Bump Stops And bump stop length, With some math

ADSRACINGSHOCKS

Active member
Joined
Jan 16, 2023
Messages
79
We get a lot of requests for 2.5" x 4" bump stops, and the first question i ask is “what trophy truck you are building?” A common misconception is that you need to run a huge bump stop to get best performance. The longer the better Right? Wrong! and here is why.


Engaging the bump too early can upset the vehicle in rough terrain and limit your traction, you can overheat the bump from relying on it too much, and it can add harshness to the overall ride.

Well how do i know what stroke and diameter is best for me?

There are a few questions we need to answer first

For front suspension applications
1. Do you have a bypass?
2. Will you be striking on the arm or on the spindle?
3. What is your overall vehicle weight?

Why does it matter if i run a bypass?
When adding a bump stop to a system, we do not want to take away the bypasses position sensitivity, every properly designed bypass will have a bump zone built into it. If we “over bump” the system with too long of a bump stop it will make tuning difficult. How do we figure out which length is going to work well with our bypass? Most bypasses will have between a 2-3” bump zone depending on manufacturer. We don’t want our bump stop to engage until after the bypass begins to get into its internal bump stage.

Time for some math and examples.


Example 1: Arm mounted bump stop

We have a long travel kit on a ranger with 10” travel shocks our bump zone in our bypass is 2.5”

The bump stop is mounted towards the chassis pivot of the lower arm. We don’t want the bump stop to engage until after the bypass bump zone so we will go with a 2” travel bump stop.



Disclaimer!! Most arm mounted bump stops will engage very soon due to mounting constraints this is not the end of the world but can cause a little harshness on choppy terrain, when space and chassis permit a spindle bump will be more ideal



Example 2: Spindle mounted bump


We have the same Ranger with 10” travel shocks but we moved the bump stop to strike the spindle we still have a 2.5” bump zone in the bypass and our motion ratio is 1.5:1, we can see what our effective bump zone is of the bypass is by multiplying the bump zone by 1.5



Shock Bump Zone x Motion Ratio= effective bump zone



2.5” x 1.5= 3.75” of effective bump zone



We can see that even spindle mounting the bump stop on this example does not warrant a 4” travel bump stop.



For this truck we would look at a 2.5” or 3” stroke bump









Rear Suspension applications Leaf/ Link
1. Do you have a bypass, how much bump zone does your bypass have?
2. What is your motion ratio? Normal applications will generally see values in these ranges (Leaf Springs 1:1-1:3, Links 1:3-2:1)
3. What is your static up travel?



Lets go over a few common examples for rear set ups


Example 1: overlander style spring over Tacoma

These vehicles are very limited in up travel and normally have about 12” total wheel travel about 5-6” being up travel, when up travel is low our tuning window closes fast we want the suspension to conform to terrain but we also do not want to bottom out, we recommend a 2” travel bump stop on these applications.



Example 2: spring under Tacoma with smoothies

Our example truck has 8” of up travel since we have no bypass helping stage off the travel we are going to “lean on” our bumps to do our stopping in this scenario a 2” bump is not going to give us very much to work with a 3” will allow us to run a “loose” enough valving set up to eat terrain and still give us 3” of total bump stage, this allows 5” of up travel before the bumps begin to come into play.



Example 3: spring under ranger with 3.0 bypasses

Our example truck has 16” travel shocks with a motion ratio of 1.2:1 and an internal bump stage of 2.5” our total up travel is 10”.

We can use our same formula from before



Shock Bump Zone x Motion Ratio= effective bump zone


2.5” x 1.2 = 3” of effective bump zone


We can see that using a 3” bump on this truck would be engaging at the same time as our bypass for this truck we would recommend a 2.5” or 2” bump stop



Example 4: trailing arm Chevy with single coil-over no bypass

Our example truck is running a 16” coil-over on a 1.45” ratio trailing arm our total up travel is 9”

This truck should have more up travel but lets walk them thru the steps any way, being similar to our smoothie Tacoma we have limited up travel and no bump stage from a bypass, but we do have some dual rate engagement to help with slowing things down. We know that with 9” of up travel a 4” bump will only give us 5” of travel before the bump kicks in not ideal, this truck we would recommend a 3” stroke allowing a decent bump stage to up travel compromise





Example 4: trailing arm center mount truck

This is the final dream for most of us a “big boy” truck, we have an 18” 4.0 bypass on a 1.6:1 trailing arm we have 14” of up travel and our bypass has a bump zone of 3.5” (insert Tim Allen sounds here) lets do some math !





Shock Bump Zone x Motion Ratio= effective bump zone


3.5” x 1.6” = 5.6” of effective bump zone



We can finally use our 4” stroke bump stop!





Ok we now have a general rule of thumb to figuring out how much bump travel we need, but what diameter do we run?



For most recreational mini trucks mid-sized and crawlers we would recommend our 2.125” diameter, if you are a full size or heavy/ race/ pre-runner a 2.5” would be better suited at taking the big hits involved with racing and large inertia situations.



As always we hope this is helpful and would be happy to help you figure out what would be best for your situation.
 
How would you determine the motion ratio of something or would that not be the biggest factor to worry about?
you will need to map out your arm between pivot point centers and the shock mounting point or cycle your suspension and compare wheel travel to shock travel 10" shock 15" of travel 15/10=1.5:1 or for a leaf spring 16" shock and 18" of travel 18/16=1.125:1 its not hyper critical but you dont want overlap if possible
 
Back at it with the solid info!

Seriously thank you guys for offering this up to the Public.
 
so good! what's your take on the Solid axle bump set up.
for solid axle stuff we can generally treat it as a limited up travel situation if you have 8" of up a 3" isnt a bad choice any thing less and i generally swing for a 2" stroke
(my personal precrawler has only about 5-6" of static up travel and i run a 2" stroke 2.5" diameter its a full size truck and gets rode pretty hard)
 
Oooh. Now do PSI… how oil fill level can change the bump… and when or if the bump needs to be tuned?

Mostly because I’m selfish and I’m already emailing you about these questions to remind you how much I’m a thorn in your side lol!
 
So i got a question on the math for the spindle mounted bump stop for the front end.

typically A-arm trucks are unequal length arms so if you use the math your saying, it would be as if the upper arm is the same length as the lower, right? if im thinking about this right with the unequal length arms, the upper arm is going to move faster than the lower which is a different motion ratio, correct? which would be more of a 1.25:1 UCA to LCA or range depending on vehicle, then you need to factor in the shock to LCA motion ratio as well for pretty accurate bump zone.

also, i can damn well attest to the shorter bump in the rear depending on bump travel. i have 7" up on a single coilover, no bypass and 4" bumps suuuuucked hitting the axle. shortened them to 2" and absolute world of difference
 
So i got a question on the math for the spindle mounted bump stop for the front end.

typically A-arm trucks are unequal length arms so if you use the math your saying, it would be as if the upper arm is the same length as the lower, right? if im thinking about this right with the unequal length arms, the upper arm is going to move faster than the lower which is a different motion ratio, correct? which would be more of a 1.25:1 UCA to LCA or range depending on vehicle, then you need to factor in the shock to LCA motion ratio as well for pretty accurate bump zone.

also, i can damn well attest to the shorter bump in the rear depending on bump travel. i have 7" up on a single coilover, no bypass and 4" bumps suuuuucked hitting the axle. shortened them to 2" and absolute world of difference
Best to use the arm that your shock is hooked too, just like a trailing arm on the rear. you are correct the upper arm swings in a different arc. if you are going to strike the spindle you can plot the 2 arms as they cycle to verify proper bump engagement compared to bypass position
 
So setting up bumpstop with minimal PSI i would guess is correct as it is just a help for suspension tuning to control the last few inches of travel. Now what I do is remove schrader, bottom out the bump stop with about 1/4" shaft showing and then fill them with oil. My experience is this requires less charge in bumpstop so you don't feel when it engages but it ramps up pressure quickly to help control the bottoming out. Do you suggest doing that or just run the factory oil volume and tune the pressure and valving in the shocks?
 
So setting up bumpstop with minimal PSI i would guess is correct as it is just a help for suspension tuning to control the last few inches of travel. Now what I do is remove schrader, bottom out the bump stop with about 1/4" shaft showing and then fill them with oil. My experience is this requires less charge in bumpstop so you don't feel when it engages but it ramps up pressure quickly to help control the bottoming out. Do you suggest doing that or just run the factory oil volume and tune the pressure and valving in the shocks?
When we build bumps at the shop they have maximum oil volume, basically over filled and then bleed until they fully compress, as you stated it allows you to run less initial pressure and still have maximum build up, from there you can tune with pressures from 65-250psi to get what you need. For dialing systems with limited up travel or limited shock packages you can also change valving in the bump MOST systems only need pressure changes but you can get tricky to squeeze out the last bit of performance with valving. generally the bump stop should be treated as your last stage of control a good shock tune is always needed for best performance, over relying on the bump stops can over heat them and degrade the service life.
 
Ok makes sense. I know fox 2.0 x 4" stroke have like 100ml and I know they can hold around 150ml. Prob 10 years ago a friend had some new radflo bumps and both of them blew the welded top off them when smashed hard, which I attributed to over filled and hydrolocked them
 
Ok makes sense. I know fox 2.0 x 4" stroke have like 100ml and I know they can hold around 150ml. Prob 10 years ago a friend had some new radflo bumps and both of them blew the welded top off them when smashed hard, which I attributed to over filled and hydrolocked them
yes we have seen people rebuild things incorrectly and blow all kinds of parts up, if it can not fully compress by hand when bleeding it is over filled and needs to be corrected
 
Yeah these ones were brand new ones he bought from radflo and we charged to 150 psi. Called them and they said sorry you probably messed with them and overfilled. Probably why companies that mass production products just set volume lower than fill since it still works as designed, just not as good and no risk of hydrolocking that way. Plus the schrader end of radflo bumps was a cap welded on, not a machined piece that threaded into end of bumpstop body like it should've been
 
Back
Top