Rocket3 Roadster Trike - Differential ratio question

Chariot · May 31, 2024

Is there anyone out there who is knowledgeable about the ford sierra diff commonly used in UK trikes built since the Rocket3 was introduced. My problem is I think my trike is pulling too high a gear through the diff. We measured the ratio at 3.125, could possibly have been 3.2. This ratio probably works ok with normal trikes but as my trike is carrying a wheelchair it is lugging 200kg between chair and passenger. As a result of this unless ridden carefully it is prone to destroying clutches in a few thousand miles. So far I have tried every clutch and ended up with Barnett with stronger springs and shims under the bolts. This is ok but you can't boot it away from the lights. i was thinking of having a lower ratio fitted to the diff. Maybe 3.6. Or is there anything available in-between 3.2 and 3.6. Any help would be greatly appreciated. I have talked to Cassava trikes but they no longer use a sierra diff having moved to BMW
Cheers
Kenny

Taz · May 31, 2024

I am no expert on trike diffs but I do have some thoughts. I absolutely agree a lower gear ratio (higher numerically) would benefit your situation. Overall you want to lower the gear ratio, at a minimum for first gear but in reality, short of changing out the gears for 1st gear in the gearbox, any other changes are going to change all the gears so to speak. There are lots of ways to accomplish this.

Based on your scenario, I don't think something like a 3.6 or so would be low for you. It will also impact top gear as well. Another way of getting a similar effect is to go with shorter tires. That will impact the gear ratio. I don't know what you are running now, but going with a lower profile tire, would also give you the same effect. That would be at the likely expense of ride quality, but it might be worth the trade-off. Use this calculator to figure out the difference. I put in a 245/60/17 and dropped the aspect ratio to 245/50/17 and that would help lower your effective gear ratio by almost 7%. That would change your 3.12 ratio to almost like a 3.33. That might be enough for you. Tires are cheaper than clutches.

I just thought I'd approach this problem from a different perspective, and it might be much cheaper and easier to go this route.

Tire Size Calculator

Free calculator to find the dimensions of a tire based on its code. It also helps in choosing alternative tire sizes based on rim size or comparing two tires.

www.calculator.net

Chariot · Jun 2, 2024

Taz said:
I am no expert on trike diffs but I do have some thoughts. I absolutely agree a lower gear ratio (higher numerically) would benefit your situation. Overall you want to lower the gear ratio, at a minimum for first gear but in reality, short of changing out the gears for 1st gear in the gearbox, any other changes are going to change all the gears so to speak. There are lots of ways to accomplish this.

Based on your scenario, I don't think something like a 3.6 or so would be low for you. It will also impact top gear as well. Another way of getting a similar effect is to go with shorter tires. That will impact the gear ratio. I don't know what you are running now, but going with a lower profile tire, would also give you the same effect. That would be at the likely expense of ride quality, but it might be worth the trade-off. Use this calculator to figure out the difference. I put in a 245/60/17 and dropped the aspect ratio to 245/50/17 and that would help lower your effective gear ratio by almost 7%. That would change your 3.12 ratio to almost like a 3.33. That might be enough for you. Tires are cheaper than clutches.

I just thought I'd approach this problem from a different perspective, and it might be much cheaper and easier to go this route.

Tire Size Calculator

Free calculator to find the dimensions of a tire based on its code. It also helps in choosing alternative tire sizes based on rim size or comparing two tires.

www.calculator.net

Hi Taz, many thanks for your excellent advice just been out checking my tyre size. Unfortunately I am running 245/45 R17 could drop to 40. Even given the state of our awful roads here might be worth a try.Would give me a 3.76 reduction. We could run lower pressures for a bit more comfort

Taz · Jun 2, 2024

Something is better than nothing... You could also try going a slightly narrower tire as you'll still have 2 of them. That will also reduce the height as well. Just going to a 225/45/17 will be a 2.76% change. If you went to a 225/40/17 that would be a 6.21% change. You'll still have a lot of tire for the bike.

Just another thing you can play around with to try and get you closer to where you need to be. Use the calculator and play around with it. Obviously for best ride comfort, you'll want the max sidewall height you can get. As you mentioned, playing with tire pressure will help. Since you have 2 tires, you have plenty of support at power pressures, given the likely load rating of each tire.

Chariot · Jun 3, 2024

Taz said:
Something is better than nothing... You could also try going a slightly narrower tire as you'll still have 2 of them. That will also reduce the height as well. Just going to a 225/45/17 will be a 2.76% change. If you went to a 225/40/17 that would be a 6.21% change. You'll still have a lot of tire for the bike.

Just another thing you can play around with to try and get you closer to where you need to be. Use the calculator and play around with it. Obviously for best ride comfort, you'll want the max sidewall height you can get. As you mentioned, playing with tire pressure will help. Since you have 2 tires, you have plenty of support at power pressures, given the likely load rating of each tire.

Hi Taz,

I've ordered 2 Bridgestone Turanza 6s in 245/40/17. This will give me half an inch less road clearance at the diff. We have sleeping policemen here (road humps) at the moment not a problem except from the odd 1 or 2 . Costing me £316 at the moment which is a lot better than changing a diff ratio, if this doesn't work I will try the 225s. The bridgestone is known to be a comfortable tyre. Thanks

Taz · Jun 3, 2024

Good luck. I hope it makes enough a noticeable difference. Just hit that policeman square on and both tires should go up and over him at the same time so you should be good.

Chariot · Jun 4, 2024

Getting tires fitted in the next couple of days, but it has turned back to winter here. Snow above 2000ft. In June!

Taz · Jun 4, 2024

Get some good all-season or mud & snow tires.

Chariot · Jun 5, 2024

Chariot said:
Hi Taz,

I've ordered 2 Bridgestone Turanza 6s in 245/40/17. This will give me half an inch less road clearance at the diff. We have sleeping policemen here (road humps) at the moment not a problem except from the odd 1 or 2 . Costing me £316 at the moment which is a lot better than changing a diff ratio, if this doesn't work I will try the 225s. The bridgestone is known to be a comfortable tyre. Thanks

Hi Taz, tyres fitted just waiting on some good days. its like winter here.

Taz · Jun 5, 2024

That rig looks pretty cool.

Rocket3 Roadster Trike - Differential ratio question

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