Bump Steer

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roymck
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Re: Bump Steer

Post by roymck »

Firstly I have a friend who runs negative camber trunions on his MG Midget, He suffers no bump steer. The front suspension geometry is the same on a A35 as it is on a Midget. The difference is in the steering as its by a rack (and also the steering arms are different) . Now if you adjust the tracking on a MIdget you adjust the outer steering rods but the center part of the rack is a fixed length inside the housing. On a A35 the tracking is adjusted by the inner rod as the outer steering rod length is fixed.
Now to induce negative camber to a A35 is one of 2 ways shorten the top wishbone or lengthen the lower. Barry King used to make lower wishbones that are slightly longer this had the effect of making the track wider so the fixed steering rod is now too short. The other method is to fit a negative camber top trunion, Peter May does a solid bushed trunion and Bug eye barn in Australia do a trunion that take the standard rubber bush or polybush. Fitting these will make the fixed steering rod too long. The frontline conversion shortens the top arm to give a smidgen of Negative camber.
So by adding the A40 adjustable outer steering rods you are able to adjust not only the inner rod length but also the outer, this MAY enable you to dial out some bump steer.
As regards the suspension bottoming out the Bump stops could be trimmed slightly but also the pressed steel pan that sits inside the spring that hits the bump stop differs between the Midget and the A30 , just cannot remember what one is shortest.
Last edited by roymck on Tue Nov 24, 2015 2:50 pm, edited 1 time in total.
Big Daddy
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Re: Bump Steer

Post by Big Daddy »

roymck wrote:Firstly I have a friend who runs negative camber trunions on his MG Midget, He suffers no bump steer. The front suspension geometry is the same on a A35 as it is on a Midget. The difference is in the steering as its by a rack (and also the steering arms are different) . Now if you adjust the tracking on a MIdget you adjust the outer steering rods but the center part of the rack is a fixed length inside the housing. On a A35 the tracking is adjusted by the inner rod as the outer steering rod length is fixed.
:iagree: I raced a Midget with MGCC from 1987 to 1992. It had Peter May's negative camber trunions which did not induce additional bump steer... :thumbs:
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Big Daddy
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Re: Bump Steer

Post by Big Daddy »

Countryboy wrote:Please educate me :wave:
Steven, last point first. I'm not the oracle and very capable of getting things wrong and frequently do. Just been around so long that I've had plenty of opportunity to learn from many mistakes... :whistle:

Countryboy wrote: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 ?
Almost right :D Still unproven but both steering arms should be adjusted to compensate for camber change. Amount depends on the vertical position where the steering arm sits on that trapezoid I attempted to explain. However this is easy to determine by measuring toe change (articulation with springs removed) and changing link length until it is minimised. Roy's explanation was helpful... :thumbs:

Countryboy wrote: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 ?
This is just my understanding so very happy to be challenged * :wink:

Dampers control speed of vehicle body displacement and dissipate that energy via heat. Energy input is a function of body mass but is independent of spring rate: 240 or 700 lbf/in makes no difference.

* Simplistic explanation but effectively correct. Proper analysis would consider mass/spring/damper effect of all components in chain between road surface and body, especially the tyres. But unless the road springs are solid (infinite rate) then dampers will still dominate.

Countryboy wrote:I would have thought that telescopic dampers would be required for this ?
Yes :thumbs: I'm fearing wrath of stalwarts, but unfortunately lever arm damper are fairly hopeless for many reasons. Telescopics work better and can allow control of rebound and bump settings, with the ability to alter damping with frequency (handling is most influenced by lower frequencies). Damper settings can have more influence over handling characteristics than either spring or ARB rates.
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roymck
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Re: Bump Steer

Post by roymck »

My thoughts would be that the stiffer the damper the less travel the front suspension would make, less travel would minimise the amount of work that they would have to do so less heat build up and less chance of failure.
My lever dampers are fitted with Moss uprated valve bodies and new standard shocker oil. I believe a period modification was to fit the valve body from a MGB, that's according to my book on ST Tuning. I don't believe filling dampers with thicker oil works long term as it would have more of a tendency to blow a seal.
Big Daddy
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Re: Bump Steer

Post by Big Daddy »

roymck wrote:My thoughts would be that the stiffer the damper the less travel the front suspension would make, less travel would minimise the amount of work that they would have to do so less heat build up and less chance of failure.
Sorry but I don't agree. Need to consider energy balance (aka amount of work done). Stiffer dampers will dissipate the same energy at shorter travel.

Can't reinvent physics. Disturbing force is proportional to vehicle body acceleration (ie sprung mass). Energy input will be the same regardless of damper travel.
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roymck
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Re: Bump Steer

Post by roymck »

DOH ! Sorry my mistake I should have said the stiffer the springs and the less travel the suspension would make.
Does that make sense?
Softer springs=More travel for a given load
Stiffer springs=Less travel for the same load

Roy
Big Daddy
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Re: Bump Steer

Post by Big Daddy »

:iagree: But energy into the whole suspension system still remains the same and must be dissipated. Damping from shock absorbers is the biggest contribution but tyres also have some effect.
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earthhist
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Re: Bump Steer

Post by earthhist »

roymck wrote:
As regards the suspension bottoming out the Bump stops could be trimmed slightly but also the pressed steel pan that sits inside the spring that hits the bump stop differs between the Midget and the A30 , just cannot remember what one is shortest.
IIRC the midget ones give about 0.5 inches less spring travel (i.e the centre of the pan is higher), which equates to about 1 inch at the wheel.
Richard Johnston, Plymtree Devon, formerly A30/A35 Club Technical Officer and President
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gazza82
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Re: Bump Steer

Post by gazza82 »

Whole load of discussion on bump steer and somebody called Ackermann, sorry Ackermann steering, on a youtube video here ...


Click here to learn how to add YouTube Videos to your phpBB forum

Skip to 1:28 ..


This is a real hybrid .. Celica GT-4 into an Mini ... and episode 11 ... so lots more where this comes from .. :whistle:
"If you're driving on the edge ... you're leaving too much room!"

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Cars: "Project 757" '59 A35 2-door bought in 1971 & Subaru BRZ SE LUX Auto plus "family fleet": Alfa MiTo, Peugeot 206, (Ex '98 Alfa Romeo 156 2.0 TS)
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gazza82
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Re: Bump Steer

Post by gazza82 »

By the way .. there's plenty in the "Project Binky" to keep everyone happy .. suspension, welding, cutting, jigs, more welding, rust (lots of rust!), more welding ...

Oh and as you would expect, the odd profanity .. they are are affectively redesigning a Mini shell! :rol:
"If you're driving on the edge ... you're leaving too much room!"

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Dave Clark
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Re: Bump Steer

Post by Dave Clark »

I have followed that project since the beginning - absolutely awesome.

I,m pleased to see the use of a laser in connection with bump steer experiments - I used a laser pen on the van when it had a rack.

When I win the lottery I am going to get those guys to do an A35 with the same mechanics -200+ bhp, four weel drive, whats not to like ?
Big Daddy
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Re: Bump Steer

Post by Big Daddy »

Dave Clark wrote:When I win the lottery I am going to get those guys to do an A35 with the same mechanics -200+ bhp, four weel drive, whats not to like ?
Possibly four wheel drive :?:
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Re: Bump Steer

Post by gazza82 »

It will need some Evo2 arches ... ;-)

I think it would be interesting to see and drive ...

I quite fancied putting an Alfa 75 TS runing gear with rear transaxle under one ... But my imagination is better than my engineering skills. :-(
"If you're driving on the edge ... you're leaving too much room!"

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Re: Bump Steer

Post by Dave Clark »

I said weel dinneye ?

I knew I should not have acquired a reputation for spelling monitor !
Big Daddy
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Re: Bump Steer

Post by Big Daddy »

Big Daddy wrote:
Dave Clark wrote:When I win the lottery I am going to get those guys to do an A35 with the same mechanics -200+ bhp, four weel drive, whats not to like ?
Possibly four wheel drive :?:
Dave Clark wrote:I said weel dinneye ?

I knew I should not have acquired a reputation for spelling monitor !
Didn't even notice your speeling mistake... :mrgreen:

Have a feeling that achieving what you want from a four wheel drive system would be extremely difficult. For instance, how would you manage dynamic distribution of F/R wheel torque :?: ... :whistle:
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