Bump Steer

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fsmlittler
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Bump Steer

Post by fsmlittler »

we are trying to get rid of bump steer on our 35 an I understand that A40 steering parts allow the adjustment needed. Can anyone tell me which parts we need from A40?
Dave Clark
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Re: Bump Steer

Post by Dave Clark »

I think you may be thinking of the A40 adjustable track rod which goes on the drivers side.

This is fitted to allow the steering to be centred on the central tight spot in the steering box and then the track adjusted afterwards, without altering the drivers side track rod.

I can,t see how this could be of any benefit in eliminating or reducing bump steer.

I can only suggest that you restrict suspension travel at the extremes so as to avoid the area where bump steer occurs,but this is hardly an elegant solution and would be unacceptable on a purely road car.

Maybe Rae Davis could offer some suggestions ?

When I fitted a rack to an A35 I had great difficulty in getting the rack low enough to avoid bump steer.

To investigate what was happening I assembled the suspension without springs, attached a laser pen to a disc and, with the steering straight ahead, cycled the suspension though its travel tracing the track of the laser on a clean sheet of ply set in front of the car. Prepare to be surprised !
fsmlittler
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Re: Bump Steer

Post by fsmlittler »

Thanks Dave. I think the may have to call Rae for some advise. We're taking the engine and box out so plan checking how much movement we've got with springs out. Like you I struggled to see how an adjustment to the track rod would cure it.
Big Daddy
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Re: Bump Steer

Post by Big Daddy »

fsmlittler wrote:we are trying to get rid of bump steer on our 35 an I understand that A40 steering parts allow the adjustment needed. Can anyone tell me which parts we need from A40?
Bump steer on Evo2 has been reduced to almost zero, but it has a Morris 1000 rack and custom wishbone suspension. However believe I understand how A40 adjustable links help although I've never proved this in practice.

Assume you have encountered excess bump steer after adopting negative camber probably by using top trunnions with larger offset :?:

From a frontal perspective, envisage front suspension to be a trapezoid with top and bottom arms running horizontally parallel (as shown below). If the steering link sits parallel between them, then its pivots must have the same lateral separation as the inclined sides of the trapezoid to eliminate bump steer. So on the diagram below, a higher steering link would be shorter than a lower link to maintain correct geometry.

This all goes wrong when the outer top wishbone pivot is moved inboard to provide negative camber. However it can be corrected by using shorter links which the adjustable A40 parts allow. Also this means the standard A35 track rod can be maintained, rather than shortened to provide correct toe with negative camber.

Many after-market links for both A35 and A40 are adjustable anyway :thumbs:

Hope this helps... :wave:
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beko
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Re: Bump Steer

Post by beko »

Would that mean you would need an adjustable link on both sides so you could replace the standard draglink with fixed centres - thus permitting adjustment of the length between the centres to compensate for the other changes?
earthhist
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Re: Bump Steer

Post by earthhist »

fsmlittler wrote:we are trying to get rid of bump steer on our 35
Good luck with that! Welcome to the 1950s, the era before anyone understood what really happened between the tyre and the road, and steering was designed on geometrical principles, and all implemented very approximately, and with large amounts of backlash. Continual steering adjustment to keep the car in line is the norm when driving.

With the original system all you can hope to do is to minimise the amount of play in all the components, and have a steering box with a single central tight point (the A40 adjustable draglink is used on the steering wheel side to compensate for the steering box tight point being off centre - it isn't needed on the left side as the cross tube link is adjustable), and get the toe-in right (this last makes the biggest difference in terms of driveability). The problem is reduced if the dampers are in top condition, and better still uprated. The problem is also reduced somewhat by using radial tyres. The condition of the rear suspension also matters, though not to the same extent.
Richard Johnston, Plymtree Devon, formerly A30/A35 Club Technical Officer and President
Big Daddy
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Re: Bump Steer

Post by Big Daddy »

beko wrote:Would that mean you would need an adjustable link on both sides so you could replace the standard draglink with fixed centres - thus permitting adjustment of the length between the centres to compensate for the other changes?
Yes shorter (ie adjustable) steering links would be needed on both sides to provide correct geometry with negative camber. But as I've never tried this in practice (as declared above), can anyone who has please comment :?: ... :whistle:
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earthhist
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Re: Bump Steer

Post by earthhist »

I used just one adjustable link as I described above, to get the tight point centralised, with an essentially standard setup, except for using midget hubs and brakes. With that setup I didn't have any obvious steering assymetry issues.
Richard Johnston, Plymtree Devon, formerly A30/A35 Club Technical Officer and President
Big Daddy
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Re: Bump Steer

Post by Big Daddy »

fsmlittler wrote:we are trying to get rid of bump steer on our 35 an I understand that A40 steering parts allow the adjustment needed. Can anyone tell me which parts we need from A40?
Can you clarify whether this bump steer problem has arisen after adopting negative camber :?: Or is front suspension geometry still standard :?:
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Dave Clark
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Re: Bump Steer

Post by Dave Clark »

Rae,s racers use quite a lot of negative camber, but I suspect that his very high spring rates reduce suspension travel enough to make bump steer less of a problem ? I suspect that there are other, much less obvious tweaks to his cars if which we know nothing.

As Richard has indicated, trying to make silk purse from a sows ear will inevitable involve compromises.

An A35 on a smooth surface is fine, but bumps do make a tremendous difference so race track practice is so different from road use as to make it of limited use -for me at least. All I know is that when my car hits a bump mid corner the rear axle is steering the car as much as I !.
Big Daddy
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Re: Bump Steer

Post by Big Daddy »

Dave Clark wrote:Rae,s racers use quite a lot of negative camber, but I suspect that his very high spring rates reduce suspension travel enough to make bump steer less of a problem ? I suspect that there are other, much less obvious tweaks to his cars if which we know nothing.
Tried negative camber trunions with 720 lbf/in front springs on my old 995 racer. The car darted around violently and was almost uncontrollable... :shock:

Suspect Rae is correcting steering geometry to accommodate revised suspension geometry, as I alluded above... :whistle:
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Countryboy
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Re: Bump Steer

Post by Countryboy »

Big Daddy wrote: Yes shorter (ie adjustable) steering links would be needed on both sides to provide correct geometry with negative camber. But as I've never tried this in practice (as declared above), can anyone who has please comment :?: ... :whistle:
Derek Minter from the A40 club can supply these for £45 a pair
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fsmlittler
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Re: Bump Steer

Post by fsmlittler »

Thanks all

Big Daddy-we've got 700lb springs but standard trunnions. Have toe in set as absolutely factory standard and running 175/50 13 Yokohamas. I will study your explanation and see if I can follow it. Thanks

Under heavy braking the car is trying to change direction and is very hard to keep straight. There is an 85mph rise at Saint Goueno hill climb that you'd hardly notice in my Clan but it's a tank slapper in the A35.

I know Rae sells replacement tubular steering components but like BD I have yet to speak to anyone who has adjusted bump steer out using the adjustment of existing steering components.

Steven can you let me have the contact info for the A40 spares? Cheers
Big Daddy
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Re: Bump Steer

Post by Big Daddy »

fsmlittler wrote:Big Daddy-we've got 700lb springs but standard trunnions. Have toe in set as absolutely factory standard and running 175/50 13 Yokohamas.

Under heavy braking the car is trying to change direction and is very hard to keep straight. There is an 85mph rise at Saint Goueno hill climb that you'd hardly notice in my Clan but it's a tank slapper in the A35.
Suspect front suspension is running out of bump travel, which sends the effective spring soaring and induces instability. On a lowered car, usual culprit is early contact with the bump stops which need to be cut down accordingly. But could be any limiting condition such as insufficient stroke on auxiliary telescopic dampers.

Beyond those obvious problems, increased damping in pitch* will calm braking instability (*front bump and rear rebound damping).

Almost forgot to mention F/R braking ratios :oops: With short wheelbase and relatively high mass centre, A35 suffers from large weight transfer to front wheels under heavy braking. If braking ratio is set too far rearwards, this will result in horrible braking instability exacerbated by bumps. Our experience from Evo2 and RR is surprisingly high front biased braking gives the best results... :thumbs:
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Countryboy
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Re: Bump Steer

Post by Countryboy »

BD
Am I correct in thinking that a passenger side adjustable steering arm should be fitted after adjusting it to the length of a standard arm less the amount the camber has been changed by ?

Standard lever arms were made to damp the movement of 240lb ? springs , do any that are uprated stand a chance of working with 700 lbs ?

I would have thought that telescopic dampers would be required for this ?

Please educate me :wave:
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