Damper lengths.....

June 18, 2019

This has come up a few times. In general I say with as much stroke as possible which never answers the question unless your going for zero drop, or dislike suspension of course. Then you need to start thinking about the restrictions to having a vast amount of stroke. These maybe as it is on the front of my car the wishbones touch the chassis if there is to much droop. But there are an infinite number of things which need looking at. Things like brake pipe lengths, driveshafts pulling out, driveshaft joints past design angles, rack joints locking, geometry sudden changes, .... The list is endless and varies from car to car. The best way of checking these things is to remove any damper on the car and jack the suspension though a full range of movement, measuring everything, eyes and ears fully open, full lock both ways ways and see where restriction is needed. The damper open length dictates the suspension fully extended length. The damper closed length unless there are external bump stops should dictate the suspension fully compressed length. These lengths are absolute and not load dependent. Within these absolutes tyres should not rub, brake pipes not stretch, chassis should not hit the floor etc and the list or real nasties is long but common sense

Recently I have read where people have rubbing of tyres on the rear of there vehicles, also I was recently contacted by some one with 85mm lowered floors which naturally rub the ground from time to time. Increasing ride height/ spring rating/ damper stiffness will reduce the likely hood of the suspension compressing so far but its not absolute. A bigger mate or mid corner bump can still push the wheel up enough to rub.

I have to design many kits for sale in countries like Germany which have rules for aftermarket modified suspension parts. There are many many considerations , reflector heights, load rating and ground clearance to name a few. So the suspension still needs to work with 1.4 x the gross axle weight while ground clearance is above 80mm and nothing else rubs. 1.4x gross load statically is supposed to take into account dynamic loadings. 80mm I believe is an arbtary figure based on a brick IIRC.

I recently fitted some new dampers to my car. I took the opportunity to run a **tape measure** against my car. Closed length metal to metal (no bumpstop fitted) 240mm giving a center wheel to wheal arch measurement of 245mm, and fully open 335 mm giving a fully open center wheel to wheel arch measurement of 365. The average Motion ration across the full range of movement is there for (335-240)/(365-245)= 95mm Damper movement over 120 wheel movement =0.8 But the reality the MR drops off when the damper fully compressed so it closer to 0.85 for most the travel.

*** tape measure measurements are approximate***

Theres at least a fingers gap between anything rubbing and in fact a shorter closed length could be used without car self contact. The ground clearance at full compression 60mm. This is with 195-50-15 tyres. Adding to the safety factor is that there will be a bumpstop but that is some what offset with the tyres compressing under load, and damper bushes squashing a little. Ground clearances depend of the type of vehicle and what you are using, and where. A LandRover used on rutted tracks needs a lot for ground clearance, where as a Westfield used on nice roads or tracks needs a lot less.

With a factory 35-40mm lowered floor would be very close to the ground but I would say most of the time would be OK.

The Compressed length of dampers can be adjusted a little to make longer by adding packers around the shaft and using different bump stops.

The above picture (don't say the damper is mounted upside down, they also have 2 knobs!) Shows a plastic bump packer resting on the bump stop. These bump packers also have vent groves to allow air trapped in the bump stop to vent without trying to force air and debris into the damper seal. They can be stacked.

Different bump stops and bump stop materials will change the fully loaded compressed length maybe. Dense expanded foam types still compress to very short usually 15-20% of the original length where as block of rubber simply change shape not volume so tend to have a relatively long loaded length 25-30% of original length.

Damper open length,

There may be little preventing you wanting a really long open length other then driveshafts pulling out etc. There will be restrictions in terms of what damper manufacturers can offer lets say you want 240 closed the max open length we can offer would be ~ 345 mm, in the case of the above dampers these are double adjustable so the maximum I could give myself was 335 open.

Also if the spring dislocates at full extension there no point in having a damper so long it allows dislocation as once the spring dislocates there is only the undamped weight on the ground not offering much grip. MOT person dislikes loose springs too.

In a normal tin top road car standard soft suspension will offer 75-100mm of wheel extension from ride height. The wheels will stay on the ground even on some fairly rough uneven surfaces while cornering bl**** hard. On a relatively stiff low center or gravity car like a Westfield used on smooth roads 50mm of droop is enough on the driving wheels, less on the non driving wheels. These figures are based on normal loaded to extended wheel position. I've driven many cars with much less droop and you'd not know.

In the example of my car.. Open length is 335 giving a center of wheel to wheel arch of 365mm, typical damper length at normal ride height is 295 mm, 40mm of damper travel is ~ a little under 50mm of droop wheel travel. In the other direction from normal ride the car sits 15mm of a 40mm bump stop .

In the case of the guy with 85mm lowered floors I'd look to extend the closed length of the dampers ~ 40mm to stop the car hitting the floor in bump situations, have the rear ride height ~ 50mm higher than mine and the open length of the dampers probably 40mm longer than mine too.. Assuming brake pipes, drive shafts can handle the extra extension... With such a large increase in ride i'd be concerned about the rear geometry as well ... Compromises are not always easy to make or agree.