I'm erring on the side of tension but am not at all sure. What do you think? I'll definately look up Carroll Smith. AMMO Link to comment Share on other sites More sharing options...
Graham Perry Posted January 19, 2001 Share Posted January 19, 2001 Yep Mike you are quite correct. Scrape is the word, we have had to weld 1/8 inch caps around the knuckle end of the A frame diagonals so that they don't wear away too quickly at the outer ends. The trick of making the radius horizontal does work though in terms of axle steer. All oversteer is now predictable. So Ammo its a worthwhile conversion Link to comment Share on other sites More sharing options...
Graham Perry Posted January 19, 2001 Share Posted January 19, 2001 Yep Mike you are quite correct. Scrape is the word, we have had to weld 1/8 inch caps around the knuckle end of the A frame diagonals so that they don't wear away too quickly at the outer ends. The trick of making the radius horizontal does work though in terms of axle steer. All oversteer is now predictable. So Ammo its a worthwhile conversion Link to comment Share on other sites More sharing options...
Graham Perry Posted January 19, 2001 Share Posted January 19, 2001 Yep Mike you are quite correct. Scrape is the word, we have had to weld 1/8 inch caps around the knuckle end of the A frame diagonals so that they don't wear away too quickly at the outer ends. The trick of making the radius horizontal does work though in terms of axle steer. All oversteer is now predictable. Having said that we will be raising the ride height 10mm next season with our new adjustable set up. So Ammo its a worthwhile conversion even if it comes at your problem from another angle Link to comment Share on other sites More sharing options...
Mike Bees Posted January 19, 2001 Share Posted January 19, 2001 Ammo - surely if you're accelerating they're in compression and if you're braking they're in tension. All of the accelerative force must go through the radius arms and the A-frame - they're the only things holding the wheels to the chassis! I don't see how anti-squat properties will reduce the tendency for the wheels to leave the ground over bumps. You're right in that in general softer springs (and softer or no anti-rollbar) will give better traction on non-smooth surfaces. Mike Link to comment Share on other sites More sharing options...
AMMO Posted January 19, 2001 Author Share Posted January 19, 2001 Mike, due to a mental block I just can't visualise this. If, say, the nearside driving wheel is turning anti-clockwise doesn't the de dion tube want to turn clockwise(together with the rest of the car)? Hence the front end lift which can't be due only to weight transfer. Think of a motorcycle pulling a wheelie. This puts the link arm in tension and the A frame in compression. I'm still not convinced I am right. I should have paid more attention at school, what was that action / reaction thing they always went on about? Maybe it does not apply here. I just don't know. I checked out Carroll Smith and according to him there are two ways of acheiving anti-squat: with convergent link arms and parallel upwards facing link arms. So maybe on this point we are both right. I know for a fact that I induced anti-squat on a shaft driven motorcycle by lowering the front pivot point of the torque arm of a parallelogram. Crawled on the floor of my garage earlier and discovered that the link arm on my car with 140 mm ride height at the rear is slightly down at the front and the A frame is roughly horizontal. So maybe I already have some anti-squat. What do you think? Has anyone observed a car on a rolling road? What does the back end do? Goes up, down or just sit there? I wish I was young again. I knew everything then. The older I get the less I understand. AMMO Link to comment Share on other sites More sharing options...
Mike Bees Posted January 19, 2001 Share Posted January 19, 2001 Ammo -it's the diff that transmits the torque to the wheel, so it's the diff that will try to turn in the opposite direction to the wheel. In the case of a live axle car this will affect which bits of suspension take which direction of strain, but not with a DD tube where the diff is bolted into the chassis - the DD tube is just a carrier for the wheel bearings. Mike Link to comment Share on other sites More sharing options...
Mike Bees Posted January 19, 2001 Share Posted January 19, 2001 PS - on the rolling road the car will behave pretty similarly to on the real moving road. Introducing a degree of anti squat at the rear or anti dive at the front doesn't usually mean that the car does the opposite of what you'd expect, it just does less of what you'd expect (i.e. squats a bit less, dives a bit less). Mike Link to comment Share on other sites More sharing options...
AMMO Posted January 19, 2001 Author Share Posted January 19, 2001 Thank you Mike for your explanation. Now I understand. AMMO Link to comment Share on other sites More sharing options...
Peter Carmichael Posted January 20, 2001 Share Posted January 20, 2001 The Watt's linkage front link mounts to the upper radius arm location. It might be easier for a welder to access the rear area to add the rear pickup than to deskin and weld in a lower mount at the front. It might be easier to upgrade to a roll neutral Watt's linkage than try and get a lower radius arm mounting. Just a thought. Link to comment Share on other sites More sharing options...
AMMO Posted January 20, 2001 Author Share Posted January 20, 2001 Thanks Peter for your suggestion. I will look into that. My main concern was one of reliability. While considering how to beef up the back end I got interested in anti-squat. As I intend to eventually stick 200 BHP through the transmission I don't want any of the life threatening failures described by some members when the link arm front bolt shears. If you were designing the back end from scratch you would put the front of the link in double shear. Adding a Watts linkage would make everything much stronger and would probably be easier to do from what Graham Perry says. I have read up my Carroll Smith and on Chris' suggestion will draw up the rear geometry to see if I can make head or tail of it. The car is for high mileage road use and about five track days a year. I want to achieve a supple responsive ride with good feed back and grip. I don't want to end up with a go-kart. I already have a Mini which is fantastic on the track but absolutely horrible on the road. That's why I decided to buy a car with proper suspension. I will tolerate some body roll as I realise you can't always have your cake and eat it. The plan is as follows: Draw rear geometry. Check amount of rear wheel movement available above and below the car's static laden position. Check spring rate at rear. The car is 140 mm at the back 120mm at the front. Im considering raising the back by either pre-loading the spring or changing for a heavier spring. This is all new to me but I do have a background in motorcycle racing so maybe some of the ideas I have may not apply. Thanks to everyone for a great response to my posting. I know I'm going to enjoy this car once I get it to my liking. AMMO Link to comment Share on other sites More sharing options...
Blatman Posted January 20, 2001 Share Posted January 20, 2001 I don't want to be picky but............................. If you were "designing from scratch", you would design an independent suspension system, complete with all it's advantages. Link to comment Share on other sites More sharing options...
AMMO Posted January 20, 2001 Author Share Posted January 20, 2001 Yes Blatman. 100% right. Carroll Smith, Tune to Win, page 41: "The De Dion axle is basically a beam axle arranged so that the final drive unit is part of the sprung mass. This is its only real advantage over the beam axle. It is not currently in use on racing cars and has not been for twenty years. (My edition is 1987)I feel safe in assuming that it will not return. Therefore we will not discuss it". Ouch. AMMO Link to comment Share on other sites More sharing options...
AMMO Posted January 20, 2001 Author Share Posted January 20, 2001 Sorry, forgot to add it was designed in 1904 by a Mr. De Dion (sounds like a foreigner to me, probably French!). Things have moved on a bit since then. However I deliberately bought a forty four year old car to go with my forty one year old car (a Mini) with the intention of a having a bit of a fiddle and I have a feeling I'm going to enjoy every minute. AMMO Link to comment Share on other sites More sharing options...
Blatman Posted January 21, 2001 Share Posted January 21, 2001 Sorry AMMO, I withdraw my picky statement aboveblush.gif. I think it is a shame that Caterham have not yet ventured furhter into the 20th (yes 20thwink.gif) century and designed a proper independent rear suspension system. Could they not get their hands on the Lotus 7 and a half (I think) plans, which had such a set upquestion.gif Does anybody else know why this hasn't happenedquestion.gif Link to comment Share on other sites More sharing options...
millsn Posted January 21, 2001 Share Posted January 21, 2001 I changed to using the lower mounting hole just to see if I could get improved ride. It definitely helped. It's odd though as this is supposed to be the racing position, I think that the dedion tube wants to rotate when it hits a bump much less if you're on the lower mount. Nigel Mills - xflow Link to comment Share on other sites More sharing options...
Paul Ranson Posted January 21, 2001 Share Posted January 21, 2001 I think it is a shame that Caterham have not yet ventured furhter into the 20th (yes 20th) century and designed a proper independent rear suspension system. A trailing arm setup akin to that of the Hillman Imp or Porsche 911 would be quite practical but pointless. Doing a 'proper' double wishbone job would require quite extensive chassis modifications to find somewhere to mount the inner ends. More interesting would be to put pushrods and inboard springs/dampers at the front. I think this would offer more promise from the handling front. FWIW Ferrari had an F1 car that could be set up as 'De Dion' in the early 1970s. Paul Link to comment Share on other sites More sharing options...
AMMO Posted January 22, 2001 Author Share Posted January 22, 2001 Double wisbone rear would be nice. Don't W*******ds have that? (Just typed in W*******d to see what happens)W*******d W*******d W*******d. AMMO Link to comment Share on other sites More sharing options...
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