With the fashion in street cars heading to ever decreasing ride heights, something's got to give. And when it comes to the chassis, it's most often spring travel that gets the first cut simply because lower springs are the fastest, cheapest and easiest solution.

The advent of air springs has been revolutionary, as for the first time a low chassis height has been possible with adequate suspension travel while driving. However, in the street rod vernacular if low is good, then really low can only be better, and this leads to the need for chassis re-engineering because bolt-on solutions have their limitations.

   • Click here to see suspension lowered via shortened coil springs


Taken to extremes, there's a limit to how far a chassis can be lowered by shortening spring height, as eventually there will be no suspension movement remaining and it will come into contact with the bump-stops (jounce bumpers), chassis, or body. At that point serious re-engineering of the chassis or uni-body is required to increase clearance, but due to cost and time factors major chassis surgery is usually a last resort.

A key function of suspension design is to help maintain contact between a vehicle's tires and the road surface....

Providing increased axle clearance provides benefits beyond simply lowering a chassis in the quest for weed-eating aesthetics, it enables longer springs to be retained and this contributes not only to better ride quality but handling also.

Suspension insulates passengers from road undulations and vibration, however maintaining contact between tires and the road surface is necessary for grip, which in turn determines how well a vehicle accelerates, brakes and corners. And while stiff springs will reduce body roll, they can also impact on the ability of a chassis to maintain tire contact with the road.

Over a bumpy surface, stiff springs and/or limited suspension travel can lead to a chassis being violently forced upward, during which time tire/road contact can be lost. Conversely short, stiff springs can affect an axle's ability to drop into a road depression - again with the potential to lose contact between the tire and road.

   • Click here to see how overly stiff or too soft springs can cause loss of tire/road contact


Contrasting this, a spring with enough extension to retain tension during suspension droop will help maintain road/tire contact as it pushes the tire against the road.

It's a popular misconception that stiff springs and heavy-duty shock absorbers automatically equate with race-car handling....

The amount of weight it takes to compress a spring a given amount is called 'Spring Load'. This is often confused with 'Spring Rate', which is the amount of weight required to compress a spring one inch.

'Spring Load' is expressed in pounds and dictates how much weight a spring can support at a given height. 'Spring Rate' is measured in pounds-per-square-inch and does not change as a spring is compressed.

A 'Spring Rate' of 100 lbs-in. will see a spring compress one inch with a 100 lb. load; compress two inches with a 200 lb. load; and three inches with a 300 pound load. Based on this principle, if a chassis is lowered using shorter springs then the 'Spring Rate' needs to be increased to prevent the suspension travel from bottoming out.

Take for example a car that has three inches of axle travel on compression (jounce), that's fitted with 100 lbs-in springs. The suspension would be fully compressed with a 300 lb. load.

Now imagine if the same car is lowered two inches; it would require a 300 lbs-in. spring to prevent the chassis from bottoming out with the same load. A lowered car that has suspension travel of only one or two inches may look cool, but it's going to ride rock-hard or bottom out on minor bumps. Either way, this involves the potential for loss of road/tire contact.

If short-stroke shock absorbers are fitted they may limit suspension compression & extension, also limiting road/tire contact....

Assuming that suspension components have adequate clearance from the chassis or body, the amount of suspension travel that springs and shocks provide is a key factor in handling. Spring and shock travel is important in terms of compression over bumps, and extension into road depressions.

   • Click here to see how short springs & short-stroke shocks can stop droop and cause loss of tire/road contact



Suspension design should also be such that the overall stroke of a shock absorber should match overall spring length and, ideally, its stroke should be divided by two to match spring compression and rebound respectively.

For example, a coil-over shock absorber that provides 5 inches of overall travel should be installed so that at ride height it provides approximately 2.5 inches of compression and 2.5 inches of rebound. The same principle applies to shock absorbers used in tandem with a separate spring, such as a transverse spring in a street rod chassis.

Springs and shocks need to be matched, in terms of 'Spring Rate', overall length, compression and rebound stroke, and compression and rebound valving. Engineering adequate suspension travel at the desired ride height will not only help assure ride comfort but a chassis' ability to maintain road/tire contact, which affects acceleration, braking and cornering.

Lowering a street rod chassis using bolt-on parts is easy, however providing adequate spring and shock travel requires real engineering....

So what exactly happens when bolt-on components are used to lower a reproduction street rod chassis? The first step is usually a dropped axle. Four inch dropped axles are the most common for good reason, as any axle with a bigger drop starts to compromise ground clearance when front tires with less than 24-inch diameter are used.

It's common practice to equip Deuce street rod chassis with a Model A front cross-member as this lowers the chassis one inch by raising the transverse spring mount the equivalent amount. Use of a reverse-eye spring will gain another inch, more if a de-arched or mono-leaf front spring is used.

However if the front spring mount is raised and overall spring height is decreased, suspension travel becomes increasingly limited by the proximity of the top of the transverse spring to the underside of the chassis rail.

Notching street rod chassis rails above the front spring to provide more suspension travel is done at the expense of weakening the chassis in a critical area....

Following the principle of the Model A front cross-member, custom flat front cross-members are becoming more common in the quest for ever-lower front ride height in cars equipped with transverse front springs. This style of cross-member further reduces the distance between the transverse spring and underside of the front chassis rails, which is why the rails are usually C-sectioned to provide more spring clearance.

Typically this C-section can be half the height of the chassis side rail, which begs the question whether removing this much steel from where a critical cross-member mounts is good engineering practice, especially when the weight of the engine and transmission are in very close proximity.

Without adequate chassis clearance a soft front spring chosen for comfort is going to continually bottom out, and a overly stiff spring that prevents bottoming is going to make the chassis ride rock-hard - in turn compromising acceleration, braking and cornering.

Street rod chassis manufacturers use a variety of techniques to lower chassis, and while all tend to reduce spring travel, little thought is often given to the shock absorber that has to work with such modifications. Usually a 'standard' length shock mount is retained and a short-stroke shock absorber is used, as this is an easy bolt-on solution.

Typical street rod shocks feature 2-3 inches of total travel, resulting in as little as 1-1.5 inches of suspension on compression and rebound....

Typically, lowered street rod chassis are fitted with front shocks with 2-3 inches of travel. Divide that by two for adequate suspension compression and extension - and even if the front spring can provide 2 inches of travel the shock will bottom out before the spring does, limiting suspension travel to 1-1.5 inches.

By engineering a street rod chassis to have adequate chassis-to-axle clearance, longer springs and shock absorbers, and bump stops can be used. And while it's common practice to remove bump stops to provide increased suspension travel they are more important than most people assume, being designed to compress with increasing resistance, which decelerates axle travel when it reaches its limit.

When adequate suspension travel is provided and spring and shock travel are matched in terms of compression and extension, this can enable the best of all worlds - a softer more comfortable ride, better handling, and a weed-eating stance..........read more


 

 

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