DIAGNOSING AND REPAIRING Issue 12/2017
BENT SUSPENSION COMPONENTS SHOCK ABSORBER, SUSPENSION, BRAKES, TOWBARS AND WHEEL ALIGNMENT SPECIALISTS
Diagnosing Bent Steering and Suspension Components Using Steering Geometry Angles
We do this by making use of alignment angles to effectively divide the suspension into two halves. The alignment figures will tell us in which half of the suspension the fault will be found.
Camber is one of the most commonly adjusted alignment geometry angles and 95% of all faults are corrected by normal alignment methods. However in the other 5% of cases, location of damaged components can prove difficult and time consuming. More importantly, incorrect diagnosis and repair of the camber faults may lead to far more serious ramifications. This issue of Tech Stop, shows how alignment angles can be used to indicate where the damaged component is, what to replace and how to achieve correct alignment geometry angles.
The alignment angles we use to do this are Camber, S.A.I. (Steering Axis Inclination) and I.A. ( Included Angle). S.A .I., also known as King Pin Inclination (K.P.I.), is the angle between the true vertical and a line drawn through the centre of the strut’s top pivot (or upper ball joint) and the lower ball joint. It is sometimes difficult to obtain an OE specification on S.A.I. and so we recommend keeping a record of SAI angles to obtain an average figure which becomes your specification for a particular vehicle.
There are a number of potential causes of camber faults including movement within the bushes; wear in the joints; ride height changes; flex from within the chassis; or a combination of the above.
I.A is the angle between the S.A.I. and camber line when viewed from the front. We cannot measure I.A. directly and so we calculate it by adding or subtracting the actual camber from the S.A.I.. This explains the term ‘Included Angle’ as you need to include camber with the S.A.I. to get the I.A.. When camber is positive, add it to the S.A.I. angle. If camber is negative, subtract it from the S.A.I. to obtain the I.A..
As we have mentioned, normal adjustment will correct the great majority of these faults. Our purpose here is to show how the use of alignment angles, can make it easy to identify problems.
Here are two examples: S.A.I. + Camber = I.A. 1. S.A.I. = 9°
Camber = + 1°
I.A. = 9° + 1° = 10° 2. S.A.I. = 11°
Camber = -1°
I.A. = 11° + -1° = 10°
Example 1
Example 2
Straight advice, specialists you understand and...
e S.A.I. and I.A. are very good wheel alignment geometry angles to use as diagnostic tools for vehicles at are not producing alignment figures consistent with the O.E. specifications. It helps you determine if a mponent is bent and where it is located.
DIAGNOSING AND REPAIRING
BENT SUSPENSION COMPONENTS
the S.A.I. is at the OE specification, (on average 13° to 14°), we can be certain that the problem is tboard of the steering axis pivots. If the S.A.I. is greater or less than the OE spec, then the problem will lie board of the steering axis pivots, one of the pivots has moved (or both). Issue 12/2017 twin lateral arm suspension the steering axis pivot is the centre line drawn between the upper and lower ll joints. On a MacPherson or Chapman strut suspensions, the steering axis pivot is TOWBARS the centre point of the SHOCK ABSORBER, SUSPENSION, BRAKES, AND WHEEL ALIGNMENT SPECIALISTS p pivot bearing and the centre of the lower ball joint.
ample 1. The S.A.I. and I.A. are very good wheel alignment
The Included Angle on the LH side is within OE specs, geometry angles to use as diagnostic tools for vehicles but the RH side is 2° out. Because the RH side S.A.I. is that are not producing alignmenthas figures specification but its Included Angle is incorrect, we car with MacPherson Strut suspension the consistent following cambertoreadings: with the O.E. specifications. It helps you determine if a know that the fault exists outboard from the steering axis is bent is located. pivots. H = -1° component LH = +1° 10' and OEwhere spec =it +1° If the S.A.I. is at the OE specification, (on average 13°
The components that are outboard from the steering
e vehicletowas involved in an accident and we need to determine what damage has been done to the 14°), we can be certain that the problem is outboard axis pivots are the ball joint stud, the strut shaft, the strut gnment . of A the visual inspection reveals no difference between the left and suspension components. steering axis pivots. If the S.A.I. is greater or less body andright the forged stub axle or the knuckle assembly. than the OE spec, then the problem will lie inboard of
y checkingthe thesteering S.A.I.,axis the pivots, following was one of thenoted: pivots has moved (or both).
H = 7° 15’, LH = 7° 00'
OE spec = 7°
We know that the fault does not exist inboard from the steering axis pivots. Thus the following components will be in correct alignment: the ball joint seat, lower control arm, lower inner bush, chassis rail, strut tower and top bearing plate.
In twin lateral arm suspension the steering axis pivot is theboth centre lineare drawn between the upper and lower ball e S.A.I. on sides within specs. joints. On a MacPherson or Chapman strut suspensions, Example the steering axis pivot is the centre of the top om the camber and S.A.I. readings, we canpoint calculate the pivot Included Angle. 2. bearing and the centre of the lower ball joint. A vehicle with twin lateral arm suspension has the H 6° LH 8° 10' following wheel alignment specifications:
Example 1. e Included Angle on the LH side is within OE but the RH side is A car with MacPherson Strut suspension has specs, the following camber A.I. is to specification butreadings: its Included Angle is incorrect, we know that the Camber 2° ering axisRH pivots. = -1° LH = +1° 10’ OE spec = +1° S.A.I.
RHout. Because LH the RH side OE 2° fault 20’ exists outboard -0° 20’ from the -O° l5’ 5° 55’
8°30’
8°35’
I.A. 8° 15’ 10’ 8°20’ The vehicle wasoutboard involved infrom an accident and weaxis needpivots are ewheel components that are the steering the ball joint stud, the8°strut shaft, the alignment geometry angles to use as diagnostic tools for vehicles to determine what damage has been done to the This vehicle obviously has a camber problem, but the ut body and the stub axle orIt the assembly. gures consistent with forged the O.E. specifications. helpsknuckle you determine if a
ocated.
alignment. A visual inspection reveals no difference between the left and right suspension components.
Included Angle is to specification. Because of this we
know that allThus the components between the two steering eation, know that the fault does not exist inboard from the steering axis pivots. the following components (on average 13° to 14°), we can be certain that the problem is pivots and the wheel are in correct alignment. By checking the S.A.I., the following was noted: ll be in correct alignment: the ball joint seat, lower control arm, lower inner bush, chassis rail, strut tower . If the S.A.I. is greater or less than the OE spec, then the problem will lie one of the pivots has moved (or both). d top bearing This means the problem is inboard of the steering axis RH =plate. 7° 15’, LH = 7° 00’ OE spec = 7°
steering axis pivot is the centre line drawn between the upper and lower S.A.I. on sides specs. amplestrut 2. The hapman suspensions, theboth steering axis are pivotwithin is the centre point of the the lower ballFrom joint. the camber and S.A.I. readings, we can calculate
pivots. The possible faults are a bent chassis, incorrectly positioned ball joints, stretched top arm, bent lower arm, worn upper or lower pivot bushes.
vehicle with twin lateral arm suspension has the following wheelalignment specifications: the Included Angle.
8° 10’ nsion has theRH following readings: RH6°camberLH LH OE
pec = +1°
mber
2° 20' -0° 20' -O° l5'
A.I.
5° 55'
8°30'
8°35'
A. pec = 7°
8° 15'
8° 10'
8°20'
ccident and we need to determine what damage has been done to the eals no difference between the left and right suspension components.
ng was noted:
specs. is vehicle
obviously has a camber problem, but the Included Angle is to ecification. Because of this we know that all the components between the gs, we can calculate the Included Angle. o steering pivots and the wheel are in correct alignment. Example 1 Example 2 de within OE but the RH is 2° out. the RH side is ismeans thespecs, problem is side inboard ofBecause the steering axis pivots. The possible faults are a bent chassis, luded Angle is incorrect, we know that the fault exists outboard from the correctly positioned ball joints, stretched top arm, bent lower arm, worn upper or lower pivot bushes.
Straight advice, specialists you understand and...
from the steering axis pivots are the ball joint stud, the strut shaft, the or the knuckle assembly.
xist inboard from the steering axis pivots. Thus the following components
DIAGNOSING AND REPAIRING Issue 12/2017
BENT SUSPENSION COMPONENTS SHOCK ABSORBER, SUSPENSION, BRAKES, TOWBARS AND WHEEL ALIGNMENT SPECIALISTS
Incorrect Diagnosis May Cause Other Problems
It is the scrub distance that effects feedback to the driver through the steering wheel. A vehicle with zero scrub distance has poor directional stability and very little feedback.
repair made, then the original problem is solved and a
Most rear wheel drive vehicles have positive scrub
A Commodore has the following angles:
change from positive to negative scrub radius resulted from the introduction of diagonal split braking systems.
May roblems rect Cause Diagnosis May Problems Other Problems If a Other problem is Cause incorrectly diagnosed and an incorrect
droblem ectly an incorrect diagnosed isnew incorrectly repair andappears. made, an diagnosed incorrect then the and repair original an isincorrect made, problem then repair the is original solved made, then and problem athe original is25solved problem and solvedfront and a problem The following an example of radius of between - 5Omm andaallismodern this. wheel drives have negative scrub radius of 2 - 6mm. The an s. problem The example following appears. of this. isThe an example followingofisthis. an example of this.
he mmodore following hasangles: the following RH angles: LH
Positive scrub radius causes the wheel to pull outwards and the opposite is true for negative scrub radius. When LH RH LH Both bearing plates are positioned for the lowest camber a front brake fails on a RWD vehicle, severe brake pull can occur. When a front brake fails on a FWD vehicle, the and highest caster. From this information and previous ber-0.50° experience, -2° -0.50° chassis would still try to pivot around the wheel which is the mechanic diagnosed it as a bent strut and it was given a camber correction of 1.50° This solves braking, but because it has a negative scrub radius the wheel will try to From turn in,this thus counteracting the ere bearing lowest positioned plates camber for areand the positioned highest lowest caster. for wear, theFrom and lowest this camber information caster. highest andthis caster. information andinformation andpivoting the camber problem andcamber tyre buthighest has caused aandFrom of the chassis. The result the vehicle stops in a straight possibly more serious one. If the S.A.I. and the I.A. had nosed ous the experience, mechanic it as a bent diagnosed the strut mechanic andititaswas adiagnosed bent given strut a camber itand as ait bent was correction strut givenand aof camber it1.50° was given correction a camber of 1.50° correction of 1.50° line. been checked, the mechanic would have found the solves erwear, problem the but camber has andcaused tyre problem wear, a possibly but andhas tyre more caused wear, serious abut possibly has one.caused Ifmore the S.A.I. aThis serious possibly and one. more If the serious S.A.I.one. andIf the S.A.I. and following: explains why an incorrect diagnosis which changes Camber
-2°
-0.50°
A. cked, would hadthe been have mechanic checked, found the would the following: mechanic have found following: havebetween found the the following: I.A. and hence the Scrub Radius can turn a simple Scrub radius or scrub distance is would the the distance
alignment fault into a far more dangerous problem. the centre of the tyre’s footprint and the point at which the steering axis meets the road surface. Zero or neutral bbance distance radius between orisscrub thethe distance distance centre of between is the the tyre's distance the footprint centre between of and thethe tyre's thecentre point footprint of at the which and tyre's thefootprint point at and which the point at which scrub radius is when the steering axis meets the road ts eering Zero the road or axis neutral surf meets ace. scrub theZero road radius or neutral is tyre’s ace. whenscrub Zero the steering or radius neutral when scrub meets the radius steering the is when axisthe meets steering the axis meets the surface at the centre ofsurf the footprint. Scrubisaxis radius iscentre positive the steering axis theradius road entre surf otprint. ace of the at Scrub the tyre's radius footprint. of iswhen positive the tyre's Scrub when footprint. radius theismeets steering positive Scrub axis when meets isthe positive steering the when axisthe meets steering the axis meets the inboard of the centre of the footprint; and it is negative nt; entre inboard andofitof the is the negative footprint; centre when of and theit the footprint; is steering negative and axis when itmeets isthe negative steering the when axis outboard the meets steering the road axisoutboard meets the road outboard when the steering axis meets the road outboard of theroad tyre’s centre. e tyre's centre.
ack he e that scrub to effects distance feedback that effects to feedback to eriver vehicle steering through wheel. the A steering vehiclewheel. A vehicle nce zero onalhas scrub poor distance directional has poor directional le ityfeedback. and very little feedback.
eitive rear vehicles wheelhave drivepositive vehicles have positive een all radius 25 - of 5Omm between and25 all- 5Omm and all drives ern scrub front have wheel negative drivesscrub have negative scrub sitive sheofchange 2 - 6mm from . The positive change from positive gative ius e resulted scrub from radiusthe resulted from the duction stems. nal splitofbraking diagonal systems. split braking systems.
oive causes pull scrub outwards theradius wheel and causes tothe pullopposite the outwards wheelistotrue and pullthe for outwards opposite negativeand scrub is true theradius. opposite for negative is true scrub forradius. negative scrub radius. nails cle, a front on severe a brake RWD brake fails vehicle, pull oncan asevere RWD occur. brake vehicle, When pull severe a front can occur. brake brakeWhen pull failscan on a front aoccur. FWD brake When fails a front on a brake FWD fails on a FWD Straight advice, specialists you understand and... would ot le,around thestill chassis the try to wheel would pivotwhich still around try is to braking, thepivot wheel around but which because the is braking, wheel it haswhich abut negative because is braking, it has buta because negative it has a negative elradius thus willcounteracting try thetowheel turn in, will the thus try pivoting counteracting to turn of in,the thus chassis. the counteracting pivoting The result of the the-chassis. pivoting the The of the result chassis. - the The result - the
DIAGNOSING AND REPAIRING Issue 12/2017
BENT SUSPENSION COMPONENTS SHOCK ABSORBER, SUSPENSION, BRAKES, TOWBARS AND WHEEL ALIGNMENT SPECIALISTS
Handy Hints One of the easiest ways of doing a preliminary check of the wheel base, is to check the gap between the tyre and the guard, using your fingers. If there is any noticeable variation then its time to get the tape measure and start checking accurately. A shorter wheel base on one side of the car usually indicates that an impact has occurred and further inspection is required. One of the indicators that there may be a problem, is that the Caster angles differ from left to right.
If you are having problems finding the source of a Camber fault, check to see that the ball joint has been bent. To confirm this, remove the stud from the taper and inspect it. Live axle housings can bend, so make sure that the rear geometry angles are checked as well. At Pedders, we look after suspension fault diagnosis on a daily basis. So talk to your local Pedders Suspension outlet for all your suspension and brake needs.
Common Alignment Problems with Double Wishbone Suspension. S.A.I
Camber
I.A.
Problem
OK
Less than specification
Less than specification
Bent spindle or upright, bent ball joint stud
Less than specification
Greater than specification
OK
Bent lower control arm, sub frame moved
Greater than specification
Less than specification
OK
Bent upper control arm, sub frame moved
Less than specification
Greater than specification
Greater than specification
Bent lower control arm as well as bent spindle or ball joint
Common Alignment Problems with Strut Type Suspensions. S.A.I
Camber
I.A.
Problem
OK
Less than specification
Less than specification
Bent spindle and/or bent body and /or Bent ball joint stud
OK
Greater than specification
Greater than specification
Bent spindle and/or bent strut body and/or bent ball joint stud
Less than specification
Greater than specification
OK
Bent lower control arm or strut out at top
Greater than specification
Less than specification
OK
Strut tower in at top
Greater than specification
Greater than specification
Greater than specification
Bent control arm or strut tower in at top as well as bent spindle or strut body
Less than specification
Greater than specification
Greater than specification
Bent lower control arm or tower in at top Plus bent spindle or strut body
Less than specification
Less than specification
Less than specification
Bent lower control arm or tower out at top plus bent spindle or strut body
Straight advice, specialists you understand and...