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Converting GS750 front end to GS850 setup,need advice please-

  • Thread starter Thread starter Tola
  • Start date Start date
T

Tola

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So from a different post I found out what the best option would be to go to a dual disc setup on my '77 GS750. I found an 850 fork w/TTT and now I need brakes.

Questions are:

Would a dual disc setup from a 750 work meaning would the caliper mounting brackets just bolt on and line up properly?

I have a '77 spoked hub/wheel, is the axle diameter the same or do I have to get a new wheel?

Thanks!
 
Its a gs850 g fork, 750 brakes. My main concern s that on my single disc setup there is a large 1mm thick spacer/washer behind the disc, can't seem to find one online and not sure of I can go without on the right side?
 
Tola said:
Its a gs850 g fork, 750 brakes. My main concern s that on my single disc setup there is a large 1mm thick spacer/washer behind the disc, can't seem to find one online and not sure of I can go without on the right side?
Click to expand...

Ok, for one, you will have to use the brake calipers off the GS850G, or a like-style front brake setup if you are going to run SLOTTED front discs off the 80's series GS bikes. You may run into an issue where the older style (L-arm) brake calipers will sit too low on the brake disc and the lower part of the round pads will hit the spokes in the brake discs.

For two... lose the 1mm spacer. Is this spacer flanged, or flat? If it's flanged, it was a dust-cover for the original 750 speedo drive, which went inside of it. The brake discs should bolt DIRECTLY to the hub.

Also, the 850 front end is 37mm... so you may need a different front fender, and the axle and spacers from the 850G.

If the lower triple doesn't match your steering locks (which it should) you will have to go with a GS1000 lower triple tree.

You'll also need head bearings...
 
Last edited:
Thanks, great info! The spacers are flat & flanged. Flat one is rubber coated and was behind the rotor. I was told by a reliable source it was for vibration control. so I'll lose that one. More parts needed than I thought, going to upgrade as soon as $$ allows, think I should get bike running well first.

So the comment about 80- dbl. disc brakes rubbing on spokes had me wondering. If I go dbl. disc with 750 calipers on my '78 750 fork could rubbing be an issue with that setup? Fork on my bike must have been replaced at one time since it has a single now.
 
No. the 77/78 only had single discs. So if your right fork has caliper mounts its been replaced.
 
New issues

New issues

I started to rebuild the forks with new seals and found 2 problems.

One, the left fork stanchion is bent!!wtf, so I'm going to have to change it out and might as well upgrade.

Second problem is that all the 850 and 750 forks on ebay have a steering crown with tapered needle bearings but my steering races are the cup style that take loose balls. Guess I need new races? can't seem to find any anywhere?
 
Z1 or all Balls sell the replacement kits for little money.

Eric
 
figured out that allballs makes a kit, should have looked harder. If I upgrade to a 850 TTT do I need to get the 850 bearing set or the 750? Not sure if the headtubes are the same i.d. or if the steering tube is different meaning will the 850 steering tube fit with the 750 bearing kit? Allballs has different part numbers for each year.
 
Call them and tell them what you are doing.
It's not like it hasn't been done thousands of times before.
They will know which kit that you need. ;)

Eric
 
Sorry for all the questions. Just spoke to the guys at allballs. They have a conversion chart but he said I have to measure steerer length to figure out which thickness I need since there are several different bearings for top and bottom which would alter stack height. Am I making this too complicated and should just order bearing for the 850 TTT?
 
7981GS said:
Call them and tell them what you are doing.
It's not like it hasn't been done thousands of times before.
They will know which kit that you need. ;)

Eric
Click to expand...

Guy at allballs was helpful but not really knowledgable for my application.
 
the race height from the steering stem determines the bearing height.
The bearing ID is determined by the triple tree diameter.
The Bearing OD is determined by the stem diameter.

So if I were putting an 850 triple into a 750 I would look up the bearings and dimensions for both the 850 and the 750.
I will make up some numbers of a bearing just for explanation.
850 35mmx20mmx5mm
750 32mmx18mmx6mm

For my top bearing I would buy a 32mmx20mmx6mm bearing. At least that is the way I would do it :)
THOSE ARE NOT REAL NUMBERS SO DON"T USE THEM.

I am sure somebody else with more experience will refute me if I am totally off base.
 
You can't buy a set off the shelf for this - at least not here. An 850 uses 47x25x16 uppers and 55x30x18 lowers. A 750 standard conversion kit is 48x25x16 upper and 48x27x14 lower.

If you go to one of the bearing manufacturers websites you can work out which ones you'll need.
 
Well that sucks. I've come across numerous posts stating its a simple bolt on upgrade?? Must be for newer 750's and not the 77 but 77-79 appear to be the same and I've seem those with 850 forks?
 
Tola said:
Well that sucks. I've come across numerous posts stating its a simple bolt on upgrade?? Must be for newer 750's and not the 77 but 77-79 appear to be the same and I've seem those with 850 forks?
Click to expand...

It is simple. Just look up what bearings you need - try the SKF site for example - order them and fit. Dreef showed you how to do the sum and I gave you the numbers.

1980 onwards 750s had the same bearings as the 850s.
 
So would the equation that dreef used for the top also work for the bottom of the tree? I'm doing the same in swapping out my front end. using 80 850 tt/forks.
 
Tola said:
Well that sucks. I've come across numerous posts stating its a simple bolt on upgrade?? Must be for newer 750's and not the 77 but 77-79 appear to be the same and I've seem those with 850 forks?
Click to expand...
I think just a bearing change qualifies as bolt on :)
 
Manzarr said:
So would the equation that dreef used for the top also work for the bottom of the tree? I'm doing the same in swapping out my front end. using 80 850 tt/forks.
Click to expand...
Exact same principle for top as for bottom. Height and OD are based on the bike frame. ID is based on the triple tree.

As long as you stick within the GS family the only issues you can have are steering stop differences, fork spacing differences, and possible rake(?) difference.

If you get the axle & spacers then all GS front wheels should drop right in. The steering stop difference will require either grinding a stop on the tree off or installing a stop onto the tree. Personally I would never modify the frame and I would probably install a stop by drilling and tapping a hole so I could screw a bolt in as my stop.

The rake difference I think is only between those with leading axle forks like the L model forks... but that might be wrong?
 
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