Lotus Elan

Hand brake tree,

PostPost by: davidj » Fri Aug 16, 2019 1:38 pm

Good afternoon,

I am just assembling a hand brake tree on a Spyder chassis. The lotus manual shows the hand break tree pivoting on the chassis. Why is this? I would have thought it would be more robust and less resulting wear on the location holes if boss and spacer were sandwiched between the plates and pivoted on the bolt, or am I missing something? Maybe they were concerned the bolt could be over tightened, squashing the boss and stopping movement?

I am sure someone can point out why it is not designed like this.

Cheers.


David
Attachments
screenshot-2019-08-16-at-2.18.17-pm.png and
davidj
Third Gear
Third Gear
 
Posts: 288
Joined: 09 Apr 2008

PostPost by: baileyman » Fri Aug 16, 2019 2:04 pm

I think it is to allow for suspension movement.

In addition, I took advantage of the lower pivot to lean the tree to the rhs so that I would have just a little more rotation available before impinging the diff cover. My new tree is oriented at 90 deg, whereas in your pick it looks like one with the upper arms rotated a bit clockwise, which should also offer more rotation. That early design seems like it should be better than the current replacement.

Also, with that suspension travel idea in mind I made sure to cinch up the nuts for the rod ends loose so they can pivot a bit at the upper tree arms.

John
baileyman
Second Gear
Second Gear
 
Posts: 113
Joined: 17 Aug 2017
Location: Boston

PostPost by: vxah » Fri Aug 16, 2019 4:21 pm

The tree has to be able to move left/right to allow for difference in the actuation rod travel. If say the left adjuster were a little slacker than the right when the brake is applied one rod would move more than the other so the tree would be able to "tilt" to the right as well as rotate...
vxah
Third Gear
Third Gear
 
Posts: 265
Joined: 08 Nov 2012
Location: Cambridge UK

PostPost by: snowyelan » Fri Aug 16, 2019 11:29 pm

Probably because it's cheaper. I think Colin used the word 'lighter' in place of 'cheaper' depending on who he was talking to.....
Scott
45/9011
Hawkestone, On, Ca
snowyelan
Second Gear
Second Gear
 
Posts: 167
Joined: 14 Sep 2003
Location: Hawkestone, Ontario, Canada

PostPost by: Quart Meg Miles » Sat Aug 17, 2019 10:46 pm

vxah wrote:The tree has to be able to move left/right to allow for difference in the actuation rod travel. If say the left adjuster were a little slacker than the right when the brake is applied one rod would move more than the other so the tree would be able to "tilt" to the right as well as rotate...

Correct! The tree self-centres between the hubs and ensures that both caliper levers get the same pull.

The pull of the cable has to be resisted by the chassis holes but provided these and all the pivot pins are well greased wear is not a problem.

If your tree is off the car, then I would recommend redrilling all the pivot holes inline with the cable and rod ends to closely fit new pins (which you may have to make or purchase specially). Smother everything in grease. I drove 100,000 miles without a working handbrake before redrilling the tree, and 120,000 with it working perfectly since, merely adjusting the calipers for pad wear.
Meg

26/4088 1965 S1½ Old and scruffy but in perfect working order; the car too.

Put your money where your mouse is, click on "Support LotusElan.net" below.
User avatar
Quart Meg Miles
Fourth Gear
Fourth Gear
 
Posts: 866
Joined: 03 Oct 2012
Location: West Sussex, UK (South coast)

PostPost by: davidj » Sun Aug 18, 2019 2:40 pm

Afternoon,

Sorry, miss leading english on my part; I was not asking why the hand brake tree pivots. I understand it has to pivot to successfully operate both brakes. Rather it was a general question about the design of the assembly, as it pivots on the two chassis holes. I would have thought a simpler design would have sandwiched the tree boss between the two chassis plates and pivot on the 2 1/4 inch bolt.

However, it is a rather academic discussion as my chassis has already been drilled to take the o/d of the tree boss.

Cheers,

David
davidj
Third Gear
Third Gear
 
Posts: 288
Joined: 09 Apr 2008

PostPost by: davidj » Sun Aug 18, 2019 3:07 pm

....... as shown in this (crude) editing of the original drawing.
Attachments
screenshot-2019-08-18-at-4.03.22-pm.png and
davidj
Third Gear
Third Gear
 
Posts: 288
Joined: 09 Apr 2008

PostPost by: Higs » Sun Aug 18, 2019 5:39 pm

Because the the bolt can be done up tight without deforming the chassis rails (it compresses onto the pivot tube) and hence it cannot be assembled wrongly.

In your modified design, if you were to tighten the bolt too far, it would deform the chassis rails and potentially could lock up the pivot tube so that it could not rotate.

You could do it in a way that pivots on the bolt - either with a shoulder on the bolt to restrict the amount of tightening or with a second tube inside the pivot tube that would be trapped between the rails - leaving the pivot to rotate. Both of these are more complex, more expensive and probably heavier by a few grams - hence the current design.

Richard
Higs
Second Gear
Second Gear
 
Posts: 159
Joined: 02 Dec 2009
Location: Bristol, England

PostPost by: baileyman » Mon Aug 19, 2019 1:53 am

That's interesting. My replacement, source from rd, is similar to the second design.
baileyman
Second Gear
Second Gear
 
Posts: 113
Joined: 17 Aug 2017
Location: Boston

PostPost by: davidj » Mon Aug 19, 2019 8:33 am

@higs I thought that may be the case as well, see my initial post. However, because of the lip design, the bolt would have to be very tight to deform the chassis rail, and with the existing design, the mounting holes wear as the handbrake operates.

However, water under the bridge now!
davidj
Third Gear
Third Gear
 
Posts: 288
Joined: 09 Apr 2008

PostPost by: rgh0 » Mon Aug 19, 2019 10:28 am

The handbrake tree is a true example of Lotus built in lightness and simplicity based on clever engineering analysis of the stresses involved. The tension in the rods to the brake levers balances the tension in the hand brake cable as the tree tilts forward when loaded . The for / aft and side to side flexibility also accommodates the suspension movement and all of this occurs without additional components or weight or complexity.

A small design challenge back in the early 60's but something that most other manufacturers struggled with then and still do now

cheers
Rohan
User avatar
rgh0
Coveted Fifth Gear
Coveted Fifth Gear
 
Posts: 7088
Joined: 22 Sep 2003
Location: Melbourne, Australia

PostPost by: RogerFrench » Mon Aug 19, 2019 2:07 pm

Not a Lotus original design, I think.
The Austin-Healey Sprite, and maybe other BMC cars, has a similar tree but as an aluminium casting. Very nice, but too expensive for Lotus!
1967 Elan S3
1972 Europa Twin Cam
2005 Elise
User avatar
RogerFrench
Fourth Gear
Fourth Gear
 
Posts: 589
Joined: 01 Dec 2009
Location: Richmond, TX

PostPost by: Bigbaldybloke » Mon Aug 19, 2019 10:08 pm

Just a note, the workshop manual includes a drawing for a modified handbrake tree that gives better leverage for the handbrake, depending on the age of your car it may already have it as standard, if not its a worthwhile mod if you have it apart.
Change is inevitable, except from a vending machine!
Bigbaldybloke
Fourth Gear
Fourth Gear
 
Posts: 592
Joined: 16 May 2017
Location: Lincolnshire

PostPost by: john.p.clegg » Tue Aug 20, 2019 5:30 am

User avatar
john.p.clegg
Coveted Fifth Gear
Coveted Fifth Gear
 
Posts: 5568
Joined: 21 Sep 2003
Location: Manchester

Total Online:

Users browsing this forum: No registered users and 9 guests