bump steer

[twincamman]

after many years of racing vees and not worrying about bump steer it is time to address the problem on the ?lan ---- what is the best way---- -- what special self manufactured tools are recommended ---ed

[marcfuller]

There is a useful tech thread regarding this subject on the on the gglc website-
http://gglotus.org/ggtech/bumpstr-how2/how2bumpstr.htm

[M100]

Despite what is mentioned by many books over the years that you want to reduce bump steer (even saying approaching zero), some is absolutely essential for stability, but it must always be in the right direction.

Basically as you turn in to a corner the car will roll, one front wheel going into bump the other into rebound. To prevent the tyres from overturning, you need the geometry that in roll will turn the wheels slightly against the direction you are turning, otherwise the car will turn in very quickly as positive steer angle is induced by the geometry. Depending on the roll stiffness this can be at a much faster rate than determined by steering wheel input alone. This can make the car turn in so hard that oversteer is induced and the car becomes very hard to drive smoothly. Introduce the correct amount of bump steer in the appropriate direction and the car will be much easier to drive.

[type26owner]

Hi Ed,
What makes you think there is a problem? Does the car dart around when traversing over some rough spots? May I suggest that if you're having a problem that you check the rack for wear damage. They are prone to quickly wear out the tooth at the straight ahead position. It's easy to spot the problem. Watch the lefthand tierod's motion is it's a LHD car as someone turns the steering wheel past center (for a RHD car it's the righthand tierod). If the tooth on the rack is wornout the tierod will move up by about 1/8" as it crosses the damaged area IIRC. Also with the car jacked up off the ground you'll be able to feel it detenting when rotating the steering wheel. That rack design is scary. The idiot that designed it probably got an award too!

As far as I know the bumpsteer solution only applies to the straight ahead position of the steering rack. Once a turn is intiated all those relationships go skew. The effect that adds steering in a turn is called Ackerman.

[twincamman]

I had a zero Ackerman set up on the vee ---worked well on fast tracks like Mosport and the Glen --poor on slow tracks like Shannonville and Nelson Ledges ---the ?lan is just a little spooky AT 90 AND 100 MPH --THE FRONT FLOATS A BIT ---MAYBE A FRONT SPOILER OR SOME AERO AID MAY HELP?- shocks seem ok ---ed

[type26owner]

Hey Ed,
The amount of ackerman is proportional to the amount of steering input. Turns out most folks don't turn in much in the really high speed corners so there's almost no ackerman to speak of. Only time ackerman is important means you're going slow as a snail such as when autocrossing.

Think in terms of weight transfer is your best friend. You screw that timing up and you fly off the track. Yup, it's spooky. Could be the coupe has totally different aero handling though.

I'll duck for cover now.

[twincamman]

it may be a tire problem ---ie -- wire winding angle or compound --I dont have big enough hollyhocks to drive past 100 to se e if it clears up or gets worse--ed

[type26owner]

Hey, the light went on as drove to work today. M100, are you referring to roll steer? IIRC, Carroll Smith and Fred Puhn said it's been tried but it tends to make the car twitchy and unnerve most drivers. That's normally done with the trailing links chassis attachment points on the rear suspension though.

Ed, as a benchmark I can let go of the steering wheel at redline in fourth gear on the straights and it tracks true. That seven degrees of castor and the amount of mechanical trail helps a bunch. As Carroll Smith states tires are the first 80% of the handling. Choose poorly and you're screwed.

[twincamman]

no the sumbich just loses it confident feel -- is as if its trying to get unstuck-as in the front end vacillates ---may try some tire pressure adjustments -----btw the zero Ackerman set up works so well in a vee because of the inherent understeer in the 60 year old design oe the front end - even carol smith didnt understand it -ed

[type26owner]

I had to pump the MX4s up to 60psi so they wouldn't scare the hell out of me. The RE92s are cheap and handle well up at that speed and the pressure only has to be set to 22psi. Man, do those sidewall stiffeners help out at the limit of the traction circle!

[type26owner]

With a Vee I'd guess it suffers from all that weight of the torsion bar frontend levering the Polar Moment of Inertia in a bad way. Nothing will turn in quickly with that kinda setup. Little wonder it has so much understeer. It's all about compromises.

I cringe when someone suggests moving the battery to the boot or the radiator up into the nose. Bad idea if you want it to handle well.

[twincamman]

that and the geometry is screwed --so you destroy the handling in the front so its as bad as the rear and the result is a good handling car ---in a way -- well it may not be a bump steer problem as that is just toe in and out --- ed

[type26owner]

Ed,
You need aero advice of a 26R guy. Seems they are all much to wimpy to post here though.

I wish I had bought that 26R Steve Griswold wanted to sell me back in 69 for $3500.

[twincamman]

thats 500 dollars LESS than I paid 2 1/2 years- ago -- they certainly don't hold their value ----did I get rooked?-- -I have found a few strange holes on the bottom front of the car --perhaps a couple of small chin spoilers on each side may help- stabilize the front ---- ed

[rgh0]

I get my Elan Coupe up to around 135 -140 mph with no aerodynamic issues and good stability. Only really have an issue in strong cross winds going over the hump in the end of the main straight at Phillip Island at top speed where the car lightens off. If the car feels unstable I would go over the whole suspension front and rear in terms of condition and alignment to ensure its all set right.

The only significant alignment differences in my car from standard is a slightly lower suspension height setting giving a little negative camber and I run the front toe in at the top end of the specification range.

If you cant find anything in the suspension and the wheels are all true then try a new set of good quality tyres.

Rohan