Tech Tips

"A reference for bump stop loads&bump stop selection"

In our last tech tip, we discussed the pros and cons of bump springs vs spring washers. In this issue, we will discuss our experience with bump stop loads. This will only be a guideline; however, for many of you, it will be a very accurate example to follow.

Most of us have a Gale Force machine and many of us measure our shock travel (if you don't, you should be!) either by using the o-ring on the shock shaft or an external travel indicating device. For must of us, this is the best way to understand what our race car is doing in a dynamic state, however, we must keep in mind that this is not an exact representation of what is happening dynamically, it is only a guideline. LET ME EXPLAIN. First off, if our RF shock travel shows us it is traveling 3" our our LF is 3 1/4", these numbers only represent our MAX travel and we have no way of knowing (without data acquisition) when or where on the race trace we achieved these travels. We can use this information, though, to better understand what is happening with our bump stops and make better decisions on tuning with our stops at the track. Let's look at the RF with its 3" of max travel - with that travel on our Gale Force - that comes out to 2200lbs. This is a combined load of 1: the preload in the spring 2: the rate of the spring and amount we travel it and 3: the load generated from the bump stop.

When selecting bump stops, we need to better understand what we are trying to do with them and in what load range we are working. In the above example of 2200lbs - that is only at max travel and that could occur anywhere on the track under braking, if there is a bump stop in play, etc. This is important to know and understand because, in actuality, when you subtract the spring from the equation, you might only see 600lbs on the stop and figure that the max travel is because there is a bump getting into turn 3, and now we might be at 500lbs that the stop might see as an average throughout the corner. Let's set up an example.

RF spring = 200lbs
RF eye to eye at ride height 18" = 1000lbs
RF travel = 3"
RF dynamic load = 2200lbs

So, if we take our 2200lbs and subtract our 1000lbs of preload and then subtract 600lbs  (200lb spring travel 3" = 600lbs), we come up with an answer of 600. 600lbs is the load that is generated through the bump stop at max travel.

Now that we have a baseline load number to work with, we need to look at what the race track is like. For example, if the track is smooth and has very few bumps, then you would want to run a harder stop and more gap, still ending up at our 2200lbs at 3" of travel, but because we have a harder bump stop and more gap, we hold our AERO platform longer and maintain optimum tire contact patch longer.

If the race track is a bit rough, you would be looking for a bit softer bump stop to be able to run less gap. This way, when the wheel wants/needs to change directions quickly (a bump or dip in the track), there is some compliance there and we maintain as much load on the stop as possible without spiking a load in the tire due to being too aggressive with the bump stop.

A track that does not generate any braking load and also does not generate any banking loads (there are a few of them!), then you need to take as much spring out as you can, run as hard a bump stop as you can and run as much gap as possible. We do this because we are not generating very much load anyway, so the load that we do get, we want to achieve it as late as we can in the travel and maximize our platform and camber curves.

Most of us see 2000lbs on the Gale Force, and that is what we use, but when selecting a bump stop, it is best if we understand how that number is generated. In this example, 2000lbs can never change, that load is a constant (and will be unless we run a lot quicker) but how we get there can be a huge variable. We can't change the number, we can only change how we achieve it.

There are other things to consider when selecting a bump stop - do we want something that is linear or are we looking for a progressive stop - but that's for another day!

For now, this was just a brief example on why it's important to understand bump stop loads. Join us in 2015 to learn more!

Stay cool this summer!
See you at GRIP 2015!

Jason & Gary