Wheel Balance Sensitivity and Bump Steer

[david.g.chapman]

I was wondering, does anyone know if there is a relationship between bump steer and how sensitive the elan (or +2) is to front wheel inbalance?

I have never tried to eliminate the bump steer on my +2, but if it reduces steering wheel wobble (not bad but could be better) I might try a bit harder...

Dave Chapman.

[Foxie]

Imho, wheel imbalance and bump steer would not compound a problem either way.

Bump steer will be most noticeable on a road with an irregular surface. Wheel imbalance may be felt at the wheel, but it will not effect the overall steering response.

If you are driving on a good regular surface, such as a motorway, bump steer will not be detectable. However wheel balance will definitely be.

if you have wheel balance problems, find a place that can check them on the car. Sometimes, even with perfectly balanced wheels on the balancing machine, the hubs can be a factor.

[RichC]

more likely to be linked to the dimensions of tyres/ wheels... i'm hazarding a guess they are non-standard?

[david.g.chapman]

Thanks for your replies.

I am running on Lotus alloy wheels from Paul Matty, so a standard set up.

My method of wheel balancing is to get them set up off the car, then I road test the car and add extra weights to try and include out of balance hubs. I do not know of anyone in Essex that balances on the car. If anyone does know of a place, I would like to know!

I am thinking that any force that translates into a vertical movement of the front suspension wishbones will deflect the steering wheel if the bump steer is not set OK. So that could be any out of roundness in the tyre, or even out of balance forces, perhaps?

Another possibility is that my steering rack pinion preload is set too low, so there is not enough dampening in the rack.

I will try and set the bump steer and pinion preload this winter, and will post if there is any improvement.

Cheers,

Dave Chapman.

[Foxie]

Thanks for your replies.

I am thinking that any force that translates into a vertical movement of the front suspension wishbones will deflect the steering wheel if the bump steer is not set OK. So that could be any out of roundness in the tyre, or even out of balance forces, perhaps?

Dave, you are perfectly correct.

However the vertical deflection caused by imbalance would be infinitesimal in relation to normal suspension deflection, and the bump steer effect would be another magnitude smaller. If you are very good at maths you could calculate a value, but it would be so small as to be humanly non-detectable .

[ericbushby]

The thing about bumpsteer is that if the shims are not set correctly, then when the suspension is deflected up or down the front wheels turn without deflecting the steering rack or the steering wheel. Which way they turn depends on whether the rack is high or low.
It is most noticable when hitting a bump or hollow, when a rapid correction is needed only for it to change
direction back after the bump is passed and the suspension returns to its normal level. Mine was quite scary until I discovered that no shims were fitted.
Incidentally the chassis was engraved with 40 thou. one side and 60 thou. at the other. I finished up with 130 and 180 thou.
Eric in Burnley
1967 S3 SE DHC (now with C/V drive shafts, as of today)

[denicholls2]

Steering wobble can be attributed to bad wheel balance but the following are equally likely:

Unround or out-of-true wheels.
Disc warp.
Wheel bearing adjustment too loose. There isn't a lot of leeway in the allowable runout for wheel bearing adjustment, and many run them looser than spec which would be a contributor to wobble.

If you don't have a dial gauge, it's a cheap investment in proving both discs and wheels are true.

Bump steer I wouldn't worry much about, wobble will wear things out (shocks, tires, bearings) much more quickly. Find it and fix it.

[david.g.chapman]

Thanks again.

My front wheel bearings are on the tight side (just a trace of play). Lateral run-out on the front wheels is less then 0.5 mm. Front disc run out is OK too - no judder when braking.

Yes, I suppose it would take quite a large vertical movement on the suspension to give a steering wheel deflection. I will still try and get the bump steer fixed (there is some).

Wheel wobble even varies depending on where the front tyres were resting before a trip - quite sensitive.

My money is on the steering rack pre-load - watch this space.

Dave.

[RedBarnMan]

If wheel wobble is also dependant on where you last parked it seems that you have a tyre problem - they are flat spotting. That will normally reduce after you have driven a few 10's of miles and got some heat into them. No point in trying to balance a wheel after a car has been sat on it over winter. Have you tried seeing how bad the bare wheels are (no tyre in place) as some cast wheels have pronounced 'heavy' areas.

[david.g.chapman]

I forgot to say that generally if there is a wobble after a week standing, then it gets better after a few miles, but does not disappear. Sometimes there is no wobble at all - could be I have parked in the right place.

I has a Ford Capri in the 70's (same front wheel bearings funnily enough) that did this. In those days the Churchill on the car balancer was still in use, but not any more it seems.

Just to repeat, if anyone does know of an on the car balancing service near me in Writtle, Essex, that would be helpful.

Dave Chapman.

[david.g.chapman]

I have found someone who does wheel balancing on the car.

They are Goodwin Tyres in Green Lane, Ilford.

I will see them on Monday.

Dave Chapman.

[Bigbaldybloke]

Good luck with your on the car wheel balancing, make sure you have done a decent length drive before they are balanced to eliminate any possible parking flat spots, and remember to mark the wheels / hubs so you put them back on in the same place if you take them off at some stage in the future.

[david.g.chapman]

Goodwin Tyres, Ilford have balanced my front wheels on the car. They brought out an ancient front wheel balancer, blew the dust off of it and gave it a self test - OK.

I was then asked to jack the front of the car up (I preferred that) and the operator marked a line on the tyre and spun up the wheel with the machine. A strobe light was used to determine the position of the wheel, and a sensor was placed on the lower wishbone to detect vibration. He than added 30 grams on the passenger side wheel to balance it. The process was repeated on the other wheel but no additional weight was required.

The vibration on the steering wheel has virtually disappeared, so I am a happy man. It costs ?12 a side if anyone else is interested. Phone up first and check that Jim is in, as he is the one who operates the machine.

Cheers,
Dave Chapman.