Diff Movement

[Guest gdc]

i spoke to richard some time ago at gbs, he told me of a lightweight that had passed sva and the owner had mentioned that he thought the diff was moving in the diff support structure.ie up and down in relation to acceleration and deceleration.

at that time mine did not have that prob, it now has developed into a very woring problem .

under acceleration there appears to be no problem ,however when decelerating in gear there is now a noise that appears to be the propshaft clouting the tunnel.

taking that into account i have now added a further support to the rear of the diff and mated it to the top of the diff support housing on my daughters car which is still being built .

if sucessusfull i will add to my car during the winter rebuild.and dans .

my real question is have any other lightweight builders had the same problem .the real prob is with so few on the road the feedback is not going to be that good ,so am i just talking to my self ,poss!

the prob appears to be that the diff under load is twisting in the diff support structure.

it is housed between the diff upwrights two pieces of ally sheet.

spaced in rh fashion with washers or spacers,using the two front nose bolts and the two large shall we say diff body studs,

this has played on my mind and on my daughters car i have bonded large plate washers to the ally in an attempt to reduce the action of the inherrent forces and also added the rear stay to the top of the diff.

does any one have any other ideas however wacky that might over come this twisting action.

a steel cradle comes to mind but the limmited space may make it difficult to fabricated in situe.

 

regards graham

[Guest ScotMac]

Sorry, you are right...i don't have any feedback, yet. However, since my strengthening is soo much different than yours, i don't know how useful any feedback i give would be anyway. Now that i have the fiero back from Washington, and thus have my tools back, i hope to get a bit more work done on the hood.

 

I am just hoping that i have covered that area, since my strengthening of the diff box is very extensive, since that was the first area brought up as a possible problem.

 

Graham, is adding support to the top enough? I don't know how you did it, and am not doubting your ability, but just wondering. If the side mounts were not strong enough, by the adding the top strengthener, might you end up w/ a rocking motion when the side mounts begin to go again?

 

ie, i agree that your attempts to strengthen the diff box where it bolts up on the sides, by bonding the washers, is the more appropriate solution. If you remember, my solution was to use a thick 3/16" slab of flat 6061-t6 ally for the entirety of both diff box sides, combined w/ 1/8" angled sections on both sides of the angled sections that connect to the boxed cross members. That gave me a total of 3/8" of ally for the "bottom" of the two side diff bolts to bolt up to, combined w/ the big RH washers.

 

Hope you can figure it out.

 

Cheers, -Scot

[Guest gdc]

hi scott

i wasnt having a go at you ,but it seems i end up with the probs and in the end sort them out and the other lightweight boys watch and see the result. i would just like to see some input from other sources.

i dont have all the answers.

but i may have cracked this one.

the rear top mount is in and today i have sorted a front bottom mount (not what your thinking)

that can be retrofitted after the build is complete if nessary.

i have fitted it to my daughters car and made a pattern so that the other two cars can be sorted withought to much hassle.

if any members want details i am happy to oblige however non members will have to pay the £15 membership or sort it them selves.

pm me if you want details.

i may even be persuaded to make them if required.

scott nice to see you back after you fierro experiance on track.

i just sold my fiero for a pittance ,to make space here.

ohh did you see zoom zoom has ordered a zero 7.

so the bears will be back.

regards graham

[Guest diyer]

Guys,

 

I have not noticed any diff movement. No visible deformation of the diff box and no noises while driving. I will complete a full inspection and report on my findings.

 

cheers

Simon

[E.Hunt]

Damm can't find my picture

 

Graham not on road so can't say if this will help.I've built a 'u' shapped structure that is bolted to the bottom front diff/wishbone tube going up to support front diff support, then it goes forward along the tunnel to take the seat belt fixing, It's bolted to the main tunnel bottom over the hole cut out in the floor and the rivets in the sides were replaced with 6mm nuts & bolts. Hopefully the sides are tall anouth to stop the propshaft leaving the confines of the tunnel if it gives way. all from 5MM steel not saving weight but saving worry

[E.Hunt]

finally found picture on garage pc

diff seat

in situ

 

dosn't show the plates going up the side to keep the prop from escaping

[Guest gdc]

cheers colin

the pics are good and clear looks good and strong a bit like i have done on my daughters ,you might want to concider fitting the origonal top rear mount as well even if you do away with the rubber and mount the top solid to the boot floor and brace it across to the upwrights.

by the way i am having a party on the 3nov if you want to get s t faced and stop over your more than welcome.

regards graham

ps all are welcome.

[Guest gdc]

hi

i spoke to richard at gbs yesterday and he told me of a phone call from a lightweight owner whos diff had spun in the diff housing .

the guy was putting close to 200bhp throught it and it was unmodified.

 

i think we need to address this prob as a matter of urgency to ovoid further probs .

as you know mine is modified and i think its moving ,but with all the other things going on i havnt had chance to check it out.

richard has said that the forces when decelerating in gear are larger than when accelerating , this is when i have the problem. the car that spun its diff has a large ammount of horses , and mine has a high compression engine which may account for my problem.

when i spoke to richard we discussed a frame work based on the zero to hold the diff in place and braced to the rest of the car,my idea so far is to try to build a cradle to hold the diff and mayby support the rear wishbones, doing away with the ally box sections.

prob is where do you stop with the steel , could end up with a full teel chassis?

 

any thoughts?

 

regards graqham

 

ps i want to keep the lightweight as an alloy monococue, and not a locust or a 07.

so if we have to add some steel ,then i think it needs to be limmited.

[Guest gdc]

so i finaly got around to stripping my car to check the diff out, result is no movement at all.

 

the prob appears to be the front uj on the prop, whitness marks to the edges.

when i fitted the engine i moved it to accomodate the carbs ,so they didnt foul the peddle box,

was fine while i was getting used to the car but as the confidence grew the prob appeard .so all in all not a bad result my fault for moveing the engine over and putting the prop out of allingment.

new type nine with long first going in and the bike carbs are smaller so i should be able to straighten out the prob .

but i am fitting the origonal type diff top mount to bolt to the boot floor, just in case.

 

regards graham

[Guest zoomzoom]

Thats good new graham,

The zero chassis has the plate in place for the 'top' mount of the diff and I was not going to bother but have now had a rethink!

 

Marcus

[peter_m7uk]

Hey Lightweighters,

 

Just wanted to show you what I did in response to these worries - I decided to use the

holes in the back of the diff for a bit of extra support. I got a couple of bits of 3mm

mild steel and made up a bracket. Was a bit fiddly to retrofit given that you can't see

what you're doing in the diff box and that I already had the fuel tank in!

 

Anyway, please click on the links to check out the Diff Brackets and Diff Rear Mount.

 

I reckon that should do it! My only worry is that the original Sierra rear mount used rubber

to connect the chassis and diff. Presumably this was done to account for movement of the

rear suspension subframe relative to the chassis and would damp out any shocks? The Lightweight

does not use separate subframes, so hopefully this method shouldn't be too stiff! After all, it's there

in the first place to stop movement.

 

Cheers,

Pete

[peter_m7uk]

"richard has said that the forces when decelerating in gear are larger than when accelerating , this is when i have the problem. the car that spun its diff has a large ammount of horses , and mine has a high compression engine which may account for my problem."

 

Graham, not sure that this is true.. In the worst case, engine braking from high rpm in 1st gear with a type 9 box, the maximum torque applied to the diff is 3.65 times the torque of an engine spinning with a closed throttle at high rpm. (The engine naturally decelerates rapidly anyway, so if you brake hard a lot of the work in slowing down the engine is done by the engine) I doubt if these forces

are greater than accelerating hard in any gear other than possibly 5th, where the torque applied

to the diff is only 0.82 times the torque of the engine.

 

Any comments?

 

Pete

[Guest gdc]

hi pete

it would appear that my diff has not moved , and the problem car had not been modified.

my problem related to a miss alignment of the prop due to the carbs fouling the peddle box. the whitness marks on the prop were the giveaway that was another reason to fit the bike carbs so that i could re align the prop, re your figures , i have no idea what they mean not cos i am thick but i dont have the relevent knowledge.

 

hows the build going?

regards graham

[peter_m7uk]

Hi Graham,

 

I saw your comment about the propshaft, just wanted to tell people what I'd done as a precaution

against diff movement. Hope your problem is now solved?!

 

On my figures, I was just taking the gear ratios for a Type 9. A gearbox acts as a torque multiplier,

so it's straightforward to see the difference between engine torque and gearbox output torque.

 

The build is coming along slowly but steadily! I am getting close to the stage of fitting the engine and

gearbox into my rolling chassis now. Would love to be ready for the summer, but God knows if that's

possible! Check out the rest of my Build Pictures.

 

Pete

[Guest timswait]

My only worry is that the original Sierra rear mount used rubber

to connect the chassis and diff.

You'll be fine bolting it straight up without the rubber. It's how it's done in the 2B as standard. In the Sierra the whole rear subframe is free floating on 3 rubber mounts (including the one on the diff) This is to isolate the cabin from noise and vibration from the road and from the diff itself. In a Hood you want it all bolted up firmly and aren't too bothered about noise and vibration being transmitted to the chassis, so what you've done sounds fine.

As for whether the forces are highest under acceleration or deceleration, surely either way the torque on the diff will be highest in 1st gear? Whether it's highest under power or engine just depends whether your engine produces more torque when it's under power or engine braking from high revs. I'm not sure which would be the greater, but I'd guess they'd be of fairly similar magnitude, and it probably depends on the exact details of the engine, state of tune, compression ratio, etc.