MGB V8 front crossmember

3xpendable · Post by **3xpendable** » Thu Sep 23, 2010 7:36 am

Can someone help me? I had planned to use the original front crossmember from my rubber bumper shell for my V8 conversion as i've been told it has the extra clearance required for the V8 sump. However, in my 'build' thread someone has stated that if you use chrome bumper springs (2 inches lower) with a rubber bumper crossmember you mess up the suspension geometry. Is this correct? I'm about to rebuild my front suspension so want to know which one to use before I do.

Thanks

mgbloke · Post by **mgbloke** » Fri Sep 24, 2010 7:32 am

Welshie
As you know the rubber bumper crossmember has a 1" spacer welded to the top. You can use a chrome bumper crossmember with a few modifications. This puts the sump pretty close to the rack but it will work.
The modifications include cutting and re welding the rack mounts and extending the rack steering shaft. There is an easy way to make sure you dont change the rack position and so keep the correct geometry.
let me know if you need further details.

Mark

3xpendable · Post by **3xpendable** » Fri Sep 24, 2010 7:56 am

mgbloke wrote:Welshie
As you know the rubber bumper crossmember has a 1" spacer welded to the top. You can use a chrome bumper crossmember with a few modifications. This puts the sump pretty close to the rack but it will work.
The modifications include cutting and re welding the rack mounts and extending the rack steering shaft. There is an easy way to make sure you dont change the rack position and so keep the correct geometry.
let me know if you need further details.

Mark

Hi Mark,

Many thanks for that. Yes I was aware the rb crossmember has a spacer, but if this is between the crossmember and the body then the suspension geometry should be the same as a cb right? To be honest I have 2 rb crossmembers and the only cb I have is on my current road car so can't pinch that. If I can use the rb crossmember with no geometry issues i'd rather use that. Can I just fit 2 inch lowering springs to this crossmember?

mgbloke · Post by **mgbloke** » Fri Sep 24, 2010 9:35 am

The Rack mounts for both crossmembers are totally different
The RB crossmember has to be one of BL's biggest bodges. If you look at the rack mountings you will se that one of them sticks further forward than the other. They did this instead of changing the angle of the rack steering shaft. Not only did they fit the spacer to the crossmember but they sunk the steering UJ back up into the bulkhead to clear the V8 exhaust hence the odd rack mounts.
Yes you can lower a RB crossmember by 2 " but you will have issues with bumpstops and the A arms will be at an extreme angle (possible bumpsteer) not really what you want. It is dooable but not your best option.
It would be easier for you to stick with the RB but if you decide to go CB I could give you more info.

Mark

3xpendable · Post by **3xpendable** » Sat Sep 25, 2010 7:05 am

mgbloke wrote:The Rack mounts for both crossmembers are totally different
The RB crossmember has to be one of BL's biggest bodges. If you look at the rack mountings you will se that one of them sticks further forward than the other. They did this instead of changing the angle of the rack steering shaft. Not only did they fit the spacer to the crossmember but they sunk the steering UJ back up into the bulkhead to clear the V8 exhaust hence the odd rack mounts.
Yes you can lower a RB crossmember by 2 " but you will have issues with bumpstops and the A arms will be at an extreme angle (possible bumpsteer) not really what you want. It is dooable but not your best option.
It would be easier for you to stick with the RB but if you decide to go CB I could give you more info.

Mark

Well now i'm getting confuzzled! I was aware that the rack mounts had been altered to keep the steering rack/arm angle the same as they raise the ride height but welding a 25mm spacer on the top of the crossmember to body mounts. Ok fair enough BUT the mounting for the damper is unchanged and the same height so this in theory means that if the kingpin length is the same as a cb car (which it all is) then fitting cb springs would make all this geometry the same as a std chrome bumper car and the reason its all to pot is because they stick 2 inch longer springs in there. So do you mean what is messed up is the relationship between the steering rack/arms and the front suspension? If the rack is raised higher then presumably the length of the steering arms moves too much under load and causes the bump steer?

Another issue is someone has just told me this:

I put lower front springs on the RB cross member and, whilst the steering is light and accurate, I have no self centreing. I suspect that lowering the front without shimming the crossmember/chassis mounting point might have affected the castor angle.

So...?

mgbloke · Post by **mgbloke** » Sat Sep 25, 2010 7:35 am

Ah I se we are not quite thinking along the same lines.
The Chrome bumper springs will not give you a drop of 2" from my experience more like 1/2" to 3/4" depending how heavy the car is (rubber bumpers still fitted)
Free length of pre 1975 springs is 9.875" and post 1975 being 10.125" with slightly different spring rates. If you fit the CB spring it will be just fine.
I thought you meant 2" lowered springs which would have a much higher rate and put the A arms pointing upwards.

Depends on how much lowering you want really.

Mark

3xpendable · Post by **3xpendable** » Sat Sep 25, 2010 11:14 am

That makes sense then! Yes basically my intention is to bring the car down to cb ride height. so if I drop it by say the 1 inch raise given in the crossmember, will that be ok? or will I still have geometry issues?

mgbloke · Post by **mgbloke** » Wed Sep 29, 2010 6:23 am

A 1" drop will be fine but dont forgetyou might need to alter your bump stops.

Cheers
Mark