Bump Stop Clearance

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theunz
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Bump Stop Clearance

Post by theunz »

How much lower bump stop clearance should there be? I have Deans Super Comp Springs and the proper bump stops he sells to go with them. There is only about a quarter inch or 7mm clearance. That doesn’t seem like much.
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GeoffM
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Re: Bump Stop Clearance

Post by GeoffM »

Here's mine for comparison. I went with the drawing from the Bob Sharpe book. Not sure why he specifies the ridge shape, but I was just following orders :)
Datsun suspension.jpg
Datsun suspension 2.jpg
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todd lorber
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Re: Bump Stop Clearance

Post by todd lorber »

With that kind of clearance, you are better off either cutting them down, or pulling them out altogether. I have the same springs and I currently have the bump stops out. I plan on changing out the springs and putting in modified bump stops. Somewhere on this site there is a diagram showing fronts and rears cut down. I have it on my phone but the file size is way to large to post here.

I think the reason for that ridge shape is to make the resistance more gradual-rather than having an abrupt stop.
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GeoffM
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Re: Bump Stop Clearance

Post by GeoffM »

Thats what I'm thinking too todd...although just specifying a cone shape with the same total height would probably satisfy a similarly shaped spring rate curve that brings on the resistance gradually. Anyway...who's gonna argue with Mr. Sharpe?
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Re: Bump Stop Clearance

Post by rwmann »

A ridge (or a taper) is easier to cut with a hacksaw...
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theunz
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Re: Bump Stop Clearance

Post by theunz »

Thanks Geoff, it looks like you have about an inch of clearance. Dean says that mine are a replica of the Datsun Comp short bump stops and the same for the springs, so one would think that the clearance was correct. A quarter inch just doesn’t seem like enough movement, but since they are designed for competition and most tracks are relatively smooth maybe that’s sufficient.
Would still like to hear from someone with the same setup as mine.
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Re: Bump Stop Clearance

Post by JT68 »

It’s a fundamental problem with the spring height and geometry as illustrated very well by the photos.

a. - Bob Sharp was setting up track cars AND WASN'T running "super comp" springs. It isn't even close to the same recipe for a good street car.

b. Todd whatever you do ***Do not remove your bumpstops -- if you do your tie rod becomes the bumpstop**** Dangerous at best.
If you cut them too low, you have the same problem, especially on hard bumps.

You run into the same problem when lowering the spring perches or cutting coils off the springs.

The "comp" bumpstops are also a fairly bad plan for street use- they come on way too hard and are too blunt. Cutting the stock bumpstop at a gradual angle is much better.

c. For great handling and ride, you need to stay out of the bumpstops except at railway crossings where they protect your steering, suspension and frame from damage.

Think about it...as soon as the blunt relatively hard comp bumpstop is on the frame, the total spring rate has gone through the roof--you have the very high 860 pounds/inch of the SC spring added to the minimal elasticity and travel of the bumpstop and ultimately steel.

Whenever you are "well into" the bump stop your total spring rate has tripled/quadrupled,+++. All the suspension loading changes in that instant which provides unpredictable response and is very unsettling.

Let us know if you need any further help or clarification. You guys have experienced the result of the problem already.
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Re: Bump Stop Clearance

Post by JT68 »

Also, to clarify, the step profile of the Bob Sharp mod was intended to help with the same problems mentioned above...the small step serves to "engage gradually", the 2nd step (the blunt transition) is to protect the suspension components from damage w/hard impacts.

The Sharp bumpstop mod is fine, as is an angled cut of the stock bumpstop.
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Re: Bump Stop Clearance

Post by GeoffM »

JT68 wrote: Mon May 03, 2021 3:32 pm Also, to clarify, the step profile of the Bob Sharp mod was intended to help with the same problems mentioned above...the small step serves to "engage gradually", the 2nd step (the blunt transition) is to protect the suspension components from damage w/hard impacts.

The Sharp bumpstop mod is fine, as is an angled cut of the stock bumpstop.
One would conjecture.

There are actually three different general shapes in the Bob Sharpe manual. Based on his shapes and how much each corner can roll before reaching the bumpstops is a way to tune oversteer/understeer. His "oversteer" fronts are only 1/2" tall....I'm wondering if the tie-rod would hit the spring perch by then? If not, perhaps the bumpstop tower (sans bumpstop) would be the first obstruction before the tie-rod hits.
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Re: Bump Stop Clearance

Post by JT68 »

[/quote]
His "oversteer" fronts are only 1/2" tall....I'm wondering if the tie-rod would hit the spring perch by then? If not, perhaps the bumpstop tower (sans bumpstop) would be the first obstruction before the tie-rod hits.
[/quote]

probably not with the springs Bob Sharp was running, like I said, they were not SC springs-he was smart enough to understand these important issues and avoid them.

either way, its a bad formula if the spring is so low that you have this problem. You don't want the sheet metal bumpstop tower to serve as the bump stop-in that case the the spring rate on the offending corner becomes nearly infinite (steel-on-steel, equivalent to coil bind of the spring on a hard bump) and the sheet metal tower would crack/fatigue/deform under that abuse. Fail.
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Re: Bump Stop Clearance

Post by jr02518 »

I would like to thank everyone for this line of questions and insightful answers. I don't have a copy of Mr. Sharp's book yet, do you know what spring rate he was working with and the free length of the the springs?
I am building a car to compete with in auto crossing events on the current 200 tread ware of tires. I currently am driving a 1982 BMW 320i on H&R springs and Beilstein Sport struts/shocks with the stock spring hats and the biggest front bar, 29mm, I can find. Lots of body roll and it sticks. In the world of local SCCA class's this combination is set up to run H Street. My Roadster will also run in H Street. I have the big bar on the car. I would like to avoid the shorter springs on the market with a 860 lb springt rate, is there an alternative that is a bit taller with a softer rate?
The size and shape of the bump stops is very cleaver, I am very use to working with the limitations of my current combination and look forward to the entertainment my new car will give the crowd!

David
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Re: Bump Stop Clearance

Post by JT68 »

jr02518 wrote: Mon May 03, 2021 9:40 pm I would like to thank everyone for this line of questions and insightful answers. I don't have a copy of Mr. Sharp's book yet, do you know what spring rate he was working with and the free length of the the springs?
I am building a car to compete with in auto crossing events on the current 200 tread ware of tires. I currently am driving a 1982 BMW 320i on H&R springs and Bilstein Sport struts/shocks with the stock spring hats and the biggest front bar, 29mm, I can find. Lots of body roll and it sticks. In the world of local SCCA class's this combination is set up to run H Street. My Roadster will also run in H Street. I have the big bar on the car. I would like to avoid the shorter springs on the market with a 860 lb spring rate, is there an alternative that is a bit taller with a softer rate?
The size and shape of the bump stops is very cleaver, I am very use to working with the limitations of my current combination and look forward to the entertainment my new car will give the crowd!

David
Hi David yes, we have helped several guys setup for track use. The MY (Mike Young) springs will give you enough spring rate and clearance to avoid the issues discussed on this thread. We keep them in stock. We can also have a set of Bilsteins custom valved for the track for you (there is no additional charge for custom valving on new orders :D )

If you are aiming for neutral handling, You will still need to cut the bump stops for neutral or oversteer as Mr. Sharp describes because if you add a large front sway bar you will be making the car more prone to understeer. You can pm/text/email/call is you want to discuss specifics on the custom valving/options depending on what you want.

go fast!
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Re: Bump Stop Clearance

Post by Datsunparts.comInc »

Theunz, Like the instruction sheet says you must cut the bump stops. Remember that the bump stops we had made are dead on copies of the Nismo comp bump stop made for the original, taller, Datsun comp spring which was 9 3/16" in height. We offer them as they are new rubber and supple and not hard as a rock like the 50 year old ones people have on their cars. They are a starting point and easy to cut, grind or belt sand, whittle away as per the Bob sharp recommendation.
.750 to 1 inch best for clearance. You can also add the spacer under the spring which is free when you ask for it. It will raise the car up though.

www.datsunparts.com/2174

Dean
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Re: Bump Stop Clearance

Post by Datsunparts.comInc »

theunz wrote: Mon May 03, 2021 11:32 am How much lower bump stop clearance should there be? I have Deans Super Comp Springs and the proper bump stops he sells to go with them. There is only about a quarter inch or 7mm clearance. That doesn’t seem like much.
See my note above and the website for clearance information.

Dean
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theunz
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Re: Bump Stop Clearance

Post by theunz »

Thanks Dean, that explains a lot. I guess I didn’t read the instructions or thought it was for OEM bump stops. I’ll check your web site and cut my bump stops. It should make those big dips and bumps in the road a little more bearable😁

When I was in school my progress reports usually had a check mark in the “usually does not” box for the “listens to and follows directions well” category🤔
Last edited by theunz on Tue May 04, 2021 6:06 pm, edited 1 time in total.
Mike M

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