Rear Wheel Bump Steer

[Guest jimthesail]

Ahoy !

Just rejoined. Find your site difficult !

Have just put No.1 son's 2B back on road well pleased with the result, however ! Find it leaps and points at either the verge, or more disconcertingly, at the oncoming traffic when one hits a bump ! Suspect that the rear wishbones, well depressed to get wheel clearance right, are the culprit. Understand there are tapered spacers available but they correct the inclination only surely ? What is the remedy ?

Jim

[nicko]

What tyre pressure are you running?

Mine calmed down a lot when I changed the rear coil overs to gas.

Nick

[Dave R]

peter bell does double taper spacers to correct toe and camber

[fry61]

Used between two flat surfaces the wedges must be a single taper -- or ??

[Bob Tucker]

Check ride height at the rear.

It should be around 7 inches from the bottom of the side panel

right in front of the rear wing.

The 2B was "designed" to have a large gap above the rear wheels.

If its lower than that it increases rear camber and rear toe in.

Both of these will provoke bump steer.

The double taper shims will fix it.

TP should be between 18 & 20 psi front & rear.

[IanS]

the angle of the taper is what counts 0 45 90? ( pure camber / half and half/ pure toe)

But as the angle of the pivot for the trailing arm is approx 30 degrees there will always be bump steer from the back, ford deliberatly designed it this way.

The BMW Z axle also operates this way.

In a PURE back axle the rear wheels track inside the front wheels whilst turning and the front wheels take a greater side load ( due to the bigger diameter circle they take) and this leads to understeer in the limit (BAD) haveing some rear steer due to roll ( the inside wheel is lightly loaded and just skids) evens out the side load and makes a safer car.

The cheep modified mini dampers that became zimerides could not control the rear end as they were designed for a rear suspension where the trailing arm pivot was across the car and there was no rear roll steer.

[Guest 2b cruising]

I get the same reaction when my suspention bottoms out.

Really dangerouse on country roads and nearly put it in the hedge once.

It was ok before I changed the leaking rear gaz shockers.

The chap from Gaz, I think dampertec, sent me the ones that he recommended. They had a shorter stroke than the old ones.

I had to adjust the rear top shock support to get the required ride hight.

Now if I have them set on a softer ride for road use they bottom out easily.

I have got new ones same as original to fit now, and put rear top mounts back to as was.

I prefer a softer ride for normal running because I have seriouse back problems.

My seats are fat and comfy but not good enough to ride on stiff settings.

Edited November 14, 2015 by 2b cruising

[Guest jimthesail]

Many thanks to all for quick replies ! When 'me lad' picked it up he ran it aground on a slight bump in the road and obtained the help of 6 burly firemen from a nearby station to lift it off. On inspection ground clearance was about 2". We have progressively raised it to about 4 but it still grounds on speed bumps. Reminds me of a dog with worms ! It seems that another inch is in order even at the risk of gaping gaps under the arches. Might try Mr Bells wedges too? Solves the Christmas present problem.

Other than having to detach my left leg when cruising it is an amazing drive. Reminds me of the Ford engined Morgan I had in my youth that I thought was the bee's knees but in fact was a mechanical nightmare ! However if I stick my right buttock to starboard I can just wedge my port knee between the wheel and the dash support ! OK till we need to change course !

[Bob Tucker]

What rear suspension do you have?

Sierra coils & shockers can work just fine if they are not clapped.

But they can be too soft & allow the rear to ground over bumps & on hard acceleration

But coilovers work better for adjustable damper settings & height.

 

BTW I cruise with my left foot hooked under the clutch pedal.

The steering wheel can usually be raised quite simply depending

on how the builder assembled it. Or moving the seat further back?

Edited November 14, 2015 by Bob Tucker

[Guest 2b cruising]

I also hook my foot under pedal.

It's not ideal, but changing its possition now and again suffices.

Other people engineer a small shelf in the tunnel panel, but this must be a small one.

I intend trying a friends car to see if it's worthwhile before going to the trouble of doing it myself.

[cb750]

Check the pivot bushes in the swing arms. If these are worn you will get rear wheel steering. I had this on a car I owned.

[tractor]

i get over the left foot problem by adjusting seat so that clutch bite point is just within reach which works for me with a zetec cos fewer gear changes needed when cruising cos of engine torque

[megadodo]

Peter Bell's double chamfer plates cured my rear wheel bump steer, fully recommended.

Also made the car far less "skittish" in a straight line.

[Bob Tucker]

Jim,

let me know if you think about changing the rear wishbone bushes.

I have a full set of Polybushes that I never used (or needed)

Bought in a bout of Creditcarditis visiting Rally Design in the early days.

[Guest peter2b2002]

my 2b use to hop when going over bumps but the wedge plates fixed it

peter2b