Has anyone cut their bumpstops when lowering?

[jumboplay]

I just found out this entire concept. My car ride is mad harsh with my Eibach + ZX2s and I'm starting to figure out why - the bumpstops are factory size. I found this article on the net about Miata bumpstops http://www.virkki.com/jyri/miata/bumps.html

When changing my spring and struts, I left my bumpstops at normal size. Apparently you're not supposed to...there's an amount you cut off based on the change in height and spring rate. If anyone has any ideas, theories or experience, please let me know. I'd like to do something about this...

Also, anyone who has Eibachs still have the manual or papers? Supposedly it tells you how much to cut off the bumpstop, but I didn't see that. If anyone can help, it'd be awesome. Stuff like this drives me crazy and then I can't sleep at night...

<small>[ November 15, 2004, 02:24 AM: Message edited by: Tommy v2 ]</small>

[babyblueMX3]

I forgot to put mine on when I changed my springs and didn't notive a difference..I guess it depends on how low your car will be

[quicksliver]

I got ZX2's paired with ST Lowering Spring. I didn't put in my bumpstops either lol. Why? Cause your not supposed to when useing zx2's.

[Casino]

ive seen a few miatas that have had this done, it helps out if done right, just dont start cutting your stock suspension lol

[ProtegeSTS]

you are ALWAYS supposed to use bumpstops.

And you should cut them if you're lowering the car.

They are a saftey device to protect your struts when you hit something hard.

[jschrauwen]

Makes me want to ask A-Spec about the bumpstops after installing KYB struts and B&G springs. Does this set-up require a bumpstop modification????

[Flashpoint2]

I have ST springs, but stock struts, and I didn't do anything to the bumpstops. Maybe I should have?

[johnboi85]

where are the bump stops located? sorry for a stupid question

[jumboplay]

The bumpstops are located directly on the strut shaft, they are black rubber boots, kinda look like stacked donuts? .
Being creative, you can access them when the car is jacked up without taking the strut assembly out.

[babyblueMX3]

Originally posted by Tommy v2:
The bumpstops are located directly on the strut shaft, they are black rubber boots, kinda look like stacked donuts? .

mine were yellow/orange

[quicksliver]

Mine were Green and Blue

[bubbaflaat]

You should always trim the bumpstops in half. Because more of the strut travel is lower now since you lowered it. It will increase the useable travel of the strut on a lowered car. All lowering kits instructions tell you to do so.

[tatsu]

Two things determine how long your bumpstops need to be: maximum travel of the strut at the desired ride height and the spring rate vs. spring length.

The maximum travel of the strut is determined by the length of the piston rod(s) inside the strut vs. the length of the strut housing. Let’s say for example that you have a stock-length strut, and it has 4” of travel from neutral (when the car is not moving) to full compression. If you lower the car by 1” without changing anything inside the strut, you now have 3” of travel. How do you gain back that lost suspension travel? By shortening the length of the piston rod. If you shorten the piston rod by 1”, you have now effectively increased the maximum travel of the strut by 1”, bringing the travel back up to 4”. Note that I would NOT advise “modding” your own struts to shorten the rods – to get more travel from a lowered ride height, you need to replace the struts with new ones that have shorter rod vs. strut housing lengths.

How this affects the length of your bumpstops ties in with your springs. This is just a theoretical example, but if you were using your stock struts with 1” shorter springs, and the spring rate (the amount of force required to compress the springs by a measured amount) of the new springs was the same as stock, you would actually need to INCREASE the length of your bumpstops to prevent damage to the struts. You would, in effect, not be lowering the car at all, merely reducing the suspension travel.

In practice, MOST “Sport” or performance lowering springs increase the spring rate by a corresponding amount to the reduction in height. Basically, if it takes 1000 lbs to compress the stock spring 4”, then the 1” shorter performance lowering spring would ideally require 1000 lbs. or more to compress it by 3”. This would allow you to use the stock bumpstop length without damaging your struts.

If you combine a high-enough-rate, shorter spring with a strut that has a shorter rod vs. housing length, THEN you can cut your bumpstops to regain some lost travel – otherwise, you are risking damage to the struts from the rod end colliding with the housing. Now, how can you figure out how much to cut your bumpstop? Well, it’s not easy if you don’t have access to some specific information about your stock struts and springs and your replacement struts and springs. If you assume that the new springs have a high-enough rate (as described above), you can just work with your strut travel – otherwise you need to know everything listed in the next two paragraphs.

First, you need to know how much travel your stock struts allow vs. the replacements. To measure it yourself, you'll need to find a way to compress the struts (stock and new) without springs slowly and very gently until they bottom out - this may not be as hard as you think without springs installed, but it may be very difficult. Always be safe and use a spring compressor to take the load off of the strut before disassembling the spring from the strut! Also, be extremely careful to push the rod straight into the strut housing so you don't bend it or damage any of the internal seals, etc. Measure the distance between the top end of the strut and the lower spring seat, and the difference (assuming the new strut's measurement is smaller) is the added travel that the new struts allow.

Next, you need to know what the spring rates (in pounds/inch) of both the stock springs and the new springs are. You can either use manufacturer data (if available) or you need to find someone who has the equipment to measure them. With progressive-rate springs, the following is more difficult to calculate – for this simplified example, we’ll assume a linear spring rate (I.E. the same along the entire length of the spring), or an average spring rate for a progressive spring. Multiply the stock and new spring rates by the travel allowed by the stock and new struts, respectively.

If the result with the new combination is the same or lower, leave your bumpstops alone. Note that if the result is lower, you are risking over-compressing the strut unless you increase the length of your bumpstops. If the result with the new combination is higher, then you can cut your bumpstops.

Take the difference between the result of the new combination and the result of the stock/old combination, divide that by the new spring rate, and you should have the amount you can cut your bumpstops, in inches.

Any questions, please ask.

<small>[ November 26, 2004, 12:54 PM: Message edited by: JP ]</small>

[tatsu]

I just thought of a quick-and-dirty method (I.E. not necessarily accurate, but much simpler than my previously posted method) of determining the difference in strut travel.

With the struts out of the car and no springs, the strut's piston rod should be fully extended out of the housing. Measure the length of the new strut body and subtract the exposed rod length on the new strut. Compare that with the same measurements from the stock/old strut body and rod length.

If the result from the new struts is greater than that from the stock/old struts, you can cut the bumpstops by that amount - assuming of course that your new vs. old spring rate/ride height allows this, as described in my previous post.

Be aware that the internal measurements of the two struts may be different, so the more complicated procedure will be more accurate, but this should give you a good "guesstimate".

<small>[ November 26, 2004, 01:23 PM: Message edited by: JP ]</small>