Bump Stops And bump stop length, With some math

Joined
Jan 16, 2023
Messages
21
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.
 
Joined
Jan 16, 2023
Messages
21
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
 

theycallmeWALLY

Administrator
Joined
Mar 30, 2022
Messages
825
Back at it with the solid info!

Seriously thank you guys for offering this up to the Public.
 
Joined
Jan 16, 2023
Messages
21
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)
 
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