Progressive bump stops

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Progressive bump stops

Postby GregSwan » Wed Jun 12, 2019 6:52 am

Hi, does anyone know of a good source for progressive bump stops?
With thanks,
Greg
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Re: Progressive bump stops

Postby F1300Tony » Wed Jun 12, 2019 9:03 am

Cast your own?
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Re: Progressive bump stops

Postby Martin Kemp » Wed Jun 12, 2019 9:05 am

I use the track surface.
As some bit of the car hits the track in roll or braking it more or less stops any increase in load on the nearest wheel. If the other side of the car or the back end continue to rise then those wheels will be progressively unloaded. The driver should be able to cope with this.

If the damper hits a stop then there will be a rapid rise in stiffness which is likely to be too quick for the driver to respond to.

I guess this is why you are looking for a progressive stop, but I think it would be difficult to get one to suit the very short suspension travel on a 750 formula car. Or perhaps I am assuming too much and you need it for a different purpose.

Anyway, I always remove the standard bump stop that comes with new dampers and if the suspension bottoms out before the car hits the track, I put polypropylene packers under the front corners of the nose (keeping to 40mm ground clearance) to make sure that the chassis does hit the track first. I did try titanium bolts but the sparks nearly got me black flagged. The polypropylene is surprisingly durable on tarmac.

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Re: Progressive bump stops

Postby GregSwan » Wed Jun 12, 2019 10:01 am

Thanks for the replies.

Not sure I'd know where to start casting my own, and certainly not by Cadwell. Proper old school 750F solution though Tony! One day.

I was thinking something along the lines of 5-10mm elastomer discs in various shore hardnesses just to keep the rear right from rubbing in the wheel arch... we had it at Croft in the hairpin and Graham Hill at Brands, so definitely going to be an issue at Cadwell. Although it rolls a fair bit it's not a chassis grounding issue. The Ingham already has slightly harder springs at the rear this year (Jon previously had the same issue at Silverstone Loop and I've also added a stack of ballast plates in the passenger space since). I don't want to upset the balance by going harder again, and looking to avoid the all or nothing effect of a generic stop for the reasons Martin points out. Single adjustable Avos at the rear so might try a couple of clicks but don't want more rebound, and have no idea of B/R ratio. No ARBs front or rear to play with either and don't want to alter the rake. I'm pretty sure I've seen aero cars which use the springs to take a set in the corner and the elastomers to hold the corner up. I'll keep looking. Taking the car to Blyton in July with Danny so might get my thinking cap on and try a couple of things there. Thanks again.

Greg
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Re: Progressive bump stops

Postby F1300Tony » Wed Jun 12, 2019 10:18 am

For sheet to cut discs out of try 'Engineering and Design Plastics' which are just NW of Cambridge.
Having built a monocoque car I hate grounding. With one person standing on the car I like the car to just hit the ground when a second jumps on it! I think Martin and I agree 95% of the time!
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Re: Progressive bump stops

Postby GregSwan » Wed Jun 12, 2019 11:33 am

I'll have a chat with them... thanks Tony.
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Re: Progressive bump stops

Postby tdriscoll » Wed Jun 12, 2019 12:02 pm

I’ve FB messaged Greg, but as I’ve now been able to log back in, I’ll share with the group.

Demon Thieves sell some Polyurethane bump stops which are ‘universal’, but of unknown properties. Powerflex are the common aftermarket brand name for vehicle tuning, a car set looks like £20.

To find the stiffness curve, load a bump rubber up in stages with a press or gym equipment or something of set mass (water bottles maybe), measuring the height as you go. You can then plot the load/deflection curve.

The appropriate range of values or material could be the subject of a thesis, test out thoroughly! Hand calculations should get you a basic range to aim for to start with - 1/2 to full car mass on one corner, divided by acceptable travel.

Another option is a progressive spring - a variable pitch coil spring will have increasing stiffness as it compresses, so no sudden change in weight transfer. A spring could be nice and soft to start with and then much stiffer as bodywork/chassis approaches the ground/wheels.
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Re: Progressive bump stops

Postby F1300Tony » Wed Jun 12, 2019 12:35 pm

Some industrial air con and water chiller units use suitable rubber discs on the mounts, about 8mm thick, around 80 Shore.
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Re: Progressive bump stops

Postby Martin Kemp » Wed Jun 12, 2019 12:36 pm

Cutting discs is a good idea but rather than cutting discs in different hardnesses it might be easier to cut them in increasing diameter, remembering to account for the hole in the middle, to give the effect of a tapered stop. Standard Avo bump stops are tapered but not enough as they would spin you off at Mansfield if the car dropped onto them (in my experience). If the taper is longer and more gradual and starts to act earlier in the corner, then it would act as a rising rate spring and the driver will be better able to cope although I don't really want to see Alex go any faster.

I think that the aero cars may be using the soft springs to pass the ride height test and the bump stops act as the real springs under aero loadings.
I considered this for the Falcon but I didn't think that the downforce would be sufficient to make it worth the complication or that my 70 year old neck would stand the bumpy ride.

Another way you can measure the characteristic of your new composite spring by standing a volunteer in the car and measuring the deflection. Then get him to drink a pint of beer - the specific gravity is marked on the bottle - and measure the new deflection. Continue to plot a graph until you reach the end stop, run out of money or the volunteer falls out of the car. At least you should be able to get another volunteer easily enough

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Re: Progressive bump stops

Postby GregSwan » Wed Jun 12, 2019 5:24 pm

:) !!!
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