Removing bump stops question

[64etype]

I'm currently reassembling the rear suspension and am planning for removal of the factory bump stops to allow for center laced wheels and a wider tire. The original bump stops will be removed and replaced by foam rubber compression stops on the shock absorber shafts. Unfortunately, the suspension is out of the car, so I am unable to easily determine the exact shock length/compression when the factory bump stop initially contacts the suspension. This measurement would define the length of the bump stop(s) on the shock absorbers (the point where the rubber begins to compress) ...and ultimately the stiffness of the rubber. Anyone happen to have this obscure bit of information? Thanks.

[64etype]

OK then.....I'll ask the question differently. Does anyone know the typical downward angle from horizontal of the wishbone link with the car on it's wheels? Also, any opinions as to the best characteristics for a bump stop on a shock absorber shaft....relatively soft foam with perhaps a 1.5 inch length (full travel is about 4.5 inches)....or something very short and shaped like a hocky puck, but obviously not that dense? Better to have a stop on both shocks on each side or would just one be adequate.

[Moeregaard]

Eric, you mention using "foam rubber compression stops" on the shock-absorber shafts. Due to the forces involved, I think you'll want to use something having a similar durometer (stiffness) as the factory bump stops. In any event, you obviously want something that will prevent the alloy hub from hitting the steel bump-stop mount.

Since your suspension travel will be determined by that of the shocks, you can get this figure by measuring the length of the shocks in their collapsed and extended configurations, measuring the distance of the shock mount on the lower wishbone to the alloy hub carrier, and doing a little math to get the suspension travel.

At this time there is not an E-Type in my garage; otherwise I'd be happy to go down and take some measurements. Perhaps one of the forum members will be willing to do this.

[64etype]

Thanks, Mark. I guess I'll go a head and mount one of the hub carrier/half shafts in the suspension frame and measure exactly where the top of the hub ends up at full shock compression. Just trying to avoid that step if someone already had some measurements. The next unknown is the gap between the mounting framework in the car and the top edge of the IRS cage as it will define where the bump stop sits relative to the fully deflected hub carrier. But I can figure that out assuming the IRS to frame mounts don't compress significantly.

As for the bump stop material and shape, I've been speaking with RE Suspension tech guys and they asked for some more data...hence my inquiry here. They'll help me with a selection once I give them the compression distance and some more detail. Specs on the original Metalastic stops would be the cat's meow. Most of the designs depicted on the RE Suspension web site include durometer graphs of compression versus force. It's pretty clear that the tall, corrugated, tower-shaped foam rubber stops I was considering won't work as the stroke on the shock is so short. So given the domed shape of the Boge shocks that I'm using, it's starting to look like a simple medium-hard donut shape 2" diameter X 1" high will work better than a flat puck shape as the contact surface would be distributed more evenly. There are five levels of stiffness offered, but any of them will result in a jolt given the short compression distance. As they'll have to be spilt at the radius before inserting in the assembly, I'll super glue the ends and count on the metal dust sleeve to maintain the shape if I use 2" diameter donuts. Any advice is appreciated.

[neil4444]

What size are you looking to go to ?
I have 5.5" centrelaced on mine & there's room for the bumpstops.

[64etype]

Neil,

I'm planning for the 6" Dayton with 215/65 tires.

[PeterCrespin]

Eric, you need to be careful. Replicating the OEM durometer would not be the cat's meow and would be wrong in principle, even if your choice ended up the same.

1) The OEM open loop bumpstops deflect differently to a solid chunk during the first half inch or so of compression while the hole closes up. Shock bumpstops won't deflect much more that that in total, if you use typical concentric hard rubber pucks.

2) The OEM bumpstops operate at 1:1 ratio with minimal lever effect since they are close to the wheel centre line. Damper bump stops will sustain considerably higher forces due to the top mount being many inches inboard.

3) To ensure an element of progressive restraint I'd always go for a tapered stop, which firms up as the travel approaches its limit.

4) Damper mounts were not designed for the heavy loads like landing from airborne. They will probably be fine but your eye rubbers will deteriorate much quicker if you actually end riding on the stops much.

5) No amount of calculation can substitute for mounting the suspension as planned but sans springs. That way you actually get to observe clearances with/without wheels/tyres based around your planned ride height. It's the only reliable way to spot unexpected fouling, such as at the top or bottom of stroke or related to radius arms, underslung exhaust etc. you will be able to see the clearance between top eye and damper body if it fouls somewhere else first.

6) Whatever clearances you observe and engineer, leave yourself at least half an inch of extra margin to allow for flex of the body, cage and mounts etc.

7) In your shoes I wouldn't dream of using anything but adjustable spring perch dampers to work around your issues. Remember when choosing dampers that extended length relates to closed length. Concentrating on the latter for your bumpstops might leave you with suspension that doesn't extend far enough to remove the oversize wheel, or too far so that the top rubs the bodywork at full droop.

I'm going through all of this with my D, front and rear. with huge 16" original high-profile 650/16" rears the issues are even trickier with a solid rear axle which twists as it drops on one side and rises to the max on the other. On the other hand, I do have they luxury of building the alloy skin to suit. :-)

Pete

[64etype]

Thanks, Pete.

I've done some previous research regarding the planned tire/wheel combination. Dan Mooney has used it with no operational issues, but he said he does remove the factory bump stops and grinds a little on the chassis bracket.

The bump stop design will depend in part on the shock design. I have new welded Boge units and don't plan to trade them in, so I'll have to split the bump stops to mount them. That's a problem with a conical stop in that it wouldn't be held in place by the dust shield. Available shock travel dedicated to the bump stop function is another as I suspect it is very small.

When I mentioned the specs on the Metalastic stops I was thinking in terms of a force versus distance curve like those that RE Suspension provides for the products they sell. You are correct that durameter numbers are only a (small) part of the story for a design like the Metalastic unit. Good point about the force considerations at the upper shock mount, although I don't anticipate much, if any bump stop contact on my sedate cruiser restoration.

The RM Suspension site has some very useful data on various designs...might be useful for your "D" project.

Regards

[64etype]

Some observations/measurements with the half shaft, hub and dampers (no springs) mounted in the IRS cage...measurements are not exact but as close as I could get...

With the Boge shocks fully compressed the half shaft is a hairs breadth from contact with the structure of the IRS cage. Don't even consider running the car without some sort of bump stop.

Full shock travel is 4 3/8", which nets 9" of vertical travel at the end of the hub splines.

The Metalastic bump stop is about 1/2" below the horizontal IRS frame mounting structure in the tub. The top of the hub carrier is about 1/2" above the top of the IRS cage structure at full shock compression.

Assuming zero gap between the IRS cage and the chassis frame, there would be about 1 inch of bump stop travel before the shock is fully compressed. In fact, there is a gap, so the working distance for the bump stop is less than an inch. It appears than when the hole in the factory bump stop is fully compressed, the bump stop action is finished.

A donut style, shock mounted stop of 1" thickness gives a fairly linear response over about 1/2" of compression. The remaining 1/2" of rubber equates to about 1" of lost travel at the wheel. I suspect the Metalastic stop actually leaves at least a 1/4" gap between the half shaft and the IRS cage, which translates to slightly more distance at the hub....probably in the range of 3/8". So a shock mounted stop would decrease available suspension travel roughly 5/8-3/4" more than the Jaguar unit. The remaining unknown is the maximum design force for the stop. I'll use one on each shock to absorb half the load, so the durometer number should be relatively low.

What am I missing?

[ralphr1780]

A short note to say have seen cars running for years with bumpstoptectomy not having any issue.

[jag68]

Hi Eric. I too have been running without bump stops for many years due to larger wheels and tires. Currently I'm running a set of 245/45 - 17" tires on Dayton wire wheels. I've had three E types over the last 20 years all without bump stops, all of which have been tracked at various times. Combined mileage over this period would exceed 75k. I've never had a problem. Never seen or felt any evidence of hard contact between suspension parts. Currently I'm using Gaz fully adjustable shocks in the back of my E Type. These come with bump stops that by eye appear to be about 1" thick. I've also used Koni and Spax - all without bottoming issues, and also without bump stops. Two of the cars had stock rear springs.

[64etype]

For the record, I just received four Ohlins bump rubbers #00563-03. They're .75 thickness and I cut them down to .5" to retain maximum possible suspension range , while also allowing for a reasonable distance for bump stop compression. It looks like a good compromise during a trial fit (IRS on the bench).

The conical shape of this serial number bump stop tucks up perfectly into the domed upper spring platform of a Boge shock. A large spacer/washer from the local hardware store sits in a conveniently positioned tray/depression at the top of the original Girling dust shield. The washer holds the bump rubber snugly up in the dome when the springs are fitted over the dust shield. The spacer is 2.25" OD with a ~1.5" ID. The hole size allows the narrow domed top portion of the shock body to pass through and contact the bump stop. The full width of the shock shoulder eventually catches on the spacer during the last bit of compression...at least that's the concept. The fact that the stop rubber has to be cut radially in order to fit it onto the shock absorber shaft appears to be a non-event as the shape is held captive on all sides and cannot slide down the shaft.

A differently shaped shock body (flat top, for example) will obviously require a different bump rubber shape and a different retention solution. Also, this particular bump stop will not work on an original Girling shock as the dome area is smaller, however, the retention method should apply. By the way, the Ohlins U.S. tech guys were incredibly helpful and sold the parts at the lowest price I could find anywhere.

Overboard...yes, probably.......it appears that I'm over thinking this by a country mile.

[Heuer]

Overboard...yes, probably.......it appears that I'm over thinking this by a country mile.

No you are not! One of the UK's top restorers, CMC, always insist on fitting bump stops to the rear shocks if the standard ones have been removed. Ohlins make a set that will fit the Boge shocks and Koni offer their own version. Jaguar thought bump stops were a necessary part of the car so lopping them off and not putting in replacements on the grounds lots of people have done it seems somewhat churlish. They only take a couple of hours to fit (less if the shocks are off the car) and they are not expensive so no excuse really.