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| Hydro front top arms https://www.ausmini.com/forums/viewtopic.php?f=2&t=99978 |
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| Author: | ausdino [ Sat May 09, 2020 7:06 pm ] |
| Post subject: | Hydro front top arms |
Just a little FYI for anyone who has a problem like I found on my mk2 S. After removing the bearings, cleaning and painting the arms, to my horror, one of them was a dry top arm (different suspension ratio with the knuckle hole closer to the centre line of the arm). Sometime before 1983, it must have been replaced as I have owned it since then, but never driven it. It took ages to find the correct part numbers : 21a1483 for the Lhs, 21a1480 for the Rhs. I was lucky enough to have a spare, which was the correct side hydro arm. A couple of pics below showing the differences. |
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| Author: | winabbey [ Sun May 10, 2020 3:18 pm ] |
| Post subject: | Re: Hydro front top arms |
Thanks for taking the time to post about that situation and the good clear images. Will no doubt be of use to others in future. |
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| Author: | Bill B [ Mon May 11, 2020 2:33 pm ] |
| Post subject: | Re: Hydro front top arms |
Bits of information like this are probably needed as more owners are considering converting from hydro to dry suspension and think you just buy a set of rubber cones and some Hi-Los. For instance, I read recently that the rear subframe for a wet setup is engineered differently to a dry subframe because of differing load factors, so even the subframes ought to be swapped over. |
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| Author: | drmini in aust [ Mon May 11, 2020 2:59 pm ] |
| Post subject: | Re: Hydro front top arms |
I don't think there's much difference in the subframes, we did a Mini Deluxe to dry a year or so ago using wet frames and arms front and back. I tapped the rear arms 3/8 UNF and used grade 8 bolts for shock mounts. Need to bend the bayonets in top of front frame so the rubber cones seat ok. |
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| Author: | Bill B [ Mon May 11, 2020 5:05 pm ] |
| Post subject: | Re: Hydro front top arms |
drmini in aust wrote: I don't think there's much difference in the subframes, we did a Mini Deluxe to dry a year or so ago using wet frames and arms front and back. I tapped the rear arms 3/8 UNF and used grade 8 bolts for shock mounts. Need to bend the bayonets in top of front frame so the rubber cones seat ok. You could be right. I don't have enough technical knowledge to make a decision but did notice the comment about there being a difference. I am interested in knowing more. Considering that the hydro system had about 300psi when stationary + the weight of the car; and the dry system only has the weight of the car. |
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| Author: | drmini in aust [ Mon May 11, 2020 5:21 pm ] |
| Post subject: | Re: Hydro front top arms |
That pressure is inside the bag. At a given ride height the upward thrust onto the bag or rubber cone will be equal. Assuming same top arm in use. |
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| Author: | Bill B [ Thu May 14, 2020 8:06 pm ] |
| Post subject: | Re: Hydro front top arms |
drmini in aust wrote: That pressure is inside the bag. At a given ride height the upward thrust onto the bag or rubber cone will be equal. Assuming same top arm in use. Not quite convinced of your argument: the hydro setup has a static pressure when the car is at rest but it isn't just contained inside the displacer. The bellows end of the displacer is pushing out towards its suspension arm and equally back into the subframe. Even when the hydro car is put on axle stands and the suspension arms are at their lowest, there is still about 100psi in the system so there is still a load on the subframes. A dry suspension car on axle stands would be putting very little pressure on the rubber units and/or the subframe. |
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| Author: | IndigoBlueCooperS [ Thu May 14, 2020 8:52 pm ] |
| Post subject: | Re: Hydro front top arms |
And what pressure does the hydro spike to when you hit a bump??? I have no idea but am curious as I suspect it is notably more than the 300psi. |
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| Author: | winabbey [ Thu May 14, 2020 10:20 pm ] |
| Post subject: | Re: Hydro front top arms |
Not sure if this helps or hinders in the discussion - viewtopic.php?f=49&t=96413 |
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| Author: | Bill B [ Thu May 14, 2020 10:27 pm ] |
| Post subject: | Re: Hydro front top arms |
IndigoBlueCooperS wrote: And what pressure does the hydro spike to when you hit a bump??? I have no idea but am curious as I suspect it is notably more than the 300psi. At a pressure to give normal ride height, the suspension is at around 280psi. When a wheel is deflected by a bump, some of the energy goes into the rubber 'spring' (the same as with dry suspension) and some goes into displacing fluid through a valve (the same as a shock absorber) and the movement of the fluid also absorbs some of the shock. I don't know what maximum pressures might be reached. |
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| Author: | 1071 S [ Fri May 15, 2020 11:02 am ] |
| Post subject: | Re: Hydro front top arms |
Don't forget that a fluid is incompressible .. by definition. The springing motion is not primarilly a function of the fluid, rather it is of the rubber components in the hydro unit. Back when hydro was introduced and racing regs mandated that the stock suspension had to be retained, it was common practice to shut off the fluid transfer so the only suspension movement possible was through the rubber components... With the fluid transfer blocked, the fluid acts as a solid allowing no movement. The shock caused by hitting a bump is partly absorbed by the rubber and partly by the restriction in flow of the fluid to the other hydro unit. ..and partly by the rear spring; it resists the extension of the rear unit and the compression of the front unit (assuming the front hits the bump). The laws of mechanics demand that the load on the subframe mounting unit is determined by the weight of the car not the pressure in the unit. Cheers, Ian |
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| Author: | Bill B [ Fri May 15, 2020 3:36 pm ] |
| Post subject: | Re: Hydro front top arms |
My reason for asking about wet to dry conversions is due to this comment I read recently and have now found again. From: http://www.amvcnsw.com.au/austin1800/WS ... lastic.pdf "Mini also has the rubber option but to perform this conversion properly is rather involved. Both subframes are structurally different. A wet front can be modified but the rear would require a dry donor unit for (welded) structural changes. Most conversions tend to be more than a little half-hearted. The legal and safety aspect is a current day concern. Unless done to the letter any wet to dry conversion can be considered unsafe and illegal. Conversion shortcomings basically come down to work ethic issues." As the article was lacking in any specific details I am curious to know more. |
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| Author: | slide [ Fri May 15, 2020 5:48 pm ] |
| Post subject: | Re: Hydro front top arms |
I converted my S to dry using the front wet subframe with dry upper arms. But at the back I used a dry frame rather than the wet one. At the rear I followed Calver's recommendation, which I quote: Again, as with the front, the hydro rear ‘frame can be modded to take the rubber springs... However I don’t advise this unless the Min’s used solely for normal road use, carrying two people in the front, minimal luggage, and totally standard wheels/tyres and standard ride height only. Why? Coz that curious metal cone shape on the dry type acts as a variable spring platform - progressively increasing the spring rate as more load is applied. Carrying more load than the minimal amount mentioned above will have the tyres bashing the wheel arches. Trust me in this, that’s all the explanation you’re going to get, as it’s a bit complicated (not enough room here for full dissertation). |
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| Author: | 1071 S [ Fri May 15, 2020 6:55 pm ] |
| Post subject: | Re: Hydro front top arms |
I don't think the makes sense... as the spring rate increases the suspension won't compress as much for a given load so the tyres will be kept further from the arches....????? Maybe the subframe shape interferes with the inner section of the cone keeping the mounting edge of the cone off the subframe ... which may initially reduce the spring rate. Like the front, the rear trailing arm is a different shape and you do need to jack up the Hilos if you fit them to a dry car .. ... And not all dry subframes are created equal... Cheers, Ian |
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| Author: | simon k [ Fri May 15, 2020 7:15 pm ] |
| Post subject: | Re: Hydro front top arms |
I think I read somewhere once that the cone shape in the rear subframe effectively works as a bump stop - when the inside of the cone hits it the 'spring' in the cone becomes a rubber block I converted my blue S using dry rear arms but hydro for everything else. I ground slots in the cones to fit over the bag locating tangs... 1071 S wrote: it was common practice to shut off the fluid transfer so the only suspension movement possible was through the rubber components... With the fluid transfer blocked, the fluid acts as a solid allowing no movement. this works really well in motorkhana cars too |
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