Soft rf and which bumpstop?

[Dirtmod13]

Looking for a starting point for a rocket blue front. Can't use stacks, but can use a bump stop or bump spring. Limited motor.

[Mr.Kennedy777]

Depends on how much weight you are seeing on that tire, the consistency of the track surface, the traction on the race surface, the banking etc etc. Loading on the RF isn't always cut and dry all the time. If you are running at the same track a lot, you might be able to get away with "winging it" sort of. You need to know how much loading is going onto the RF tire. So knowing what your pre load on the spring is, and the shock travel left over at ride height and the travel you get during a race will give you an idea of what you are looking at in terms of numbers unless you are coil binding the RF and then you can't know for sure unless you run a stiffer spring that doesn't bind or bottom out the shock. For example. If you run let's say a 500 RF spring that is 12" tall on a shock with a 7" stroke,, when it's installed, the spring is 10.5" tall, and you have 4.5" of shock travel left. You go out for a run and for this examples purpose the track is nice and smooth. You find you have .5" before the shock bottoms out. That 500# spring was compressed 1.5" at ride height, and then an additional 4" under race conditions giving you 5.5" of compression total, or 2750 pounds of force. (Relatively speaking because if the shock is on an angle this also changes the actual "loading" at the tire, but this technique should work to start because we are looking at shock travel and loading at that coil over not the tire itself) So you know you have 2750 pounds of loading at the RF tire. Now you want to select a ride spring and a bump. Let's say we decide to go 250# 12" tall ride spring. We install it on the car before we select the bump spring to determine a)how much pre load we need on the spring and its height when we reset the cars ride heights. The reason for this is if the spring is too soft, we need to pre load it so much that we might end up needing to get a)a stiffer spring or b) a taller spring or c) a spring of the same height with a higher travel rating, in order to avoid going to coil bind before we get on the bump. Let's say we install the spring and find that now at ride height the spring is compressed 3" with 4.5" of shock travel remaining. We know the 3" of a 250# spring gives us 700#. Now we know the total load the tire sees is 2750#. If we want to install a 2" bump spring that offers us 1.25" of travel, then we know the shock needs to travel another 2.5" in order to touch the bump spring. So we take 2.5" x 250# which gives us another 625#. So we would then have 1400# (750+650) on the ride spring right at the same moment we are just contacting the bump spring (not taking into consideration the thickness of the caps or anything like that, and assuming the bottom of the bump spring sits at the bottom of the shock for this example). Knowing this we have 1450# of force already taken up out of 2750 total loading of the bump spring. If we want to travel into the bump spring 1" then that spring needs to be able take 1300# of loading at 1" of travel, PLUS it doesn't hurt to give yourself a little bit more just in the event of a rut or bump on the track. So we could decide on a 1500# bump spring, (this is just for quick math). Then knowing that the bump spring is 1500# and the ride spring is 250# pounds you need to be able to control the total spring rate during chassis movement so you're looking at needing a shock with very high rebound to keep yourself from coming off the bump spring too hard. You'd need 1750# of force at 3"/second or so, to control that total equivalent spring rate. Obviously the stiffer RF ride spring you choose the softer and/or shorter bump spring you can get away with, and also you don't need to run as insane of shock rebound numbers. Rule of thumb is softer springing is in fact better for handling on dirt, simply because of how much weight you can absorb per inch of spring movement, you aren't unloading and loading that corner as much, but you still get the benefit of the increased loading on the RF without binding, and the enhanced forward bite from the increased RF spring rate as well. It takes some time there's not an exact number in terms of shock valving or spring rates because every track is different, and I prefer bump springs over bump stops. But if you decide to go to bumps you need to take into account the rate of a bump is not linear, so as you travel further into the stop the rate goes way up and it becomes more difficult to set everything correctly. Bump springs are far more forgiving so I would say if you're just trying this type of setup out, then run the bump spring and if you have success then maybe look into the stops, just be ready to put the RF coil over assembly with the stop on a shock dyno, it will be the best bet in terms of making sure you have everything set up properly. In the above example you can see how, a slick track will change RF loading, a track with more banking will effect RF loading, a wider track with faster turns will affect RF loading, so you can't just use one and call it good. Hope this makes sense or helps you with your question.

[Dirtmod13]

Very good explanation. Thank you. We run only one track, every week track is fairly smooth. Rarely any ruts. Once in a while there will be a little moisture left on bottom. Medium banking. Good momentum.