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Author Topic: UK Lakester build G/GL  (Read 105345 times)

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Offline ggl205

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Re: UK Lakester build G/GL
« Reply #390 on: October 15, 2019, 11:44:48 AM »
John:

Passing along a tidbit from Tom Burkland about my four-link front axle; Tom reckons that if you have less that 10 degrees of freedom in axle movement, there could be binding. A solution would be to triangulate either upper or lower links to form a three link with a high misalignment rod end (a big one). I checked mine and with just one inch of travel at either corner, I have more than 10 degrees.

As it turns out, 1.5:1 is going to be my motion ratio. 2:1 was a bit too much.

John

Offline Lemming Motors

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Re: UK Lakester build G/GL
« Reply #391 on: October 23, 2019, 04:38:31 AM »
Thanks for the heads up John.

Continuing to work up the rear end; spring rate is a wild (insert rear end synonym of your choice) guess but since I had one unused 400 lb spring left over from the '56 project that seemed like a good starting point; buy one more of those and see how much deflection I get with and without various pre-load and see what that says. I will probably then have two unused 400's to trial the front end.  :lol:
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Offline ggl205

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Re: UK Lakester build G/GL
« Reply #392 on: October 23, 2019, 07:51:20 AM »
John:

Things are really coming along well on your lakester. Love the location planned for rear dampers too.

I think you will find that first blush on spring rates will be woefully too low. I ran into this as well. I come from a road racing background so I used springs I had from those cars. 350 lbs at front and 700 lbs at rear. I weighed the car and had 275 lbs right an left front and about 750 lbs per corner at rear. That was far too little spring rate and I paid a big price for this mistake. I was shooting for less than one inch travel but had way more than that. So, I will now use 400 lbs springs up front through a 1.5:1 rocker ratio that should yield around 600 lbs per corner with the same 700 lbs springs at rear from last time and 900 lbs springs in reserve, just in case.

John

Offline Lemming Motors

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Re: UK Lakester build G/GL
« Reply #393 on: October 23, 2019, 10:36:08 AM »
John
I suspect you are right about the rates - its a place to start - I went for 2.25" shock bodies as the available spring rate range is much greater than the 1.9" bodies.

However, I want to challenge your math (for my own understanding).

If you have one inch of push rod / rocker movement and your rocker is 1.5:1 are you then moving the shock 1.5" or are you moving it 1/1.5 i.e. less travel which is implied by your suggestion a 400lb spring is giving you an effective 600lb?

1" of single wheel bump in my application is a little over 1/2" at the push rod (it is inboard and I have wide-ish track) which is why I am multiplying up.
Complicating factor is that 1" at both wheels in bump (i.e. both of them over a ridge) is 1" at the push rod and therefore 1.5" at the shock so I have to leave room for that two wheel bump possibility to have spare stroke in the shock and / or put bump stops on the axles.

I haven't got a clue about vehicle weight (unsprung) yet. I think I am adding way too much structural steel tube but if some is good more might be better - I will not need to add lead and it should be strong.
Assuming it was, for ease of math, 2000 lb and 50:50 weight distribution that's 1000 lb each end or 500 lb per wheel.  With my bell crank leverage advantage that would need a 750 lb spring, less some pre-load by winding the shock collar up so I am guessing around 600 - 650 lb inch springs per corner in this scenario.

As an aside - I read somewhere with push rod / pull rod suspension adjust the ride height on the push rod, not the spring seat as that would alter the lever / bellcrank starting position and therefore the arc it travels potentially changing from rising rate to falling rate etc..

My scenario seems consistent with the rates you are proposing though you have more weight rearward than I am expecting - I will have a driveshaft at ~ 14" so my engine is a bit further forward - you are using a transaxle I think.

Anyone else with a similar setup please feel free to share you experiences here as its not too late to be told I have misunderstood  :cheers:
A Bonneville Lakester please barman.
Certainly sir; a lick of salt, a sip of gas and a twist of Lemming. More Lemming sir?
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Offline ggl205

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Re: UK Lakester build G/GL
« Reply #394 on: October 23, 2019, 11:00:11 AM »
Your understanding of my spring rates is correct. Also, a 2000 pound total weight assumption is going to be close. If you can create some level ground, four good bathroom scales that weigh up to 600 pounds should give you decent corner weights. If you can borrow digital scales, all the better.

My front rockers are built to put dampers as close to 90 degrees as possible. If not, I would have to calculate sine of the angle to get a proper rocker ratio. Probably why engineers are telling you to adjust the push rod (hypotenuse) which is the longest part of their suspension links. Take a look at Stainless?s front suspension. Everything is in right angles so he will enjoy damper movement (spring compression) for the rocker ratio he has chosen. The rest of us often have to compromise this because the car was originally built with a different suspension in mind.

John

Offline Interested Observer

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Re: UK Lakester build G/GL
« Reply #395 on: October 23, 2019, 12:51:30 PM »
It might be good to review a couple of things at this point.
Spring RATE and Spring LOAD are two entirely different things.  To hold up the car you need spring load.  To control how quickly the load varies with suspension movement, one would choose the appropriate spring rate.  The spring load is adjusted by the amount of static preload in the spring (if the set up allows preload adjustment, otherwise the load is simply the load that results from holding up the car).

Also, if the suspension system utilizes linkages that interpose a leverage ratio between the wheel and the spring, the load goes as the leverage ratio, but the spring rate goes as the SQUARE of the leverage ratio.

Offline Rex Schimmer

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Re: UK Lakester build G/GL
« Reply #396 on: October 23, 2019, 01:52:37 PM »
Good comment IO, as you said the rate is the sq. of the ratio. Makes a difference. My preference is for a soft and subtle suspension so our rear shocks, which are attached directly to the rear axle assembly use 125# springs and I set the ride height such that the bump rubber is about 1 inch from the end of travel. The thinking here is that we will have a soft travel for +or- about an inch and then if we get on the bump it will prevent the suspension from going full hard.

Our front suspension is actually a falling rate arrangement, not my actual plan but mainly to get the shocks inside the body. Once I got the shocks mounted inside the body I worked out a lever/link/rocker system that connected the shocks to the front axle and it turned out falling rate. To counter that I again use a fairly stiff bump rubber and 400 # springs with about 1 inch of travel at the selected ride height. One of the good things about having shocks that uses  an adjustable link is that you can set the spring preload and then change the ride height with the link and not affect the preload.

Our car is quite light, 175 # per wheel on the front and about 375#per wheel on the rear for a total weight of around 1100# less Duke. Good for quick acceleration.

Rex
Rex

Not much matters and the rest doesn't matter at all.

Offline ggl205

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Re: UK Lakester build G/GL
« Reply #397 on: October 23, 2019, 03:29:12 PM »
It may also be good to note that cars designed for higher ground clearance require a very different suspension setup than those with low ground clearance. I will run between 2 and 2 1/2 inches of static ground clearance so my springs must hold desired ride height at speed and still give me maybe a little more than one half inch travel. I use rocker ratio to gain a bit more damper travel so they have half a chance of controlling spring occilations. When I ran with a 1:1 front motion ratio, I needed one inch of spring travel for dampers to work at their best. I thought 700 pounds of total front spring was enough but I was grossly mistaken. The car compressed springs more than one inch, forcing us to keep raising ride height until it stopped bottoming. This was a big worry. At some point, if the nose is too high, too much air finds its way under the car. Not sure what actually caused the spin but elevating front ride sure did not help.

Lastly, we hardly speak of wheel rates and that is probably more important than corner weight.

John
« Last Edit: October 23, 2019, 03:32:24 PM by ggl205 »

Offline Lemming Motors

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Re: UK Lakester build G/GL
« Reply #398 on: October 24, 2019, 03:02:03 AM »
Brilliant, thanks for the discussion guys.

I/O - I hadn't yet clocked the spring rate leverage point so that's good to catch now  :cheers:
Darn there are some clever people on this forum and that is why the build diary is invaluable - it reduces the degree to which one does idiot stuff - a bit like the building regulations in the UK - there to (try and) protect idiots from themselves.

A nominally softer spring with more pre-load (these shocks have inches of adjustment) is probably the right balance to get the load and rate close to start. Very Colin Chapman. My '69 Europa had a better ride than most modern 'sporting' cars especially the Germans who have immaculate autobahns and dial the suspension to Flintstone, yet the Europa cornered with enthusiasm. I guess if you are already on the ground you cant have body roll.
A Bonneville Lakester please barman.
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Offline ggl205

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Re: UK Lakester build G/GL
« Reply #399 on: October 24, 2019, 11:45:17 AM »
John, it is not an easy task designing and building a workable suspension for our narrow cars. Lots and lots of compromises. Two different approaches can be seen in the Das Bullet and Seth Hammond lakesters. Das Bullet has a more contemporary formula car push rod while Seth utilizes a direct rocker arrangement. Das Bullet cannot fare in his suspension but Seth can and does. So, it appears the compromise is how much perfection you can generate for your suspension without sacrificing aero. Always a quandary.

John

Offline Lemming Motors

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Re: UK Lakester build G/GL
« Reply #400 on: November 04, 2019, 10:11:02 AM »
My design (and I use that term loosely) has the rearmost chassis (around the rear axle) narrower than the main chassis so that the maximum width is afforded for the 4-bars, mounted outboard of that structure.

My sketches accommodated this by acting on the rear shocks via a pivot that takes the load from the axle (outboard) and transfers across the chassis tube to the shock (inboard). There is not enough room either side of the axle pigs head to mount the shocks vertically or simply laterally across the chassis.

Now that I have mocked that up and stared at it I am nervous of the torsional load through the 5/8" pivot arm. The advantage of this set-up is that the end of the 5/8" rod is a spline and a standard steering joint is used to attach the shock lever arm - that means the relationship between the shock arm and the axle arm can be altered.

Mock-up pics follow.
A Bonneville Lakester please barman.
Certainly sir; a lick of salt, a sip of gas and a twist of Lemming. More Lemming sir?
Just a squeeze.

A Squeeze of Lemming it is sir.

Offline Lemming Motors

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Re: UK Lakester build G/GL
« Reply #401 on: November 04, 2019, 10:17:19 AM »
Plan B was then mocked up which uses a more traditional push rod style rocker mounted diagonally across the top of the chassis rail.
This is much more direct but it means that some structure will have to sprout from the chassis rail to pickup the outside load (i.e. that the 5/8" pivot bolt  is supported by). All this has at the moment is the bracket, badly scalloped, tacked to the rail.

I could take the push rod from the inboard side of the chassis rail but that would be picking up too far toward the centre line for my taste - fine for two wheel bump but not great for single wheel bump.

Pics of this crude mocked up follow.

Plan C might just be a horizontal shock that runs across the chassis (laterally, not longitudinally) with each offset, one facing rearwards slightly and the other forwards.
A Bonneville Lakester please barman.
Certainly sir; a lick of salt, a sip of gas and a twist of Lemming. More Lemming sir?
Just a squeeze.

A Squeeze of Lemming it is sir.

Offline kiwi belly tank

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Re: UK Lakester build G/GL
« Reply #402 on: November 04, 2019, 11:38:52 AM »
Plan C would seem the most logical & create considerably less load on the chassis mount point plus remember the KISs theory Mate! With the narrow suspension width & high CG, you'll need a big a$$ sway bar to eliminate body roll/yaw. Now's the time to design that in.
  Sid.

Offline Jack Gifford

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Re: UK Lakester build G/GL
« Reply #403 on: November 04, 2019, 10:53:43 PM »
I also like plan C. If you have a reason to prefer plan A, how about just up-sizing the 5/8" shafts?
M/T Pontiac hemi guru (or does guru status expire after 30 years?)

Offline ggl205

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Re: UK Lakester build G/GL
« Reply #404 on: November 04, 2019, 11:05:15 PM »
John, Plan C looks like rockers are in single shear. Is that correct? If so, can you run dampers at 90 degrees to centerline of chassis? That way, you could put rockers in double shear with well supported brackets.

John