$2 for an improved suspension…

Posted on December 11th, 2008 in Driving Emotion,Honda,Opinion,Suspension by Julian Edgar

The topic of bump stops does not attract much interest. But especially in cars with lowered suspension, and in light-weight cars, bump stops form an important part of the springing system.

A bump stop is the (usually) rubber buffer that is compressed as the suspension reaches full bump. (Some cars also have full droop buffers as well.)

Traditionally, bump stops were impacted only rarely, but more and more often in current cars, the suspension is designed in such a way that the bump stops are frequently contacted.

Let’s look at light weight cars first.

In a light weight car, the variations in possible loads make up a greater proportion of the overall vehicle mass. This means that, to avoid bottoming-out, the suspension must be set up more stiffly to cope with the potential load variation.

Or – and here’s the key point – the bump stops can be designed to be increasing rate (but still relatively progressive) springs that are brought into operation when the car is carrying full loads over bumps. That way, the spring rate of the suspension during ‘normal’ load carrying can be set much softer, giving a better ride.

And in cars with lowered suspension? In these cars, the amount of bump travel is decreased. Even with the stiffer springs that are used, the bump stops are likely to be hit much more often than with the unmodified suspension.

This pic shows two very different rear suspension bump stops. Both are from front-wheel drive cars with torsion beam rear suspensions. The bump stop on the right is from a Honda Insight and the one on the left is from a Daewoo Matiz. Both are tiny, light weight cars.

The two-seater Insight has a short travel rear suspension – the designers get away with it by specifying a small load carrying capacity.

On the other hand, the Matiz is a four seater – its potential load variation is much greater than in the Honda. So the Matiz designers have done two things differently to the Honda. They use stiffer rear springs – and a much better design of bump rubber.

Both the Honda and Daweoo bump rubbers are progressive – that is, their spring rate (measured in pounds/inch or Nm/mm) increases as they are compressed. This increasing rate is achieved in two ways – the shape of the bump stops and also the fact that they are made of rubber, a substance that naturally has an increasing spring rate.

But as the pic shows, the Daewoo bump rubber is far more progressive – the thinner parts of the rubber initially compress more easily, bringing together the thicker, stiffer parts. Not visible in the pic is the fact that the Daweoo bump rubber also has a hole through the middle into which the compressed rubber can expand.

If carrying loads in the Honda, the rear suspension can easily bottom-out with a jarring thump. To overcome this, I originally fitted Daewoo Matiz springs to the Honda (they go straight in), a swap that gives a stiffer rear suspension. However, the standard Honda rear dampers are then a bit soft – to get best results, they should also be upgraded, something I didn’t do.

I ran with the Daweo rear springs and standard dampers for about a year, but then recently wondered what would happen if I went back to the original soft Honda springs – but also fitted the Daweoo bump rubbers.

And the result? It’s very good – a softer rear suspension but without the jarring bottoming-out over big bumps when carrying a load. In fact, it’s rather hard to tell when the suspension is hitting the bump rubbers – exactly what is desirable.

Especially in cars where the springs are easily removable, it makes sense to explore different bump rubber options. After all, there’s nothing to stop you – for example – fitting much longer than standard bump rubbers, so giving you a more rapidly rising rate spring. (In a front-wheel drive, doing this to the back suspension is likely to reduce understeer as rear roll stiffness increases.)

Or, as I have done, simply fitting a better design bump rubber to give improved full travel behaviour.

At my wrecker, bump rubbers are just a few dollars each – well worth exploring!

4 Responses to '$2 for an improved suspension…'

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  1. Ross said,

    on December 11th, 2008 at 2:11 pm

    This was a common trick on Minis, particularly the Hydrolastic ones, where it was difficult to put in stiffer springs because of the rubber cone suspension.

    People would put big soft bumpstops on to give a stiffer suspension, then reduce the hydrolastic pressure to lower the whole thing onto the stops.

    Apparently it reduced the characteristic pitching of hydrolastic Minis substantially.

  2. Richard said,

    on December 12th, 2008 at 9:18 am

    This solution is used in the NA/NB MX5 (the look like the Matiz stops). It can make it difficult to lower the suspension unless you cut the bumpstops (which form an integrated part of the suspension).

    It’s very easy to ruin a MX5’s ride/handling if you dont take the bumpstops into consideration when modding the suspension.

  3. Ben said,

    on January 5th, 2009 at 5:56 am

    This would be an interesting thing for my friends cars. Both are happy with the suspension on their cars (Both have twin turbo legacy’s, one is a sedan, the other is a wagon) except for the fact that they both scrape tyres when cornering at moderate speeds with weight in the back. It has to be worth at least looking to see what kind of bump stops are fitted.

  4. John said,

    on September 9th, 2009 at 4:36 pm

    I’ve lately been experimenting with home made ‘bump stops’ that are designed to be in light contact with the damper at static ride height, and to act as auxilliary ‘helper’ springs in order to substantially increase the total spring rate in bump (‘jounce’).

    These ‘stops’ (‘jounce snubbers’?) are made out of modified rubber spring shackle bushes that I’ve partially tapered to provide a degree of progressive ‘spring rate’, and fit onto the damper shaft in place of the stock bump stops. They are long enough so that at ride height they have some immediate affect with any bump motion.

    On my car (CB7 Accord) they are ftted to the quite soft OE coils and to Koni ‘Sports’ dampers and so far have been working remarkably well. For a ridiculously small $ outlay (for the spring shackle bush rubbers) and a morning or twos work I’ve substantially improved the roll motion, steering / handling response and chassis balance of my old Accord, without causing any stability or other handling problems (and I drive on less than smooth rural roads, including dirt).

    The car is a joy to drive with it’s new ‘spring rates’ (and some other suspension / geometry mods and chassis braces). The only issue with using rubber for these ‘stops’ (as opposed to the urethane ‘foam’ that is typical of a modern bump stop) has been that the ‘spring rate’ is somewhat temperature sensitive, so the ‘springs’ are noticably stiffer on colder days than hotter days (which affects ride harshness more than handling). I also don’t as yet have a good idea as to how long the rubber will last…