Spring & Shock length?
Discussion
When designing suspension (dual wishbone in my case), is it generally better to have longer shocks and springs, or can short ones do the job just as well? Obviously they have to be the right length for any pre-existing mount points, but if designing a suspension from scratch, with the freedom to locate the mounts in a variety of places, is longer better, worse, or no difference? I'm asking because at some point it looks like I'm going to have to redesign the suspension on my kit.
Thanx, Pum.
Thanx, Pum.
Good question that.
I guess it depends on how much suspension travel you are planning to allow for. You obviously don't want to be hitting the bump stops. I reckon for reasons of weight and strength the spring and damper should be as short as is possible for the amount of suspension travel that will occur.
I guess a short damper will overheat faster than a long one.
I guess it depends on how much suspension travel you are planning to allow for. You obviously don't want to be hitting the bump stops. I reckon for reasons of weight and strength the spring and damper should be as short as is possible for the amount of suspension travel that will occur.
I guess a short damper will overheat faster than a long one.
First point has to be what is your defined bump travel.
Second point relates to how you intend to use your damper. Just damping, as a travel stop (in either bump, rebound or both). Are you inteding to incorporate a bump stop as a damper collar, and does it act as a bump stop or a spring assister.
You are likely to be using the damper as a rebound stop - if you don't, you can create a situation where the spring completely decompresses, rattles around, and potentially snags on something. You are probably not going to use it as a bump travel stop, as the impact loads get pretty horrendous.
Now get your damper manual. Every damper design can be defined as a fixed length, plus twice the travel. Draw out your suspension articulation, fix a hub mount point (for the damper), choose a frame mount point, and draw two arcs. See whether it meets your articulation rules. Repeat as necessary.
Broadly, I would suggest you go for a longer damper, mounted higher on the frame:
- More flexibility with your choice of damper, and positions. More likely to find a setup that stays within the parameters you set.
- Articulation angle of the damper will be smaller. Spring and damper rates (as wheel rates) will remain more consistent through bump travel (at its most extreme, a horizontal spring has no bump stiffness at all and is just stressing your wishbones). Indeed you may be able to tailor it to give some gain. Whatever, you have more flexibility to tailor it.
- If you are using oil dampers, the more vertical format reduces the risk of aeration, which degrades performance (and also saves you buying unnecessarily expensive dampers that counteract a problem you could have designed out)
As an example, take a look at the front end of a Caterham. The units are short and fitted at quite a flat angle. You end up with lowering-rate. Practically, you could raise the frame mount by 3". This would minimise the lowering rate. In turn, that might allow you to soften the springs, whilst retaining the same wheel rate (smoother ride, less NVH transmission). Alternatively, you could soften off the roll bar, which offers the potential for additional 'free' grip.
Second point relates to how you intend to use your damper. Just damping, as a travel stop (in either bump, rebound or both). Are you inteding to incorporate a bump stop as a damper collar, and does it act as a bump stop or a spring assister.
You are likely to be using the damper as a rebound stop - if you don't, you can create a situation where the spring completely decompresses, rattles around, and potentially snags on something. You are probably not going to use it as a bump travel stop, as the impact loads get pretty horrendous.
Now get your damper manual. Every damper design can be defined as a fixed length, plus twice the travel. Draw out your suspension articulation, fix a hub mount point (for the damper), choose a frame mount point, and draw two arcs. See whether it meets your articulation rules. Repeat as necessary.
Broadly, I would suggest you go for a longer damper, mounted higher on the frame:
- More flexibility with your choice of damper, and positions. More likely to find a setup that stays within the parameters you set.
- Articulation angle of the damper will be smaller. Spring and damper rates (as wheel rates) will remain more consistent through bump travel (at its most extreme, a horizontal spring has no bump stiffness at all and is just stressing your wishbones). Indeed you may be able to tailor it to give some gain. Whatever, you have more flexibility to tailor it.
- If you are using oil dampers, the more vertical format reduces the risk of aeration, which degrades performance (and also saves you buying unnecessarily expensive dampers that counteract a problem you could have designed out)
As an example, take a look at the front end of a Caterham. The units are short and fitted at quite a flat angle. You end up with lowering-rate. Practically, you could raise the frame mount by 3". This would minimise the lowering rate. In turn, that might allow you to soften the springs, whilst retaining the same wheel rate (smoother ride, less NVH transmission). Alternatively, you could soften off the roll bar, which offers the potential for additional 'free' grip.
Good starting point is to make sure you go for a length that you can actually get.
When I visited AVO in Northampton they said they've had a few people trying to get dampers for kit cars whose suspension design simply doesn't provide enough room for a viable damper.
AVO supply a number of kit car manufacturers and are dead helpful, so suggest you have a word with them.
When I visited AVO in Northampton they said they've had a few people trying to get dampers for kit cars whose suspension design simply doesn't provide enough room for a viable damper.
AVO supply a number of kit car manufacturers and are dead helpful, so suggest you have a word with them.
Interesting stuff so far - thanx.
I spoke to some spring/shock suppliers at the Donnington Kit Show recently, and for an existing suspension they were saying if I specify the shortest and longest length between mount points (ie: when the suspension is at the extremes of its travel) then they can supply a generic adjustable shock that will probably do the job. I guess that is based on assumptions that my unspecified requirement are not very usual, which is probably true. WRT spring rates, they suggested looking at corner weights and picking likely springs, and then changing if necessary on a semi-trial-and-error basis, as springs are not too expensive, which sounds fairly OK to me.
From what you chaps are saying, for my new suspension design, it sounds like one approach for me is to work out the up/down range of travel of my proposed bottom mount point, and the max and min height I could place the top mount point, and then ask the suppliers what best spring/shock lengths would fit that range of possibilities; sound sensible? I'm not looking for the ultimate race setup, but rather something that will work decently for road and trackdays. From what you're saying the required travel will impose a minimum length (ie: shorter shocks and springs can do less travel). Beyond that, shorter may give a more convenient design, but longer is probably better for geometry, heat dissipation, etc.
GreenV8S: Good point re: geometry most important. Am I right in assuming that the more leaned over the shock/spring is to the direction of suspension movement, the worse that is geometry-wise?
BTW, this is a project for the future rather than something I'm doing now, and I'm planning on reading some of the suspension design material mention elsewhere before getting into this properly. I'm asking now because the body will be off the chassis for only a short time longer; after the body is fixed on, chassis mods (eg: for suspension changes) become more difficult, so I'm trying to get a quick basic idea of the issues.
Thanx, Pum.
>> Edited by Pum on Friday 11th November 17:38
I spoke to some spring/shock suppliers at the Donnington Kit Show recently, and for an existing suspension they were saying if I specify the shortest and longest length between mount points (ie: when the suspension is at the extremes of its travel) then they can supply a generic adjustable shock that will probably do the job. I guess that is based on assumptions that my unspecified requirement are not very usual, which is probably true. WRT spring rates, they suggested looking at corner weights and picking likely springs, and then changing if necessary on a semi-trial-and-error basis, as springs are not too expensive, which sounds fairly OK to me.
From what you chaps are saying, for my new suspension design, it sounds like one approach for me is to work out the up/down range of travel of my proposed bottom mount point, and the max and min height I could place the top mount point, and then ask the suppliers what best spring/shock lengths would fit that range of possibilities; sound sensible? I'm not looking for the ultimate race setup, but rather something that will work decently for road and trackdays. From what you're saying the required travel will impose a minimum length (ie: shorter shocks and springs can do less travel). Beyond that, shorter may give a more convenient design, but longer is probably better for geometry, heat dissipation, etc.
GreenV8S: Good point re: geometry most important. Am I right in assuming that the more leaned over the shock/spring is to the direction of suspension movement, the worse that is geometry-wise?
BTW, this is a project for the future rather than something I'm doing now, and I'm planning on reading some of the suspension design material mention elsewhere before getting into this properly. I'm asking now because the body will be off the chassis for only a short time longer; after the body is fixed on, chassis mods (eg: for suspension changes) become more difficult, so I'm trying to get a quick basic idea of the issues.
Thanx, Pum.
>> Edited by Pum on Friday 11th November 17:38
Shock angle has no bearing on the effective spring rate. You are actually altering the suspension load paths with is when you get into the complicated maths! It may have the same effect of reducing the spring rate on some characteristics but you will also be getting different amounts of camber change throughout the suspension travel.
Shock lengths should be determined by your suspension travel, full bump to full travel. Bearing in mind if the cars for track use you never want it to be on the bump stops. If you are planning to go to a shorter shock then make sure you dont alter where it attatches unless you do the appropriate calculations so you know it will be an improvement/the same.
If you are planning on re-designing the suspension then I would say to look into using a push/pull rod style with the dampers mounted inboard. If its done properly it will look very neat. Best to get some suspension books and look at how Camber Change, Roll Centres, Instantaneous centres, motion ratios and all the other bit, change when the suspension design changes. To do it properly will take a lot of work but will definitely be worth it in the end
Shock lengths should be determined by your suspension travel, full bump to full travel. Bearing in mind if the cars for track use you never want it to be on the bump stops. If you are planning to go to a shorter shock then make sure you dont alter where it attatches unless you do the appropriate calculations so you know it will be an improvement/the same.
If you are planning on re-designing the suspension then I would say to look into using a push/pull rod style with the dampers mounted inboard. If its done properly it will look very neat. Best to get some suspension books and look at how Camber Change, Roll Centres, Instantaneous centres, motion ratios and all the other bit, change when the suspension design changes. To do it properly will take a lot of work but will definitely be worth it in the end
nightdriver said:
Shock angle has no bearing on the effective spring rate. You are actually altering the suspension load paths with is when you get into the complicated maths! It may have the same effect of reducing the spring rate on some characteristics but you will also be getting different amounts of camber change throughout the suspension travel.
I suspect the original question was in the context of twin wishbone suspension. In that context, I don't think the situation is as complicated as described above.
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