Adjusting the suspension stiffness on a 7?

[philso]

I would like to ask how easy or difficult it is to adjust (soften) the suspension stiffness on a 7.

 

I have been in (and out) of the market for a 7 for some time now. I have found I have been mainly attracted to some of the more hard-core examples (often with race-prepped engines) that appear to have been built for mainly track or autocross use. Many of these are using GAZ adjustable coil-overs or similar.

 

My intended usage would be mostly on the road with 2 or 3 track days a year. One of my friends warned me that I might find the suspension on the track-oriented cars too harsh for my intended use. Previously, he had graciously let me take one of his Caterhams for an hour-long drive on some local country roads. I think it was one of the lower end models. I was quite happy with the ride quality in that car and probably could have tolerated a somewhat stiffer ride. I am aiming to go have a closer look at some of these cars to see how one of these track-oriented cars feel on the road.

 

If possible, I would aim to do the suspension work myself.

[MV8]

In general, a dial caliper and tape measure (for determining the old rate) and a coil over specific spring compressor (for installing a softer rate, longer "free length" spring) are required but you may not need any of these tools. Details are important for deciding what rate is appropriate for touring. Springs are universal and cost around $100-250/pair depending on the vendor and coil diameter.

An adjustment to the coil over damping rate can also cause a rough ride. Bigger wheels, tires, brakes, and higher rate springs usually weigh more and may require more damping and/or rate to control this unsprung weight trying to stay in constant contact with the road.

Depending on how similar your car will be to your buddies tourer, you may be able to use the same rate springs. Deciding what to buy is the hard part but a bad choice can be reversed easily.

[Davemk1]

There are of course three ways to add comfort/compliance to your car -

 

 - you can change the spring rate

 - you can change the damping

 - you can change the height of the tire sidewall

 

Springs - One needs to keep in mind that most Sevens have very limited suspension travel and that fitting soft springs might allow the suspension to bottom out more easily. The stiffness of the spring will dictate how far the suspension moves when it encounters a bump and not how easy it will move. This is very often misunderstood.

 

Damping - where spring stiffness dictates how far the system will move it's the damping that will dictate how quickly the suspension moves and this is something that can easily be felt. It matters little how stiff the spring is if the damping is set so stiff that the system can't move quickly enough to adsorb a bump. A car that is "over damped" will feel hard, harsh, and will lack a connected feel to the surface. Traction will be compromised. If, on the other hand, the damping is too soft the car will wallow and float as the suspension will still be reacting to one bump when it hits the next. This is where the time factor of damping comes in. Also...if the damping it too soft the car will have a greater tendency to run through all of its travel and bottom out.

 

The system will work best when the spring rates and the damping are working well together....stiff springs and soft damping will suck just as much as too soft springs and too hard damping. The two work together in conjunction and it's best to look at them as a system. A given spring rate needs a given amount of damping to control that force.

 

 

Tire sidewall - if we are talking about a traditional Seven like a Caterham, Westfield, Birkin...etc many of these cars will come with 15" wheels and a tire with a low profile sidewall. The height of the sidewall will have a real effect on ride comfort. It makes sense when you think about it...tires are an air-spring and a smaller volume spring will have a higher spring rate and it will be more progressive. One thing that can be done is to replace the 15" wheels with 13's and a tire with a taller sidewall. The tire will be the same outside diameter and this means that the sidewall is a full 1" taller and this does a great job of taking the buzz out of the ride by absorbing road imperfections before the suspension can even deal with it. There are also real-world handling benefits of the taller sidewall with a lightweight car and that is a topic of it's own. One of the best things I ever did with my Westfield was to ditch the 15's and put on 13's with a taller tire...better ride quality, better handling and lower un-sprung weight. A win-win-win if ever there was one.

 

Just my 2 cents. Have fun with it.

 

dave

 

 

[philso]

Thanks for your feedback. It appears that you are saying that the suspension is something that can be adjusted (with reasonable cost and effort) after purchase, unlike cockpit size.

[MV8]

Cockpits are adjustable too, but usually smaller. Depends on current seats. Originals had squabs.

[JimmyG]

Hi Phil, The regular production Caterham 7s up to and including the 420 only offer suspension adjustment for ride height by turning the spring preload collars on the shock bodies. But even with the base Bilstein spring/shock setup they are dialed-in up really well for spring rate and damping, so you won't need to change anything between street and occasional track days and autocross. You do want to keep the ride height as low as possible for a lower center of gravity and better handing, but in my case I have a lot of speed bumps and rough canyon roads where I live, so I have to keep the ride hight set fair high to not hit and bottom my engine sump.

Unless you are tracking and autocrossing your car full time, you won't need or want to make any other suspension changes to stiffen up your car. Better tires like Toyo Proxies R888R will give you a big handing advantage.