Sus­pen­sion Ge­om­e­try

Con­fused by cam­ber, caster, toe-in and whether wheel align­ment is worth­while? Fol­low our guide to sus­pen­sion math­e­mat­ics for clas­sic cars.

Classics Monthly - - Contents - Words and pho­tog­ra­phy Rob Hawkins

We cut through the mys­tery sur­round­ing caster an­gle, cam­ber, wheel align­ment and sus­pen­sion math­e­mat­ics.

The ride qual­ity, han­dling and sta­bil­ity of a car can be ad­versely af­fected if fac­tors such as the align­ment and an­gles of the road wheels are in­cor­rect. So it’s worth­while know­ing what’s in­volved in set­ting up your car’s sus­pen­sion ge­om­e­try, even if you in­tend to hand the job over to a spe­cial­ist.

A good start­ing point for en­sur­ing your car’s sus­pen­sion ge­om­e­try is cor­rect is to check the tyre pres­sures reg­u­larly and have the wheel align­ment (track­ing) checked at least once a year. Tyre pres­sures are im­por­tant be­cause they not only af­fect the grip un­der cor­ner­ing, ac­cel­er­at­ing and brak­ing, but also the ride height (use­ful if you can al­ter the ride height), which in turn can make a mess of other sus­pen­sion set­tings. Hav­ing the wheel align­ment checked at reg­u­lar ser­vice in­ter­vals can save on un­even tyre wear, mak­ing it po­ten­tially cheaper than hav­ing to re­new tyres.

Wheel align­ment isn’t par­tic­u­larly so­phis­ti­cated in most cases. It con­cerns the di­rec­tion of the road wheels, which for the ma­jor­ity of cars, should be al­most point­ing straight ahead (when the steer­ing wheel is straight for the front wheels). Some cars rec­om­mend a small amount of toe in or out, de­pend­ing on whether the wheels are driven or not. Toe in/out is mea­sured in de­grees and min­utes, mil­lime­tres or inches. Imag­ine look­ing at the car from above, and if the lead­ing edge of a pair of wheels is point­ing to­wards each other, this is called toe-in. This is usu­ally rec­om­mended for the front wheels of a rear wheel drive car. The op­po­site, toe-out, is where the lead­ing edges of the wheels are point­ing away from each other, which is usu­ally rec­om­mended for the front wheels on a front wheel-drive car.

The an­gle of the toe in/out is usu­ally al­tered via the track rod ends on the front of a ve­hi­cle, which con­nect the end of the steer­ing rack or steer­ing links to the part of the hub (steer­ing arm) that helps to move the wheels from side to side. The track rod ends have a threaded sec­tion and lock nut, which al­lows ad­just­ments to be made to change the amount of toe.

At the rear of some ve­hi­cles, toe in and out can be ad­justed via a track con­trol arm, which is sim­i­lar in de­sign to a track rod end. These are found on sports cars such as the Mk1 Toy­ota MR2 and MGF/ TF.

Bud­get for up­wards of £30-£40 to have the wheel align­ment checked by a spe­cial­ist. Some main­stream wheel and tyre

spe­cial­ists of­fer free wheel align­ment checks, but charge to make ad­just­ments. DIY track­ing kits are also avail­able from around £50 or more, but in most cases, you’ll need a flat sur­face to check the mea­sure­ments.

Caster

The amount of caster at a front road wheel is an im­por­tant as­pect of sus­pen­sion ge­om­e­try that can be checked at the same time as the wheel align­ment. It’s mea­sured in de­grees and rep­re­sents the an­gle to which the steer­ing pivot axis is tilted for­ward or rear­ward from ver­ti­cal, when viewed from the side of the wheel. It’s usu­ally mea­sured as an an­gle of the road wheel when the steer­ing is turned 20 de­grees left and right from straight-on: the dif­fer­ence be­tween the two fig­ures is the amount of caster. Up to four de­grees of pos­i­tive caster is ideal for many clas­sic cars. Neg­a­tive caster makes a car un­sta­ble when driv­ing in a straight line. Caster can usu­ally be ad­justed when the track­ing is ad­justed, or if ad­justable tie-rods are fit­ted.

The cam­ber of each road wheel can help or hin­der the ride qual­ity and han­dling of a car and scrub tyres if it’s wrong. It rep­re­sents the ver­ti­cal an­gle of a road wheel when po­si­tioned on a level sur­face. Mea­sured in de­grees, one de­gree of neg­a­tive cam­ber for in­stance, means the top of the wheel is an­gled fur­ther in to the car than the bot­tom of the wheel by one de­gree.

Cam­ber can be mea­sured us­ing a spirit level type of gauge (bub­ble or dig­i­tal) fit­ted against a flat sur­face on a road wheel, or across the rim.

Cars with MacPher­son strut sus­pen­sion may fea­ture a means of ad­just­ing the top mounts or one of the mount­ing bolts where the strut is se­cured to the up­right, which in turn changes the cam­ber of the road wheel.

The MX-5 has cam­ber ad­just­ment via the in­ner mount­ing bolts for the lower sus­pen­sion wish­bone arms. The Mini has no means of ad­just­ing cam­ber as stan­dard, but cam­ber brack­ets for the rear ra­dius arms and longer front lower arms are avail­able in ad­justable and fixed lengths.

Balanc­ing

There are a num­ber of DIY checks that can be con­ducted to mea­sure how well-bal­anced your car is. If there’s a suit­able jack­ing point at the front and rear of the car which is in the cen­tre (e.g. MGF and the rear diff of many RWD cars), then raise the car with a trol­ley jack and mea­sure the ground clear­ance at the op­po­site cor­ners. This helps to mea­sure the cor­ner weight­ing of the car. It’s not as ac­cu­rate as us­ing weigh­ing scales, but it can help and if you have ride height ad­justable sus­pen­sion, it can be al­tered.

An­other bud­get method is square mea­sur­ing, which en­tails cre­at­ing a rec­tan­gle with 90 de­gree an­gles and equal op­po­site sides around the car us­ing axle stands, fish­ing wire and straight lengths of wood or metal. The car must be on level ground and the fish­ing wire or straight lengths of wood or metal needs to pass by the cen­tre of each wheel. Once this has been set up (which may take a long time), mea­sure­ments can be made for wheel align­ment and to see how square the cen­tre of each wheel is in re­la­tion to the rest of the car. Laser equip­ment is more ac­cu­rate, but not as cheap or ed­u­ca­tional.

Sus­pen­sion ge­om­e­try is a fas­ci­nat­ing sub­ject that isn’t solely for race car en­gi­neers, but it does re­quire time and pa­tience to achieve worth­while re­sults.

The MGF has cen­tral jack­ing points at the front and rear, mak­ing it ideal for a spot of cheap DIY cor­ner weight­ing.

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